[v3] rcu/kvfree: Prevents cache growing when the backoff_page_cache_fill is set

  Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
executed before kfree_rcu_monitor() to drain page cache, if the bnode
structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
page cache again in kfree_rcu_monitor(), this commit add a check
for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
if the krcp structure's->backoff_page_cache_fill is set, prevent page
cache growing and disable allocated page in fill_page_cache_func().

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
---
 kernel/rcu/tree.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
  

On Sat, Apr 08, 2023 at 10:25:17PM +0800, Zqiang wrote:
> Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> executed before kfree_rcu_monitor() to drain page cache, if the bnode
> structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> page cache again in kfree_rcu_monitor(), this commit add a check
> for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> if the krcp structure's->backoff_page_cache_fill is set, prevent page
> cache growing and disable allocated page in fill_page_cache_func().
> 
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>

Much improved!  But still some questions below...

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index cc34d13be181..9d9d3772cc45 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2908,6 +2908,8 @@ static inline bool
>  put_cached_bnode(struct kfree_rcu_cpu *krcp,
>  	struct kvfree_rcu_bulk_data *bnode)
>  {
> +	if (atomic_read(&krcp->backoff_page_cache_fill))
> +		return false;

This will mean that under low-memory conditions, we will keep zero
pages in ->bkvcache.  All attempts to put something there will fail.

This is probably not an issue for structures containing an rcu_head
that are passed to kfree_rcu(p, field), but doesn't this mean that
kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
This could seriously slow up freeing under low-memory conditions,
which might exacerbate the low-memory conditions.

Is this really what we want?  Zero cached rather than just fewer cached?

>  	// Check the limit.
>  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
>  		return false;
> @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
>  	int i;
>  
>  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> -		1 : rcu_min_cached_objs;
> +		0 : rcu_min_cached_objs;
>  
>  	for (i = 0; i < nr_pages; i++) {

I am still confused as to why we start "i" at zero rather than at
->nr_bkv_objs.  What am I missing here?

>  		bnode = (struct kvfree_rcu_bulk_data *)
> -- 
> 2.32.0
>
  

> Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> executed before kfree_rcu_monitor() to drain page cache, if the bnode
> structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> page cache again in kfree_rcu_monitor(), this commit add a check
> for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> if the krcp structure's->backoff_page_cache_fill is set, prevent page
> cache growing and disable allocated page in fill_page_cache_func().
> 
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
>
>Much improved!  But still some questions below...
>
>							Thanx, Paul
>
> ---
>  kernel/rcu/tree.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index cc34d13be181..9d9d3772cc45 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2908,6 +2908,8 @@ static inline bool
>  put_cached_bnode(struct kfree_rcu_cpu *krcp,
>  	struct kvfree_rcu_bulk_data *bnode)
>  {
> +	if (atomic_read(&krcp->backoff_page_cache_fill))
> +		return false;
>
>This will mean that under low-memory conditions, we will keep zero
>pages in ->bkvcache.  All attempts to put something there will fail.
>
>This is probably not an issue for structures containing an rcu_head
>that are passed to kfree_rcu(p, field), but doesn't this mean that
>kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
>This could seriously slow up freeing under low-memory conditions,
>which might exacerbate the low-memory conditions.

Thanks for mentioning this, I didn't think of this before😊.

>
>Is this really what we want?  Zero cached rather than just fewer cached?
>
>
>
>  	// Check the limit.
>  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
>  		return false;
> @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
>  	int i;
>  
>  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> -		1 : rcu_min_cached_objs;
> +		0 : rcu_min_cached_objs;
>  
>  	for (i = 0; i < nr_pages; i++) {
>
>I am still confused as to why we start "i" at zero rather than at
>->nr_bkv_objs.  What am I missing here?


No, you are right, I missed this place. 

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2908,6 +2908,8 @@ static inline bool
 put_cached_bnode(struct kfree_rcu_cpu *krcp,
        struct kvfree_rcu_bulk_data *bnode)
 {
+       if (atomic_read(&krcp->backoff_page_cache_fill))
+               return false;
        // Check the limit.
        if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
                return false;
@@ -3223,7 +3225,7 @@ static void fill_page_cache_func(struct work_struct *work)
        nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
                1 : rcu_min_cached_objs;

-       for (i = 0; i < nr_pages; i++) {
+       for (i = krcp->nr_bkv_objs; i < nr_pages; i++) {
                bnode = (struct kvfree_rcu_bulk_data *)
                        __get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);


thoughts?


Thanks
Zqiang

>
>  		bnode = (struct kvfree_rcu_bulk_data *)
> -- 
> 2.32.0
>
  

On Tue, Apr 11, 2023 at 04:04:45AM +0000, Zhang, Qiang1 wrote:
> > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> > executed before kfree_rcu_monitor() to drain page cache, if the bnode
> > structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> > page cache again in kfree_rcu_monitor(), this commit add a check
> > for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> > if the krcp structure's->backoff_page_cache_fill is set, prevent page
> > cache growing and disable allocated page in fill_page_cache_func().
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> >
> >Much improved!  But still some questions below...
> >
> >							Thanx, Paul
> >
> > ---
> >  kernel/rcu/tree.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index cc34d13be181..9d9d3772cc45 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2908,6 +2908,8 @@ static inline bool
> >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> >  	struct kvfree_rcu_bulk_data *bnode)
> >  {
> > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > +		return false;
> >
> >This will mean that under low-memory conditions, we will keep zero
> >pages in ->bkvcache.  All attempts to put something there will fail.
> >
> >This is probably not an issue for structures containing an rcu_head
> >that are passed to kfree_rcu(p, field), but doesn't this mean that
> >kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
> >This could seriously slow up freeing under low-memory conditions,
> >which might exacerbate the low-memory conditions.
> 
> Thanks for mentioning this, I didn't think of this before😊.
> 
> >
> >Is this really what we want?  Zero cached rather than just fewer cached?
> >
> >
> >
> >  	// Check the limit.
> >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> >  		return false;
> > @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
> >  	int i;
> >  
> >  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> > -		1 : rcu_min_cached_objs;
> > +		0 : rcu_min_cached_objs;
> >  
> >  	for (i = 0; i < nr_pages; i++) {
> >
> >I am still confused as to why we start "i" at zero rather than at
> >->nr_bkv_objs.  What am I missing here?
> 
> 
> No, you are right, I missed this place. 
> 
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2908,6 +2908,8 @@ static inline bool
>  put_cached_bnode(struct kfree_rcu_cpu *krcp,
>         struct kvfree_rcu_bulk_data *bnode)
>  {
> +       if (atomic_read(&krcp->backoff_page_cache_fill))
> +               return false;
>
This is broken, unfortunately. If a low memory condition we fill
fill a cache with at least one page anyway because of we do not want
to hit a slow path.

>         // Check the limit.
>         if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
>                 return false;
> @@ -3223,7 +3225,7 @@ static void fill_page_cache_func(struct work_struct *work)
>         nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
>                 1 : rcu_min_cached_objs;
> 
> -       for (i = 0; i < nr_pages; i++) {
> +       for (i = krcp->nr_bkv_objs; i < nr_pages; i++) {
>                 bnode = (struct kvfree_rcu_bulk_data *)
>                         __get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
> 
> 
IMHO, it should be send as a separate patch explaining why it
it is needed.

--
Uladzislau Rezki
  

> > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> > executed before kfree_rcu_monitor() to drain page cache, if the bnode
> > structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> > page cache again in kfree_rcu_monitor(), this commit add a check
> > for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> > if the krcp structure's->backoff_page_cache_fill is set, prevent page
> > cache growing and disable allocated page in fill_page_cache_func().
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> >
> >Much improved!  But still some questions below...
> >
> >							Thanx, Paul
> >
> > ---
> >  kernel/rcu/tree.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index cc34d13be181..9d9d3772cc45 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2908,6 +2908,8 @@ static inline bool
> >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> >  	struct kvfree_rcu_bulk_data *bnode)
> >  {
> > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > +		return false;
> >
> >This will mean that under low-memory conditions, we will keep zero
> >pages in ->bkvcache.  All attempts to put something there will fail.
> >
> >This is probably not an issue for structures containing an rcu_head
> >that are passed to kfree_rcu(p, field), but doesn't this mean that
> >kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
> >This could seriously slow up freeing under low-memory conditions,
> >which might exacerbate the low-memory conditions.
> 
> Thanks for mentioning this, I didn't think of this before😊.
> 
> >
> >Is this really what we want?  Zero cached rather than just fewer cached?
> >
> >
> >
> >  	// Check the limit.
> >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> >  		return false;
> > @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
> >  	int i;
> >  
> >  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> > -		1 : rcu_min_cached_objs;
> > +		0 : rcu_min_cached_objs;
> >  
> >  	for (i = 0; i < nr_pages; i++) {
> >
> >I am still confused as to why we start "i" at zero rather than at
> >->nr_bkv_objs.  What am I missing here?
> 
> 
> No, you are right, I missed this place. 
> 
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2908,6 +2908,8 @@ static inline bool
>  put_cached_bnode(struct kfree_rcu_cpu *krcp,
>         struct kvfree_rcu_bulk_data *bnode)
>  {
> +       if (atomic_read(&krcp->backoff_page_cache_fill))
> +               return false;
>
>This is broken, unfortunately. If a low memory condition we fill
>fill a cache with at least one page anyway because of we do not want
>to hit a slow path.

Thanks remind, please ignore my v4 patch,   how about the following?

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 41daae3239b5..e2e8412e687f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3238,6 +3238,9 @@ static void fill_page_cache_func(struct work_struct *work)
                        free_page((unsigned long) bnode);
                        break;
                }
+
+               if (atomic_read(&krcp->backoff_page_cache_fill))
+                       break;
        }

        atomic_set(&krcp->work_in_progress, 0);



>
>         // Check the limit.
>         if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
>                 return false;
> @@ -3223,7 +3225,7 @@ static void fill_page_cache_func(struct work_struct *work)
>         nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
>                 1 : rcu_min_cached_objs;
> 
> -       for (i = 0; i < nr_pages; i++) {
> +       for (i = krcp->nr_bkv_objs; i < nr_pages; i++) {
>                 bnode = (struct kvfree_rcu_bulk_data *)
>                         __get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
> 
> 
>IMHO, it should be send as a separate patch explaining why it
>it is needed.

Agree.

Thanks
Zqiang

>
>--
>Uladzislau Rezki
  

On Tue, Apr 11, 2023 at 02:42:27PM +0000, Zhang, Qiang1 wrote:
> > > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> > > executed before kfree_rcu_monitor() to drain page cache, if the bnode
> > > structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> > > page cache again in kfree_rcu_monitor(), this commit add a check
> > > for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> > > if the krcp structure's->backoff_page_cache_fill is set, prevent page
> > > cache growing and disable allocated page in fill_page_cache_func().
> > > 
> > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > >
> > >Much improved!  But still some questions below...
> > >
> > >							Thanx, Paul
> > >
> > > ---
> > >  kernel/rcu/tree.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index cc34d13be181..9d9d3772cc45 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2908,6 +2908,8 @@ static inline bool
> > >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > >  	struct kvfree_rcu_bulk_data *bnode)
> > >  {
> > > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > > +		return false;
> > >
> > >This will mean that under low-memory conditions, we will keep zero
> > >pages in ->bkvcache.  All attempts to put something there will fail.
> > >
> > >This is probably not an issue for structures containing an rcu_head
> > >that are passed to kfree_rcu(p, field), but doesn't this mean that
> > >kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
> > >This could seriously slow up freeing under low-memory conditions,
> > >which might exacerbate the low-memory conditions.
> > 
> > Thanks for mentioning this, I didn't think of this before😊.
> > 
> > >
> > >Is this really what we want?  Zero cached rather than just fewer cached?
> > >
> > >
> > >
> > >  	// Check the limit.
> > >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> > >  		return false;
> > > @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
> > >  	int i;
> > >  
> > >  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> > > -		1 : rcu_min_cached_objs;
> > > +		0 : rcu_min_cached_objs;
> > >  
> > >  	for (i = 0; i < nr_pages; i++) {
> > >
> > >I am still confused as to why we start "i" at zero rather than at
> > >->nr_bkv_objs.  What am I missing here?
> > 
> > 
> > No, you are right, I missed this place. 
> > 
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2908,6 +2908,8 @@ static inline bool
> >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> >         struct kvfree_rcu_bulk_data *bnode)
> >  {
> > +       if (atomic_read(&krcp->backoff_page_cache_fill))
> > +               return false;
> >
> >This is broken, unfortunately. If a low memory condition we fill
> >fill a cache with at least one page anyway because of we do not want
> >to hit a slow path.
> 
> Thanks remind, please ignore my v4 patch,   how about the following?
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 41daae3239b5..e2e8412e687f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3238,6 +3238,9 @@ static void fill_page_cache_func(struct work_struct *work)
>                         free_page((unsigned long) bnode);
>                         break;
>                 }
> +
> +               if (atomic_read(&krcp->backoff_page_cache_fill))
> +                       break;
>         }
It does not fix an "issue" you are reporting. kvfree_rcu_bulk() function
can still fill it back. IMHO, the solution here is to disable cache if
a low memory condition and enable back later on.

The cache size is controlled by the rcu_min_cached_objs variable. We can
set it to 1 and restore it back to original value to make the cache operating
as before.

--
Uladzislau Rezki
  

> > > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> > > executed before kfree_rcu_monitor() to drain page cache, if the bnode
> > > structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> > > page cache again in kfree_rcu_monitor(), this commit add a check
> > > for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> > > if the krcp structure's->backoff_page_cache_fill is set, prevent page
> > > cache growing and disable allocated page in fill_page_cache_func().
> > > 
> > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > >
> > >Much improved!  But still some questions below...
> > >
> > >							Thanx, Paul
> > >
> > > ---
> > >  kernel/rcu/tree.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index cc34d13be181..9d9d3772cc45 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2908,6 +2908,8 @@ static inline bool
> > >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > >  	struct kvfree_rcu_bulk_data *bnode)
> > >  {
> > > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > > +		return false;
> > >
> > >This will mean that under low-memory conditions, we will keep zero
> > >pages in ->bkvcache.  All attempts to put something there will fail.
> > >
> > >This is probably not an issue for structures containing an rcu_head
> > >that are passed to kfree_rcu(p, field), but doesn't this mean that
> > >kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
> > >This could seriously slow up freeing under low-memory conditions,
> > >which might exacerbate the low-memory conditions.
> > 
> > Thanks for mentioning this, I didn't think of this before😊.
> > 
> > >
> > >Is this really what we want?  Zero cached rather than just fewer cached?
> > >
> > >
> > >
> > >  	// Check the limit.
> > >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> > >  		return false;
> > > @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
> > >  	int i;
> > >  
> > >  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> > > -		1 : rcu_min_cached_objs;
> > > +		0 : rcu_min_cached_objs;
> > >  
> > >  	for (i = 0; i < nr_pages; i++) {
> > >
> > >I am still confused as to why we start "i" at zero rather than at
> > >->nr_bkv_objs.  What am I missing here?
> > 
> > 
> > No, you are right, I missed this place. 
> > 
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2908,6 +2908,8 @@ static inline bool
> >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> >         struct kvfree_rcu_bulk_data *bnode)
> >  {
> > +       if (atomic_read(&krcp->backoff_page_cache_fill))
> > +               return false;
> >
> >This is broken, unfortunately. If a low memory condition we fill
> >fill a cache with at least one page anyway because of we do not want
> >to hit a slow path.
> 
> Thanks remind, please ignore my v4 patch,   how about the following?
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 41daae3239b5..e2e8412e687f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3238,6 +3238,9 @@ static void fill_page_cache_func(struct work_struct *work)
>                         free_page((unsigned long) bnode);
>                         break;
>                 }
> +
> +               if (atomic_read(&krcp->backoff_page_cache_fill))
> +                       break;
>         }
>It does not fix an "issue" you are reporting. kvfree_rcu_bulk() function
>can still fill it back. IMHO, the solution here is to disable cache if
>a low memory condition and enable back later on.
>
>
>The cache size is controlled by the rcu_min_cached_objs variable. We can
>set it to 1 and restore it back to original value to make the cache operating
>as before.

A good suggestion.  a question, when need to restore  rcu_min_cached_objs?
after the execution of kfree_rcu_shrink_scan() ends?

Thanks
Zqiang

>
>--
>Uladzislau Rezki
  

On Tue, Apr 11, 2023 at 03:09:13PM +0000, Zhang, Qiang1 wrote:
> > > > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> > > > executed before kfree_rcu_monitor() to drain page cache, if the bnode
> > > > structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> > > > page cache again in kfree_rcu_monitor(), this commit add a check
> > > > for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> > > > if the krcp structure's->backoff_page_cache_fill is set, prevent page
> > > > cache growing and disable allocated page in fill_page_cache_func().
> > > > 
> > > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > > >
> > > >Much improved!  But still some questions below...
> > > >
> > > >							Thanx, Paul
> > > >
> > > > ---
> > > >  kernel/rcu/tree.c | 4 +++-
> > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index cc34d13be181..9d9d3772cc45 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -2908,6 +2908,8 @@ static inline bool
> > > >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > > >  	struct kvfree_rcu_bulk_data *bnode)
> > > >  {
> > > > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > +		return false;
> > > >
> > > >This will mean that under low-memory conditions, we will keep zero
> > > >pages in ->bkvcache.  All attempts to put something there will fail.
> > > >
> > > >This is probably not an issue for structures containing an rcu_head
> > > >that are passed to kfree_rcu(p, field), but doesn't this mean that
> > > >kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
> > > >This could seriously slow up freeing under low-memory conditions,
> > > >which might exacerbate the low-memory conditions.
> > > 
> > > Thanks for mentioning this, I didn't think of this before😊.
> > > 
> > > >
> > > >Is this really what we want?  Zero cached rather than just fewer cached?
> > > >
> > > >
> > > >
> > > >  	// Check the limit.
> > > >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> > > >  		return false;
> > > > @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
> > > >  	int i;
> > > >  
> > > >  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> > > > -		1 : rcu_min_cached_objs;
> > > > +		0 : rcu_min_cached_objs;
> > > >  
> > > >  	for (i = 0; i < nr_pages; i++) {
> > > >
> > > >I am still confused as to why we start "i" at zero rather than at
> > > >->nr_bkv_objs.  What am I missing here?
> > > 
> > > 
> > > No, you are right, I missed this place. 
> > > 
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2908,6 +2908,8 @@ static inline bool
> > >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > >         struct kvfree_rcu_bulk_data *bnode)
> > >  {
> > > +       if (atomic_read(&krcp->backoff_page_cache_fill))
> > > +               return false;
> > >
> > >This is broken, unfortunately. If a low memory condition we fill
> > >fill a cache with at least one page anyway because of we do not want
> > >to hit a slow path.
> > 
> > Thanks remind, please ignore my v4 patch,   how about the following?
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 41daae3239b5..e2e8412e687f 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3238,6 +3238,9 @@ static void fill_page_cache_func(struct work_struct *work)
> >                         free_page((unsigned long) bnode);
> >                         break;
> >                 }
> > +
> > +               if (atomic_read(&krcp->backoff_page_cache_fill))
> > +                       break;
> >         }
> >It does not fix an "issue" you are reporting. kvfree_rcu_bulk() function
> >can still fill it back. IMHO, the solution here is to disable cache if
> >a low memory condition and enable back later on.
> >
> >
> >The cache size is controlled by the rcu_min_cached_objs variable. We can
> >set it to 1 and restore it back to original value to make the cache operating
> >as before.
> 
> A good suggestion.  a question, when need to restore  rcu_min_cached_objs?
> after the execution of kfree_rcu_shrink_scan() ends?
> 
We do not know when a low memory condition ends :) Therefore we defer a
refill work for a certain time. In the fill_page_cache_func() we allow the
cache operate as normal again:

...
	atomic_set(&krcp->backoff_page_cache_fill, 0);
...

--
Uladzislau Rezki
  

On Tue, Apr 11, 2023 at 04:58:22PM +0200, Uladzislau Rezki wrote:
> On Tue, Apr 11, 2023 at 02:42:27PM +0000, Zhang, Qiang1 wrote:
> > > > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> > > > executed before kfree_rcu_monitor() to drain page cache, if the bnode
> > > > structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> > > > page cache again in kfree_rcu_monitor(), this commit add a check
> > > > for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> > > > if the krcp structure's->backoff_page_cache_fill is set, prevent page
> > > > cache growing and disable allocated page in fill_page_cache_func().
> > > > 
> > > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > > >
> > > >Much improved!  But still some questions below...
> > > >
> > > >							Thanx, Paul
> > > >
> > > > ---
> > > >  kernel/rcu/tree.c | 4 +++-
> > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index cc34d13be181..9d9d3772cc45 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -2908,6 +2908,8 @@ static inline bool
> > > >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > > >  	struct kvfree_rcu_bulk_data *bnode)
> > > >  {
> > > > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > +		return false;
> > > >
> > > >This will mean that under low-memory conditions, we will keep zero
> > > >pages in ->bkvcache.  All attempts to put something there will fail.
> > > >
> > > >This is probably not an issue for structures containing an rcu_head
> > > >that are passed to kfree_rcu(p, field), but doesn't this mean that
> > > >kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
> > > >This could seriously slow up freeing under low-memory conditions,
> > > >which might exacerbate the low-memory conditions.
> > > 
> > > Thanks for mentioning this, I didn't think of this before😊.
> > > 
> > > >
> > > >Is this really what we want?  Zero cached rather than just fewer cached?
> > > >
> > > >
> > > >
> > > >  	// Check the limit.
> > > >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> > > >  		return false;
> > > > @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
> > > >  	int i;
> > > >  
> > > >  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> > > > -		1 : rcu_min_cached_objs;
> > > > +		0 : rcu_min_cached_objs;
> > > >  
> > > >  	for (i = 0; i < nr_pages; i++) {
> > > >
> > > >I am still confused as to why we start "i" at zero rather than at
> > > >->nr_bkv_objs.  What am I missing here?
> > > 
> > > 
> > > No, you are right, I missed this place. 
> > > 
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2908,6 +2908,8 @@ static inline bool
> > >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > >         struct kvfree_rcu_bulk_data *bnode)
> > >  {
> > > +       if (atomic_read(&krcp->backoff_page_cache_fill))
> > > +               return false;
> > >
> > >This is broken, unfortunately. If a low memory condition we fill
> > >fill a cache with at least one page anyway because of we do not want
> > >to hit a slow path.
> > 
> > Thanks remind, please ignore my v4 patch,   how about the following?
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 41daae3239b5..e2e8412e687f 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3238,6 +3238,9 @@ static void fill_page_cache_func(struct work_struct *work)
> >                         free_page((unsigned long) bnode);
> >                         break;
> >                 }
> > +
> > +               if (atomic_read(&krcp->backoff_page_cache_fill))
> > +                       break;
> >         }
> It does not fix an "issue" you are reporting. kvfree_rcu_bulk() function
> can still fill it back. IMHO, the solution here is to disable cache if
> a low memory condition and enable back later on.
> 
> The cache size is controlled by the rcu_min_cached_objs variable. We can
> set it to 1 and restore it back to original value to make the cache operating
> as before.

It would be best to use a second variable for this.  Users might get
annoyed if their changes to rcu_min_cached_objs got overwritten once
things got set back to normal operation.

							Thanx, Paul
  

On Tue, Apr 11, 2023 at 09:42:37AM -0700, Paul E. McKenney wrote:
> On Tue, Apr 11, 2023 at 04:58:22PM +0200, Uladzislau Rezki wrote:
> > On Tue, Apr 11, 2023 at 02:42:27PM +0000, Zhang, Qiang1 wrote:
> > > > > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> > > > > executed before kfree_rcu_monitor() to drain page cache, if the bnode
> > > > > structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> > > > > page cache again in kfree_rcu_monitor(), this commit add a check
> > > > > for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> > > > > if the krcp structure's->backoff_page_cache_fill is set, prevent page
> > > > > cache growing and disable allocated page in fill_page_cache_func().
> > > > > 
> > > > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > > > >
> > > > >Much improved!  But still some questions below...
> > > > >
> > > > >							Thanx, Paul
> > > > >
> > > > > ---
> > > > >  kernel/rcu/tree.c | 4 +++-
> > > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > > index cc34d13be181..9d9d3772cc45 100644
> > > > > --- a/kernel/rcu/tree.c
> > > > > +++ b/kernel/rcu/tree.c
> > > > > @@ -2908,6 +2908,8 @@ static inline bool
> > > > >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > > > >  	struct kvfree_rcu_bulk_data *bnode)
> > > > >  {
> > > > > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > > +		return false;
> > > > >
> > > > >This will mean that under low-memory conditions, we will keep zero
> > > > >pages in ->bkvcache.  All attempts to put something there will fail.
> > > > >
> > > > >This is probably not an issue for structures containing an rcu_head
> > > > >that are passed to kfree_rcu(p, field), but doesn't this mean that
> > > > >kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
> > > > >This could seriously slow up freeing under low-memory conditions,
> > > > >which might exacerbate the low-memory conditions.
> > > > 
> > > > Thanks for mentioning this, I didn't think of this before😊.
> > > > 
> > > > >
> > > > >Is this really what we want?  Zero cached rather than just fewer cached?
> > > > >
> > > > >
> > > > >
> > > > >  	// Check the limit.
> > > > >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> > > > >  		return false;
> > > > > @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
> > > > >  	int i;
> > > > >  
> > > > >  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> > > > > -		1 : rcu_min_cached_objs;
> > > > > +		0 : rcu_min_cached_objs;
> > > > >  
> > > > >  	for (i = 0; i < nr_pages; i++) {
> > > > >
> > > > >I am still confused as to why we start "i" at zero rather than at
> > > > >->nr_bkv_objs.  What am I missing here?
> > > > 
> > > > 
> > > > No, you are right, I missed this place. 
> > > > 
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -2908,6 +2908,8 @@ static inline bool
> > > >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > > >         struct kvfree_rcu_bulk_data *bnode)
> > > >  {
> > > > +       if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > +               return false;
> > > >
> > > >This is broken, unfortunately. If a low memory condition we fill
> > > >fill a cache with at least one page anyway because of we do not want
> > > >to hit a slow path.
> > > 
> > > Thanks remind, please ignore my v4 patch,   how about the following?
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 41daae3239b5..e2e8412e687f 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -3238,6 +3238,9 @@ static void fill_page_cache_func(struct work_struct *work)
> > >                         free_page((unsigned long) bnode);
> > >                         break;
> > >                 }
> > > +
> > > +               if (atomic_read(&krcp->backoff_page_cache_fill))
> > > +                       break;
> > >         }
> > It does not fix an "issue" you are reporting. kvfree_rcu_bulk() function
> > can still fill it back. IMHO, the solution here is to disable cache if
> > a low memory condition and enable back later on.
> > 
> > The cache size is controlled by the rcu_min_cached_objs variable. We can
> > set it to 1 and restore it back to original value to make the cache operating
> > as before.
> 
> It would be best to use a second variable for this.  Users might get
> annoyed if their changes to rcu_min_cached_objs got overwritten once
> things got set back to normal operation.
> 
Agree. So we do not make it visible over sysfs interface for user that
we manipulate it.

--
Uladzislau Rezki
  

> On Tue, Apr 11, 2023 at 04:58:22PM +0200, Uladzislau Rezki wrote:
> > On Tue, Apr 11, 2023 at 02:42:27PM +0000, Zhang, Qiang1 wrote:
> > > > > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() 
> > > > > is executed before kfree_rcu_monitor() to drain page cache, if 
> > > > > the bnode structure's->gp_snap has done, the kvfree_rcu_bulk() 
> > > > > will fill the page cache again in kfree_rcu_monitor(), this 
> > > > > commit add a check for krcp 
> > > > > structure's->backoff_page_cache_fill in put_cached_bnode(), if 
> > > > > the krcp structure's->backoff_page_cache_fill is set, prevent page cache growing and disable allocated page in fill_page_cache_func().
> > > > > 
> > > > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > > > >
> > > > >Much improved!  But still some questions below...
> > > > >
> > > > >							Thanx, Paul
> > > > >
> > > > > ---
> > > > >  kernel/rcu/tree.c | 4 +++-
> > > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 
> > > > > cc34d13be181..9d9d3772cc45 100644
> > > > > --- a/kernel/rcu/tree.c
> > > > > +++ b/kernel/rcu/tree.c
> > > > > @@ -2908,6 +2908,8 @@ static inline bool  
> > > > > put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > > > >  	struct kvfree_rcu_bulk_data *bnode)  {
> > > > > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > > +		return false;
> > > > >
> > > > >This will mean that under low-memory conditions, we will keep 
> > > > >zero pages in ->bkvcache.  All attempts to put something there will fail.
> > > > >
> > > > >This is probably not an issue for structures containing an 
> > > > >rcu_head that are passed to kfree_rcu(p, field), but doesn't 
> > > > >this mean that
> > > > >kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
> > > > >This could seriously slow up freeing under low-memory 
> > > > >conditions, which might exacerbate the low-memory conditions.
> > > > 
> > > > Thanks for mentioning this, I didn't think of this before😊.
> > > > 
> > > > >
> > > > >Is this really what we want?  Zero cached rather than just fewer cached?
> > > > >
> > > > >
> > > > >
> > > > >  	// Check the limit.
> > > > >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> > > > >  		return false;
> > > > > @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
> > > > >  	int i;
> > > > >  
> > > > >  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> > > > > -		1 : rcu_min_cached_objs;
> > > > > +		0 : rcu_min_cached_objs;
> > > > >  
> > > > >  	for (i = 0; i < nr_pages; i++) {
> > > > >
> > > > >I am still confused as to why we start "i" at zero rather than 
> > > > >at
> > > > >->nr_bkv_objs.  What am I missing here?
> > > > 
> > > > 
> > > > No, you are right, I missed this place. 
> > > > 
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -2908,6 +2908,8 @@ static inline bool  
> > > > put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > > >         struct kvfree_rcu_bulk_data *bnode)  {
> > > > +       if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > +               return false;
> > > >
> > > >This is broken, unfortunately. If a low memory condition we fill 
> > > >fill a cache with at least one page anyway because of we do not 
> > > >want to hit a slow path.
> > > 
> > > Thanks remind, please ignore my v4 patch,   how about the following?
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 
> > > 41daae3239b5..e2e8412e687f 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -3238,6 +3238,9 @@ static void fill_page_cache_func(struct work_struct *work)
> > >                         free_page((unsigned long) bnode);
> > >                         break;
> > >                 }
> > > +
> > > +               if (atomic_read(&krcp->backoff_page_cache_fill))
> > > +                       break;
> > >         }
> > It does not fix an "issue" you are reporting. kvfree_rcu_bulk() 
> > function can still fill it back. IMHO, the solution here is to 
> > disable cache if a low memory condition and enable back later on.
> > 
> > The cache size is controlled by the rcu_min_cached_objs variable. We 
> > can set it to 1 and restore it back to original value to make the 
> > cache operating as before.
> 
> It would be best to use a second variable for this.  Users might get 
> annoyed if their changes to rcu_min_cached_objs got overwritten once 
> things got set back to normal operation.
> 
>Agree. So we do not make it visible over sysfs interface for user that we manipulate it.
>
>


The rcu_min_cached_objs is read-only, Users cannot be set rcu_min_cached_objs dynamically. 

-r--r--r-- 1 root root 4.0K Apr 12 01:08 rcu_min_cached_objs

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 41daae3239b5..0e9f83562823 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2909,7 +2909,8 @@ put_cached_bnode(struct kfree_rcu_cpu *krcp,
        struct kvfree_rcu_bulk_data *bnode)
 {
        // Check the limit.
-       if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
+       if ((atomic_read(&krcp->backoff_page_cache_fill) && krcp->nr_bkv_objs) ||
+                               krcp->nr_bkv_objs >= rcu_min_cached_objs)
                return false;

        llist_add((struct llist_node *) bnode, &krcp->bkvcache);


thoughts?

Thanks
Zqiang


>
>--
>Uladzislau Rezki
  

On Wed, Apr 12, 2023 at 09:14:15AM +0000, Zhang, Qiang1 wrote:
> 
> > On Tue, Apr 11, 2023 at 04:58:22PM +0200, Uladzislau Rezki wrote:
> > > On Tue, Apr 11, 2023 at 02:42:27PM +0000, Zhang, Qiang1 wrote:
> > > > > > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() 
> > > > > > is executed before kfree_rcu_monitor() to drain page cache, if 
> > > > > > the bnode structure's->gp_snap has done, the kvfree_rcu_bulk() 
> > > > > > will fill the page cache again in kfree_rcu_monitor(), this 
> > > > > > commit add a check for krcp 
> > > > > > structure's->backoff_page_cache_fill in put_cached_bnode(), if 
> > > > > > the krcp structure's->backoff_page_cache_fill is set, prevent page cache growing and disable allocated page in fill_page_cache_func().
> > > > > > 
> > > > > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > > > > >
> > > > > >Much improved!  But still some questions below...
> > > > > >
> > > > > >							Thanx, Paul
> > > > > >
> > > > > > ---
> > > > > >  kernel/rcu/tree.c | 4 +++-
> > > > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 
> > > > > > cc34d13be181..9d9d3772cc45 100644
> > > > > > --- a/kernel/rcu/tree.c
> > > > > > +++ b/kernel/rcu/tree.c
> > > > > > @@ -2908,6 +2908,8 @@ static inline bool  
> > > > > > put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > > > > >  	struct kvfree_rcu_bulk_data *bnode)  {
> > > > > > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > > > +		return false;
> > > > > >
> > > > > >This will mean that under low-memory conditions, we will keep 
> > > > > >zero pages in ->bkvcache.  All attempts to put something there will fail.
> > > > > >
> > > > > >This is probably not an issue for structures containing an 
> > > > > >rcu_head that are passed to kfree_rcu(p, field), but doesn't 
> > > > > >this mean that
> > > > > >kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
> > > > > >This could seriously slow up freeing under low-memory 
> > > > > >conditions, which might exacerbate the low-memory conditions.
> > > > > 
> > > > > Thanks for mentioning this, I didn't think of this before😊.
> > > > > 
> > > > > >
> > > > > >Is this really what we want?  Zero cached rather than just fewer cached?
> > > > > >
> > > > > >
> > > > > >
> > > > > >  	// Check the limit.
> > > > > >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> > > > > >  		return false;
> > > > > > @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
> > > > > >  	int i;
> > > > > >  
> > > > > >  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> > > > > > -		1 : rcu_min_cached_objs;
> > > > > > +		0 : rcu_min_cached_objs;
> > > > > >  
> > > > > >  	for (i = 0; i < nr_pages; i++) {
> > > > > >
> > > > > >I am still confused as to why we start "i" at zero rather than 
> > > > > >at
> > > > > >->nr_bkv_objs.  What am I missing here?
> > > > > 
> > > > > 
> > > > > No, you are right, I missed this place. 
> > > > > 
> > > > > --- a/kernel/rcu/tree.c
> > > > > +++ b/kernel/rcu/tree.c
> > > > > @@ -2908,6 +2908,8 @@ static inline bool  
> > > > > put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > > > >         struct kvfree_rcu_bulk_data *bnode)  {
> > > > > +       if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > > +               return false;
> > > > >
> > > > >This is broken, unfortunately. If a low memory condition we fill 
> > > > >fill a cache with at least one page anyway because of we do not 
> > > > >want to hit a slow path.
> > > > 
> > > > Thanks remind, please ignore my v4 patch,   how about the following?
> > > > 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 
> > > > 41daae3239b5..e2e8412e687f 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -3238,6 +3238,9 @@ static void fill_page_cache_func(struct work_struct *work)
> > > >                         free_page((unsigned long) bnode);
> > > >                         break;
> > > >                 }
> > > > +
> > > > +               if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > +                       break;
> > > >         }
> > > It does not fix an "issue" you are reporting. kvfree_rcu_bulk() 
> > > function can still fill it back. IMHO, the solution here is to 
> > > disable cache if a low memory condition and enable back later on.
> > > 
> > > The cache size is controlled by the rcu_min_cached_objs variable. We 
> > > can set it to 1 and restore it back to original value to make the 
> > > cache operating as before.
> > 
> > It would be best to use a second variable for this.  Users might get 
> > annoyed if their changes to rcu_min_cached_objs got overwritten once 
> > things got set back to normal operation.
> > 
> >Agree. So we do not make it visible over sysfs interface for user that we manipulate it.
> >
> >
> 
> 
> The rcu_min_cached_objs is read-only, Users cannot be set rcu_min_cached_objs dynamically. 
> 
> -r--r--r-- 1 root root 4.0K Apr 12 01:08 rcu_min_cached_objs
> 
You can set it as a boot parameter: rcutree.rcu_min_cached_objs=XXX

> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 41daae3239b5..0e9f83562823 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2909,7 +2909,8 @@ put_cached_bnode(struct kfree_rcu_cpu *krcp,
>         struct kvfree_rcu_bulk_data *bnode)
>  {
>         // Check the limit.
> -       if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> +       if ((atomic_read(&krcp->backoff_page_cache_fill) && krcp->nr_bkv_objs) ||
> +                               krcp->nr_bkv_objs >= rcu_min_cached_objs)
>                 return false;
> 
We can eliminate the backoff_page_cache_fill per-cpu atomic variable and
just change a new one, say, min_cached_objs, if a low memory condition.
Restore it to a default what is the rcu_min_cached_objs.

I can post here an example if it helps to make it more clear.

--
Uladzislau Rezki
  

> > On Tue, Apr 11, 2023 at 04:58:22PM +0200, Uladzislau Rezki wrote:
> > > On Tue, Apr 11, 2023 at 02:42:27PM +0000, Zhang, Qiang1 wrote:
> > > > > > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() 
> > > > > > is executed before kfree_rcu_monitor() to drain page cache, if 
> > > > > > the bnode structure's->gp_snap has done, the kvfree_rcu_bulk() 
> > > > > > will fill the page cache again in kfree_rcu_monitor(), this 
> > > > > > commit add a check for krcp 
> > > > > > structure's->backoff_page_cache_fill in put_cached_bnode(), if 
> > > > > > the krcp structure's->backoff_page_cache_fill is set, prevent page cache growing and disable allocated page in fill_page_cache_func().
> > > > > > 
> > > > > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > > > > >
> > > > > >Much improved!  But still some questions below...
> > > > > >
> > > > > >							Thanx, Paul
> > > > > >
> > > > > > ---
> > > > > >  kernel/rcu/tree.c | 4 +++-
> > > > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 
> > > > > > cc34d13be181..9d9d3772cc45 100644
> > > > > > --- a/kernel/rcu/tree.c
> > > > > > +++ b/kernel/rcu/tree.c
> > > > > > @@ -2908,6 +2908,8 @@ static inline bool  
> > > > > > put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > > > > >  	struct kvfree_rcu_bulk_data *bnode)  {
> > > > > > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > > > +		return false;
> > > > > >
> > > > > >This will mean that under low-memory conditions, we will keep 
> > > > > >zero pages in ->bkvcache.  All attempts to put something there will fail.
> > > > > >
> > > > > >This is probably not an issue for structures containing an 
> > > > > >rcu_head that are passed to kfree_rcu(p, field), but doesn't 
> > > > > >this mean that
> > > > > >kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
> > > > > >This could seriously slow up freeing under low-memory 
> > > > > >conditions, which might exacerbate the low-memory conditions.
> > > > > 
> > > > > Thanks for mentioning this, I didn't think of this before😊.
> > > > > 
> > > > > >
> > > > > >Is this really what we want?  Zero cached rather than just fewer cached?
> > > > > >
> > > > > >
> > > > > >
> > > > > >  	// Check the limit.
> > > > > >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> > > > > >  		return false;
> > > > > > @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
> > > > > >  	int i;
> > > > > >  
> > > > > >  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> > > > > > -		1 : rcu_min_cached_objs;
> > > > > > +		0 : rcu_min_cached_objs;
> > > > > >  
> > > > > >  	for (i = 0; i < nr_pages; i++) {
> > > > > >
> > > > > >I am still confused as to why we start "i" at zero rather than 
> > > > > >at
> > > > > >->nr_bkv_objs.  What am I missing here?
> > > > > 
> > > > > 
> > > > > No, you are right, I missed this place. 
> > > > > 
> > > > > --- a/kernel/rcu/tree.c
> > > > > +++ b/kernel/rcu/tree.c
> > > > > @@ -2908,6 +2908,8 @@ static inline bool  
> > > > > put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > > > >         struct kvfree_rcu_bulk_data *bnode)  {
> > > > > +       if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > > +               return false;
> > > > >
> > > > >This is broken, unfortunately. If a low memory condition we fill 
> > > > >fill a cache with at least one page anyway because of we do not 
> > > > >want to hit a slow path.
> > > > 
> > > > Thanks remind, please ignore my v4 patch,   how about the following?
> > > > 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 
> > > > 41daae3239b5..e2e8412e687f 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -3238,6 +3238,9 @@ static void fill_page_cache_func(struct work_struct *work)
> > > >                         free_page((unsigned long) bnode);
> > > >                         break;
> > > >                 }
> > > > +
> > > > +               if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > +                       break;
> > > >         }
> > > It does not fix an "issue" you are reporting. kvfree_rcu_bulk() 
> > > function can still fill it back. IMHO, the solution here is to 
> > > disable cache if a low memory condition and enable back later on.
> > > 
> > > The cache size is controlled by the rcu_min_cached_objs variable. We 
> > > can set it to 1 and restore it back to original value to make the 
> > > cache operating as before.
> > 
> > It would be best to use a second variable for this.  Users might get 
> > annoyed if their changes to rcu_min_cached_objs got overwritten once 
> > things got set back to normal operation.
> > 
> >Agree. So we do not make it visible over sysfs interface for user that we manipulate it.
> >
> >
> 
> 
> The rcu_min_cached_objs is read-only, Users cannot be set rcu_min_cached_objs dynamically. 
> 
> -r--r--r-- 1 root root 4.0K Apr 12 01:08 rcu_min_cached_objs
> 
You can set it as a boot parameter: rcutree.rcu_min_cached_objs=XXX

> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 41daae3239b5..0e9f83562823 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2909,7 +2909,8 @@ put_cached_bnode(struct kfree_rcu_cpu *krcp,
>         struct kvfree_rcu_bulk_data *bnode)
>  {
>         // Check the limit.
> -       if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> +       if ((atomic_read(&krcp->backoff_page_cache_fill) && krcp->nr_bkv_objs) ||
> +                               krcp->nr_bkv_objs >= rcu_min_cached_objs)
>                 return false;
> 
>We can eliminate the backoff_page_cache_fill per-cpu atomic variable and
>just change a new one, say, min_cached_objs, if a low memory condition.
>Restore it to a default what is the rcu_min_cached_objs.

Thanks for suggestion,  will be modified in this way.

>
>I can post here an example if it helps to make it more clear.
>
>--
>Uladzislau Rezki