[v8,2/6] memcontrol: implement mem_cgroup_tryget_online()

  This patch implements a helper function that try to get a reference to
an memcg's css, as well as checking if it is online. This new function
is almost exactly the same as the existing mem_cgroup_tryget(), except
for the onlineness check. In the !CONFIG_MEMCG case, it always returns
true, analogous to mem_cgroup_tryget(). This is useful for e.g to the
new zswap writeback scheme, where we need to select the next online
memcg as a candidate for the global limit reclaim.

Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 include/linux/memcontrol.h | 10 ++++++++++
 1 file changed, 10 insertions(+)
  

Hi Nhat,

Very minor nitpick. This patch can fold with the later patch that uses
it. That makes the review easier, no need to cross reference different
patches. It will also make it harder to introduce API that nobody
uses.

Chris

On Thu, Nov 30, 2023 at 11:40 AM Nhat Pham <nphamcs@gmail.com> wrote:
>
> This patch implements a helper function that try to get a reference to
> an memcg's css, as well as checking if it is online. This new function
> is almost exactly the same as the existing mem_cgroup_tryget(), except
> for the onlineness check. In the !CONFIG_MEMCG case, it always returns
> true, analogous to mem_cgroup_tryget(). This is useful for e.g to the
> new zswap writeback scheme, where we need to select the next online
> memcg as a candidate for the global limit reclaim.

Very minor nitpick. This patch can fold with the later patch that uses
it. That makes the review easier, no need to cross reference different
patches. It will also make it harder to introduce API that nobody
uses.

Chris

>
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> ---
>  include/linux/memcontrol.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 7bdcf3020d7a..2bd7d14ace78 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -821,6 +821,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
>         return !memcg || css_tryget(&memcg->css);
>  }
>
> +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> +{
> +       return !memcg || css_tryget_online(&memcg->css);
> +}
> +
>  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
>  {
>         if (memcg)
> @@ -1349,6 +1354,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
>         return true;
>  }
>
> +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> +{
> +       return true;
> +}
> +
>  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
>  {
>  }
> --
> 2.34.1
>
  

On Mon, Dec 4, 2023 at 4:36 PM Chris Li <chrisl@kernel.org> wrote:
>
> Hi Nhat,
>
> Very minor nitpick. This patch can fold with the later patch that uses
> it. That makes the review easier, no need to cross reference different
> patches. It will also make it harder to introduce API that nobody
> uses.
>
> Chris
>
> On Thu, Nov 30, 2023 at 11:40 AM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > This patch implements a helper function that try to get a reference to
> > an memcg's css, as well as checking if it is online. This new function
> > is almost exactly the same as the existing mem_cgroup_tryget(), except
> > for the onlineness check. In the !CONFIG_MEMCG case, it always returns
> > true, analogous to mem_cgroup_tryget(). This is useful for e.g to the
> > new zswap writeback scheme, where we need to select the next online
> > memcg as a candidate for the global limit reclaim.
>
> Very minor nitpick. This patch can fold with the later patch that uses
> it. That makes the review easier, no need to cross reference different
> patches. It will also make it harder to introduce API that nobody
> uses.

I don't have a strong preference one way or the other :) Probably not
worth the churn tho.

>
> Chris
>
> >
> > Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> > ---
> >  include/linux/memcontrol.h | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 7bdcf3020d7a..2bd7d14ace78 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -821,6 +821,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
> >         return !memcg || css_tryget(&memcg->css);
> >  }
> >
> > +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> > +{
> > +       return !memcg || css_tryget_online(&memcg->css);
> > +}
> > +
> >  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
> >  {
> >         if (memcg)
> > @@ -1349,6 +1354,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
> >         return true;
> >  }
> >
> > +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> > +{
> > +       return true;
> > +}
> > +
> >  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
> >  {
> >  }
> > --
> > 2.34.1
> >
  

On Thu, Nov 30, 2023 at 11:40 AM Nhat Pham <nphamcs@gmail.com> wrote:
>
> This patch implements a helper function that try to get a reference to
> an memcg's css, as well as checking if it is online. This new function
> is almost exactly the same as the existing mem_cgroup_tryget(), except
> for the onlineness check. In the !CONFIG_MEMCG case, it always returns
> true, analogous to mem_cgroup_tryget(). This is useful for e.g to the
> new zswap writeback scheme, where we need to select the next online
> memcg as a candidate for the global limit reclaim.
>
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>

Reviewed-by: Yosry Ahmed <yosryahmed@google.com>

> ---
>  include/linux/memcontrol.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 7bdcf3020d7a..2bd7d14ace78 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -821,6 +821,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
>         return !memcg || css_tryget(&memcg->css);
>  }
>
> +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> +{
> +       return !memcg || css_tryget_online(&memcg->css);
> +}
> +
>  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
>  {
>         if (memcg)
> @@ -1349,6 +1354,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
>         return true;
>  }
>
> +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> +{
> +       return true;
> +}
> +
>  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
>  {
>  }
> --
> 2.34.1
  

On Tue, Dec 5, 2023 at 10:03 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Thu, Nov 30, 2023 at 11:40 AM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > This patch implements a helper function that try to get a reference to
> > an memcg's css, as well as checking if it is online. This new function
> > is almost exactly the same as the existing mem_cgroup_tryget(), except
> > for the onlineness check. In the !CONFIG_MEMCG case, it always returns
> > true, analogous to mem_cgroup_tryget(). This is useful for e.g to the
> > new zswap writeback scheme, where we need to select the next online
> > memcg as a candidate for the global limit reclaim.
> >
> > Signed-off-by: Nhat Pham <nphamcs@gmail.com>
>
> Reviewed-by: Yosry Ahmed <yosryahmed@google.com>

Thanks for the review, Yosry :) Really appreciate the effort and your
comments so far.

>
> > ---
> >  include/linux/memcontrol.h | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 7bdcf3020d7a..2bd7d14ace78 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -821,6 +821,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
> >         return !memcg || css_tryget(&memcg->css);
> >  }
> >
> > +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> > +{
> > +       return !memcg || css_tryget_online(&memcg->css);
> > +}
> > +
> >  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
> >  {
> >         if (memcg)
> > @@ -1349,6 +1354,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
> >         return true;
> >  }
> >
> > +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> > +{
> > +       return true;
> > +}
> > +
> >  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
> >  {
> >  }
> > --
> > 2.34.1
  

On Mon, Dec 4, 2023 at 5:39 PM Nhat Pham <nphamcs@gmail.com> wrote:
>
> > > memcg as a candidate for the global limit reclaim.
> >
> > Very minor nitpick. This patch can fold with the later patch that uses
> > it. That makes the review easier, no need to cross reference different
> > patches. It will also make it harder to introduce API that nobody
> > uses.
>
> I don't have a strong preference one way or the other :) Probably not
> worth the churn tho.

Squashing a patch is very easy. If you are refreshing a new series, it
is worthwhile to do it. I notice on the other thread Yosry pointed out
you did  not use the function "mem_cgroup_tryget_online" in patch 3,
that is exactly the situation my suggestion is trying to prevent.

If you don't have a strong preference, it sounds like you should squash it.

Chris

>
> >
> > Chris
> >
> > >
> > > Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> > > ---
> > >  include/linux/memcontrol.h | 10 ++++++++++
> > >  1 file changed, 10 insertions(+)
> > >
> > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > > index 7bdcf3020d7a..2bd7d14ace78 100644
> > > --- a/include/linux/memcontrol.h
> > > +++ b/include/linux/memcontrol.h
> > > @@ -821,6 +821,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
> > >         return !memcg || css_tryget(&memcg->css);
> > >  }
> > >
> > > +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> > > +{
> > > +       return !memcg || css_tryget_online(&memcg->css);
> > > +}
> > > +
> > >  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
> > >  {
> > >         if (memcg)
> > > @@ -1349,6 +1354,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
> > >         return true;
> > >  }
> > >
> > > +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> > > +{
> > > +       return true;
> > > +}
> > > +
> > >  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
> > >  {
> > >  }
> > > --
> > > 2.34.1
> > >
>
  

On Tue, Dec 5, 2023 at 4:16 PM Chris Li <chrisl@kernel.org> wrote:
>
> On Mon, Dec 4, 2023 at 5:39 PM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > > > memcg as a candidate for the global limit reclaim.
> > >
> > > Very minor nitpick. This patch can fold with the later patch that uses
> > > it. That makes the review easier, no need to cross reference different
> > > patches. It will also make it harder to introduce API that nobody
> > > uses.
> >
> > I don't have a strong preference one way or the other :) Probably not
> > worth the churn tho.
>
> Squashing a patch is very easy. If you are refreshing a new series, it
> is worthwhile to do it. I notice on the other thread Yosry pointed out
> you did  not use the function "mem_cgroup_tryget_online" in patch 3,
> that is exactly the situation my suggestion is trying to prevent.

I doubt squashing it would solve the issue - in fact, I think Yosry
noticed it precisely because he had to stare at a separate patch
detailing the adding of the new function in the first place :P

In general though, I'm hesitant to extend this API silently in a patch
that uses it. Is it not better to have a separate patch announcing
this API extension? list_lru_add() was originally part of the original
series too - we separate that out to its own thing because it gets
confusing. Another benefit is that there will be less work in the
future if we want to revert the per-cgroup zswap LRU patch, and
there's already another mem_cgroup_tryget_online() user - we can keep
this patch.

But yeah we'll see - I'll think about it if I actually have to send
v9. If not, let's not add unnecessary churning.

>
> If you don't have a strong preference, it sounds like you should squash it.
>
> Chris
>
> >
> > >
> > > Chris
> > >
> > > >
> > > > Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> > > > ---
> > > >  include/linux/memcontrol.h | 10 ++++++++++
> > > >  1 file changed, 10 insertions(+)
> > > >
> > > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > > > index 7bdcf3020d7a..2bd7d14ace78 100644
> > > > --- a/include/linux/memcontrol.h
> > > > +++ b/include/linux/memcontrol.h
> > > > @@ -821,6 +821,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
> > > >         return !memcg || css_tryget(&memcg->css);
> > > >  }
> > > >
> > > > +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> > > > +{
> > > > +       return !memcg || css_tryget_online(&memcg->css);
> > > > +}
> > > > +
> > > >  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
> > > >  {
> > > >         if (memcg)
> > > > @@ -1349,6 +1354,11 @@ static inline bool mem_cgroup_tryget(struct mem_cgroup *memcg)
> > > >         return true;
> > > >  }
> > > >
> > > > +static inline bool mem_cgroup_tryget_online(struct mem_cgroup *memcg)
> > > > +{
> > > > +       return true;
> > > > +}
> > > > +
> > > >  static inline void mem_cgroup_put(struct mem_cgroup *memcg)
> > > >  {
> > > >  }
> > > > --
> > > > 2.34.1
> > > >
> >