[v3,08/10] sched/topology: Remove SHARED_CHILD from ASYM_PACKING

  Only x86 and Power7 use ASYM_PACKING. They use it differently.

Power7 has cores of equal priority, but the SMT siblings of a core have
different priorities. Parent scheduling domains do not need (nor have) the
ASYM_PACKING flag. SHARED_CHILD is not needed. Using SHARED_PARENT would
cause the topology debug code to complain.

X86 has cores of different priority, but all the SMT siblings of the core
have equal priority. It needs ASYM_PACKING at the MC level, but not at the
SMT level (it also needs it at upper levels if they have scheduling groups
of different priority). Removing ASYM_PACKING from the SMT domain causes
the topology debug code to complain.

Remove SHARED_CHILD for now. We still need a topology check that satisfies
both architectures.

Cc: Ben Segall <bsegall@google.com>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Len Brown <len.brown@intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Tim C. Chen <tim.c.chen@intel.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Suggested-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
---
Changes since v2:
 * Introduced this patch.

Changes since v1:
 * N/A
---
 include/linux/sched/sd_flags.h | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)
  

Hi Ricardo,

On Monday 06 Feb 2023 at 20:58:36 (-0800), Ricardo Neri wrote:
> Only x86 and Power7 use ASYM_PACKING. They use it differently.
> 
> Power7 has cores of equal priority, but the SMT siblings of a core have
> different priorities. Parent scheduling domains do not need (nor have) the
> ASYM_PACKING flag. SHARED_CHILD is not needed. Using SHARED_PARENT would
> cause the topology debug code to complain.
> 
> X86 has cores of different priority, but all the SMT siblings of the core
> have equal priority. It needs ASYM_PACKING at the MC level, but not at the
> SMT level (it also needs it at upper levels if they have scheduling groups
> of different priority). Removing ASYM_PACKING from the SMT domain causes
> the topology debug code to complain.
> 
> Remove SHARED_CHILD for now. We still need a topology check that satisfies
> both architectures.
> 
> Cc: Ben Segall <bsegall@google.com>
> Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Len Brown <len.brown@intel.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Tim C. Chen <tim.c.chen@intel.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: x86@kernel.org
> Cc: linux-kernel@vger.kernel.org
> Suggested-by: Valentin Schneider <vschneid@redhat.com>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> Changes since v2:
>  * Introduced this patch.
> 
> Changes since v1:
>  * N/A
> ---
>  include/linux/sched/sd_flags.h | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
> index 57bde66d95f7..800238854ba5 100644
> --- a/include/linux/sched/sd_flags.h
> +++ b/include/linux/sched/sd_flags.h
> @@ -132,12 +132,9 @@ SD_FLAG(SD_SERIALIZE, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
>  /*
>   * Place busy tasks earlier in the domain
>   *
> - * SHARED_CHILD: Usually set on the SMT level. Technically could be set further
> - *               up, but currently assumed to be set from the base domain
> - *               upwards (see update_top_cache_domain()).
>   * NEEDS_GROUPS: Load balancing flag.
>   */
> -SD_FLAG(SD_ASYM_PACKING, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
> +SD_FLAG(SD_ASYM_PACKING,  SDF_NEEDS_GROUPS)

While this silences the warning one would have gotten when removing
SD_ASYM_PACKING from SMT level, it will still result in sd_asym_packing
being NULL for these systems, which breaks nohz balance. That is because
highest_flag_domain() still stops searching at the first level without
the flag set, in this case SMT, even if levels above have the flag set.

Maybe highest_flag_domain() should be changed to take into account the
metadata flags?

Thanks,
Ionela.

>  
>  /*
>   * Prefer to place tasks in a sibling domain
> -- 
> 2.25.1
> 
>
  

On Fri, Mar 03, 2023 at 11:29:52AM +0000, Ionela Voinescu wrote:
> Hi Ricardo,

Hi Ionela!

> 
> On Monday 06 Feb 2023 at 20:58:36 (-0800), Ricardo Neri wrote:
> > Only x86 and Power7 use ASYM_PACKING. They use it differently.
> > 
> > Power7 has cores of equal priority, but the SMT siblings of a core have
> > different priorities. Parent scheduling domains do not need (nor have) the
> > ASYM_PACKING flag. SHARED_CHILD is not needed. Using SHARED_PARENT would
> > cause the topology debug code to complain.
> > 
> > X86 has cores of different priority, but all the SMT siblings of the core
> > have equal priority. It needs ASYM_PACKING at the MC level, but not at the
> > SMT level (it also needs it at upper levels if they have scheduling groups
> > of different priority). Removing ASYM_PACKING from the SMT domain causes
> > the topology debug code to complain.
> > 
> > Remove SHARED_CHILD for now. We still need a topology check that satisfies
> > both architectures.
> > 
> > Cc: Ben Segall <bsegall@google.com>
> > Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > Cc: Len Brown <len.brown@intel.com>
> > Cc: Mel Gorman <mgorman@suse.de>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Tim C. Chen <tim.c.chen@intel.com>
> > Cc: Valentin Schneider <vschneid@redhat.com>
> > Cc: x86@kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Suggested-by: Valentin Schneider <vschneid@redhat.com>
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > ---
> > Changes since v2:
> >  * Introduced this patch.
> > 
> > Changes since v1:
> >  * N/A
> > ---
> >  include/linux/sched/sd_flags.h | 5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
> > index 57bde66d95f7..800238854ba5 100644
> > --- a/include/linux/sched/sd_flags.h
> > +++ b/include/linux/sched/sd_flags.h
> > @@ -132,12 +132,9 @@ SD_FLAG(SD_SERIALIZE, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
> >  /*
> >   * Place busy tasks earlier in the domain
> >   *
> > - * SHARED_CHILD: Usually set on the SMT level. Technically could be set further
> > - *               up, but currently assumed to be set from the base domain
> > - *               upwards (see update_top_cache_domain()).
> >   * NEEDS_GROUPS: Load balancing flag.
> >   */
> > -SD_FLAG(SD_ASYM_PACKING, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
> > +SD_FLAG(SD_ASYM_PACKING,  SDF_NEEDS_GROUPS)
> 
> While this silences the warning one would have gotten when removing
> SD_ASYM_PACKING from SMT level, it will still result in sd_asym_packing
> being NULL for these systems, which breaks nohz balance. That is because
> highest_flag_domain() still stops searching at the first level without
> the flag set, in this case SMT, even if levels above have the flag set.

You are absolutely right! This how this whole discussion started. It
slipped my mind.

> 
> Maybe highest_flag_domain() should be changed to take into account the
> metadata flags?

What about the patch below? Search will stop if the flag has
SDF_SHARED_CHILD as it does today. Otherwise it will search all the
domains.

--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1773,6 +1773,12 @@ queue_balance_callback(struct rq *rq,
 	for (__sd = rcu_dereference_check_sched_domain(cpu_rq(cpu)->sd); \
 			__sd; __sd = __sd->parent)
 
+#define SD_FLAG(name, mflags) (name * !!((mflags) & SDF_SHARED_CHILD)) |
+static const unsigned int SD_SHARED_CHILD_MASK =
+#include <linux/sched/sd_flags.h>
+0;
+#undef SD_FLAG
+
 /**
  * highest_flag_domain - Return highest sched_domain containing flag.
  * @cpu:	The CPU whose highest level of sched domain is to
@@ -1781,15 +1787,19 @@ queue_balance_callback(struct rq *rq,
  *		for the given CPU.
  *
  * Returns the highest sched_domain of a CPU which contains the given flag.
- */
+*/
 static inline struct sched_domain *highest_flag_domain(int cpu, int flag)
 {
 	struct sched_domain *sd, *hsd = NULL;
 
 	for_each_domain(cpu, sd) {
-		if (!(sd->flags & flag))
+		if (sd->flags & flag) {
+			hsd = sd;
+			continue;
+		}
+
+		if (flag & SD_SHARED_CHILD_MASK)
 			break;
-		hsd = sd;
 	}
 
 	return hsd;

> 
> Thanks,
> Ionela.
> 
> >  
> >  /*
> >   * Prefer to place tasks in a sibling domain
> > -- 
> > 2.25.1
> > 
> >
  

Hey,

On Sunday 05 Mar 2023 at 11:08:11 (-0800), Ricardo Neri wrote:
> On Fri, Mar 03, 2023 at 11:29:52AM +0000, Ionela Voinescu wrote:
> > Hi Ricardo,
> 
> Hi Ionela!
> 
> > 
> > On Monday 06 Feb 2023 at 20:58:36 (-0800), Ricardo Neri wrote:
> > > Only x86 and Power7 use ASYM_PACKING. They use it differently.
> > > 
> > > Power7 has cores of equal priority, but the SMT siblings of a core have
> > > different priorities. Parent scheduling domains do not need (nor have) the
> > > ASYM_PACKING flag. SHARED_CHILD is not needed. Using SHARED_PARENT would
> > > cause the topology debug code to complain.
> > > 
> > > X86 has cores of different priority, but all the SMT siblings of the core
> > > have equal priority. It needs ASYM_PACKING at the MC level, but not at the
> > > SMT level (it also needs it at upper levels if they have scheduling groups
> > > of different priority). Removing ASYM_PACKING from the SMT domain causes
> > > the topology debug code to complain.
> > > 
> > > Remove SHARED_CHILD for now. We still need a topology check that satisfies
> > > both architectures.
> > > 
> > > Cc: Ben Segall <bsegall@google.com>
> > > Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> > > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > > Cc: Len Brown <len.brown@intel.com>
> > > Cc: Mel Gorman <mgorman@suse.de>
> > > Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > Cc: Tim C. Chen <tim.c.chen@intel.com>
> > > Cc: Valentin Schneider <vschneid@redhat.com>
> > > Cc: x86@kernel.org
> > > Cc: linux-kernel@vger.kernel.org
> > > Suggested-by: Valentin Schneider <vschneid@redhat.com>
> > > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > > ---
> > > Changes since v2:
> > >  * Introduced this patch.
> > > 
> > > Changes since v1:
> > >  * N/A
> > > ---
> > >  include/linux/sched/sd_flags.h | 5 +----
> > >  1 file changed, 1 insertion(+), 4 deletions(-)
> > > 
> > > diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
> > > index 57bde66d95f7..800238854ba5 100644
> > > --- a/include/linux/sched/sd_flags.h
> > > +++ b/include/linux/sched/sd_flags.h
> > > @@ -132,12 +132,9 @@ SD_FLAG(SD_SERIALIZE, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
> > >  /*
> > >   * Place busy tasks earlier in the domain
> > >   *
> > > - * SHARED_CHILD: Usually set on the SMT level. Technically could be set further
> > > - *               up, but currently assumed to be set from the base domain
> > > - *               upwards (see update_top_cache_domain()).
> > >   * NEEDS_GROUPS: Load balancing flag.
> > >   */
> > > -SD_FLAG(SD_ASYM_PACKING, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
> > > +SD_FLAG(SD_ASYM_PACKING,  SDF_NEEDS_GROUPS)
> > 
> > While this silences the warning one would have gotten when removing
> > SD_ASYM_PACKING from SMT level, it will still result in sd_asym_packing
> > being NULL for these systems, which breaks nohz balance. That is because
> > highest_flag_domain() still stops searching at the first level without
> > the flag set, in this case SMT, even if levels above have the flag set.
> 
> You are absolutely right! This how this whole discussion started. It
> slipped my mind.
> 
> > 
> > Maybe highest_flag_domain() should be changed to take into account the
> > metadata flags?
> 
> What about the patch below? Search will stop if the flag has
> SDF_SHARED_CHILD as it does today. Otherwise it will search all the
> domains.
> 
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1773,6 +1773,12 @@ queue_balance_callback(struct rq *rq,
>  	for (__sd = rcu_dereference_check_sched_domain(cpu_rq(cpu)->sd); \
>  			__sd; __sd = __sd->parent)
>  
> +#define SD_FLAG(name, mflags) (name * !!((mflags) & SDF_SHARED_CHILD)) |
> +static const unsigned int SD_SHARED_CHILD_MASK =
> +#include <linux/sched/sd_flags.h>
> +0;
> +#undef SD_FLAG
> +
>  /**
>   * highest_flag_domain - Return highest sched_domain containing flag.
>   * @cpu:	The CPU whose highest level of sched domain is to
> @@ -1781,15 +1787,19 @@ queue_balance_callback(struct rq *rq,
>   *		for the given CPU.
>   *
>   * Returns the highest sched_domain of a CPU which contains the given flag.
> - */
> +*/
  ^^^
  likely an unintended change
>  static inline struct sched_domain *highest_flag_domain(int cpu, int flag)
>  {
>  	struct sched_domain *sd, *hsd = NULL;
>  
>  	for_each_domain(cpu, sd) {
> -		if (!(sd->flags & flag))
> +		if (sd->flags & flag) {
> +			hsd = sd;
> +			continue;
> +		}
> +

There might be room for a comment here:
                /*
		 * If the flag is not set and is known to be shared with lower
		 * domains, stop the search, as it won't be found further up.
		 */
> +		if (flag & SD_SHARED_CHILD_MASK)
>  			break;
> -		hsd = sd;
>  	}
>  
>  	return hsd;

It looks nice and sane to me - I've not compiled or tested it :).

Thanks,
Ionela.

> 
> > 
> > Thanks,
> > Ionela.
> > 
> > >  
> > >  /*
> > >   * Prefer to place tasks in a sibling domain
> > > -- 
> > > 2.25.1
> > > 
> > >
  

On Mon, Mar 06, 2023 at 01:10:37PM +0000, Ionela Voinescu wrote:
> Hey,
> 
> On Sunday 05 Mar 2023 at 11:08:11 (-0800), Ricardo Neri wrote:
> > On Fri, Mar 03, 2023 at 11:29:52AM +0000, Ionela Voinescu wrote:
> > > Hi Ricardo,
> > 
> > Hi Ionela!
> > 
> > > 
> > > On Monday 06 Feb 2023 at 20:58:36 (-0800), Ricardo Neri wrote:
> > > > Only x86 and Power7 use ASYM_PACKING. They use it differently.
> > > > 
> > > > Power7 has cores of equal priority, but the SMT siblings of a core have
> > > > different priorities. Parent scheduling domains do not need (nor have) the
> > > > ASYM_PACKING flag. SHARED_CHILD is not needed. Using SHARED_PARENT would
> > > > cause the topology debug code to complain.
> > > > 
> > > > X86 has cores of different priority, but all the SMT siblings of the core
> > > > have equal priority. It needs ASYM_PACKING at the MC level, but not at the
> > > > SMT level (it also needs it at upper levels if they have scheduling groups
> > > > of different priority). Removing ASYM_PACKING from the SMT domain causes
> > > > the topology debug code to complain.
> > > > 
> > > > Remove SHARED_CHILD for now. We still need a topology check that satisfies
> > > > both architectures.
> > > > 
> > > > Cc: Ben Segall <bsegall@google.com>
> > > > Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> > > > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > > > Cc: Len Brown <len.brown@intel.com>
> > > > Cc: Mel Gorman <mgorman@suse.de>
> > > > Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > > Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
> > > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > > Cc: Tim C. Chen <tim.c.chen@intel.com>
> > > > Cc: Valentin Schneider <vschneid@redhat.com>
> > > > Cc: x86@kernel.org
> > > > Cc: linux-kernel@vger.kernel.org
> > > > Suggested-by: Valentin Schneider <vschneid@redhat.com>
> > > > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > > > ---
> > > > Changes since v2:
> > > >  * Introduced this patch.
> > > > 
> > > > Changes since v1:
> > > >  * N/A
> > > > ---
> > > >  include/linux/sched/sd_flags.h | 5 +----
> > > >  1 file changed, 1 insertion(+), 4 deletions(-)
> > > > 
> > > > diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
> > > > index 57bde66d95f7..800238854ba5 100644
> > > > --- a/include/linux/sched/sd_flags.h
> > > > +++ b/include/linux/sched/sd_flags.h
> > > > @@ -132,12 +132,9 @@ SD_FLAG(SD_SERIALIZE, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
> > > >  /*
> > > >   * Place busy tasks earlier in the domain
> > > >   *
> > > > - * SHARED_CHILD: Usually set on the SMT level. Technically could be set further
> > > > - *               up, but currently assumed to be set from the base domain
> > > > - *               upwards (see update_top_cache_domain()).
> > > >   * NEEDS_GROUPS: Load balancing flag.
> > > >   */
> > > > -SD_FLAG(SD_ASYM_PACKING, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
> > > > +SD_FLAG(SD_ASYM_PACKING,  SDF_NEEDS_GROUPS)
> > > 
> > > While this silences the warning one would have gotten when removing
> > > SD_ASYM_PACKING from SMT level, it will still result in sd_asym_packing
> > > being NULL for these systems, which breaks nohz balance. That is because
> > > highest_flag_domain() still stops searching at the first level without
> > > the flag set, in this case SMT, even if levels above have the flag set.
> > 
> > You are absolutely right! This how this whole discussion started. It
> > slipped my mind.
> > 
> > > 
> > > Maybe highest_flag_domain() should be changed to take into account the
> > > metadata flags?
> > 
> > What about the patch below? Search will stop if the flag has
> > SDF_SHARED_CHILD as it does today. Otherwise it will search all the
> > domains.
> > 
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1773,6 +1773,12 @@ queue_balance_callback(struct rq *rq,
> >  	for (__sd = rcu_dereference_check_sched_domain(cpu_rq(cpu)->sd); \
> >  			__sd; __sd = __sd->parent)
> >  
> > +#define SD_FLAG(name, mflags) (name * !!((mflags) & SDF_SHARED_CHILD)) |
> > +static const unsigned int SD_SHARED_CHILD_MASK =
> > +#include <linux/sched/sd_flags.h>
> > +0;
> > +#undef SD_FLAG
> > +
> >  /**
> >   * highest_flag_domain - Return highest sched_domain containing flag.
> >   * @cpu:	The CPU whose highest level of sched domain is to
> > @@ -1781,15 +1787,19 @@ queue_balance_callback(struct rq *rq,
> >   *		for the given CPU.
> >   *
> >   * Returns the highest sched_domain of a CPU which contains the given flag.
> > - */
> > +*/
>   ^^^
>   likely an unintended change

Yes! I will remove it in the patch I post.

> >  static inline struct sched_domain *highest_flag_domain(int cpu, int flag)
> >  {
> >  	struct sched_domain *sd, *hsd = NULL;
> >  
> >  	for_each_domain(cpu, sd) {
> > -		if (!(sd->flags & flag))
> > +		if (sd->flags & flag) {
> > +			hsd = sd;
> > +			continue;
> > +		}
> > +
> 
> There might be room for a comment here:
>                 /*
> 		 * If the flag is not set and is known to be shared with lower
> 		 * domains, stop the search, as it won't be found further up.
> 		 */

Sure, I can and a comment to this effect.

> > +		if (flag & SD_SHARED_CHILD_MASK)
> >  			break;
> > -		hsd = sd;
> >  	}
> >  
> >  	return hsd;
> 
> It looks nice and sane to me - I've not compiled or tested it :).

Thank you very much for your feedback!

BR,
Ricardo