[PATCH v3] sched/fair: Use all little CPUs for CPU-bound workload

[PATCH v3] sched/fair: Use all little CPUs for CPU-bound workload

[Pierre Gondois]

Running n CPU-bound tasks on an n CPUs platform:
- with asymmetric CPU capacity
- not being a DynamIq system (i.e. having a PKG level sched domain
  without the SD_SHARE_PKG_RESOURCES flag set)
might result in a task placement where two tasks run on a big CPU
and none on a little CPU. This placement could be more optimal by
using all CPUs.

Testing platform:
Juno-r2:
- 2 big CPUs (1-2), maximum capacity of 1024
- 4 little CPUs (0,3-5), maximum capacity of 383

Testing workload ([1]):
Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks
is affine to a CPU, except for:
- one little CPU which is left idle.
- one big CPU which has 2 tasks affine.
After the 100ms (step 2), remove the cpumask affinity.

Before patch:
During step 2, the load balancer running from the idle CPU tags sched
domains as:
- little CPUs: 'group_has_spare'. Cf. group_has_capacity() and
  group_is_overloaded(), 3 CPU-bound tasks run on a 4 CPUs
  sched-domain, and the idle CPU provides enough spare capacity
  regarding the imbalance_pct
- big CPUs: 'group_overloaded'. Indeed, 3 tasks run on a 2 CPUs
  sched-domain, so the following path is used:
  group_is_overloaded()
  \-if (sgs->sum_nr_running <= sgs->group_weight) return true;

  The following path which would change the migration type to
  'migrate_task' is not taken:
  calculate_imbalance()
  \-if (env->idle != CPU_NOT_IDLE && env->imbalance == 0)
  as the local group has some spare capacity, so the imbalance
  is not 0.

The migration type requested is 'migrate_util' and the busiest
runqueue is the big CPU's runqueue having 2 tasks (each having a
utilization of 512). The idle little CPU cannot pull one of these
task as its capacity is too small for the task. The following path
is used:
detach_tasks()
\-case migrate_util:
  \-if (util > env->imbalance) goto next;

After patch:
As the number of failed balancing attempts grows (with
'nr_balance_failed'), progressively make it easier to migrate
a big task to the idling little CPU. A similar mechanism is
used for the 'migrate_load' migration type.

Improvement:
Running the testing workload [1] with the step 2 representing
a ~10s load for a big CPU:
Before patch: ~19.3s
After patch: ~18s (-6.7%)

Similar issue reported at:
https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/

v1:
https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@arm.com/
v2:
https://lore.kernel.org/all/20231124153323.3202444-1-pierre.gondois@arm.com/

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---

Notes:
    v2:
    - Used Vincent's approach.
    v3:
    - Updated commit message.
    - Added Reviewed-by tags

 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d7a3c63a2171..9481b8cff31b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9060,7 +9060,7 @@ static int detach_tasks(struct lb_env *env)
 		case migrate_util:
 			util = task_util_est(p);
 
-			if (util > env->imbalance)
+			if (shr_bound(util, env->sd->nr_balance_failed) > env->imbalance)
 				goto next;
 
 			env->imbalance -= util;
-- 
2.25.1

Re: [PATCH v3] sched/fair: Use all little CPUs for CPU-bound workload

[Qais Yousef]

On 12/06/23 10:00, Pierre Gondois wrote:
> Running n CPU-bound tasks on an n CPUs platform:
> - with asymmetric CPU capacity
> - not being a DynamIq system (i.e. having a PKG level sched domain
>   without the SD_SHARE_PKG_RESOURCES flag set)
> might result in a task placement where two tasks run on a big CPU
> and none on a little CPU. This placement could be more optimal by
> using all CPUs.
> 
> Testing platform:
> Juno-r2:
> - 2 big CPUs (1-2), maximum capacity of 1024
> - 4 little CPUs (0,3-5), maximum capacity of 383
> 
> Testing workload ([1]):
> Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks
> is affine to a CPU, except for:
> - one little CPU which is left idle.
> - one big CPU which has 2 tasks affine.
> After the 100ms (step 2), remove the cpumask affinity.
> 
> Before patch:
> During step 2, the load balancer running from the idle CPU tags sched
> domains as:
> - little CPUs: 'group_has_spare'. Cf. group_has_capacity() and
>   group_is_overloaded(), 3 CPU-bound tasks run on a 4 CPUs
>   sched-domain, and the idle CPU provides enough spare capacity
>   regarding the imbalance_pct
> - big CPUs: 'group_overloaded'. Indeed, 3 tasks run on a 2 CPUs
>   sched-domain, so the following path is used:
>   group_is_overloaded()
>   \-if (sgs->sum_nr_running <= sgs->group_weight) return true;
> 
>   The following path which would change the migration type to
>   'migrate_task' is not taken:
>   calculate_imbalance()
>   \-if (env->idle != CPU_NOT_IDLE && env->imbalance == 0)
>   as the local group has some spare capacity, so the imbalance
>   is not 0.
> 
> The migration type requested is 'migrate_util' and the busiest
> runqueue is the big CPU's runqueue having 2 tasks (each having a
> utilization of 512). The idle little CPU cannot pull one of these
> task as its capacity is too small for the task. The following path
> is used:
> detach_tasks()
> \-case migrate_util:
>   \-if (util > env->imbalance) goto next;
> 
> After patch:
> As the number of failed balancing attempts grows (with
> 'nr_balance_failed'), progressively make it easier to migrate
> a big task to the idling little CPU. A similar mechanism is
> used for the 'migrate_load' migration type.
> 
> Improvement:
> Running the testing workload [1] with the step 2 representing
> a ~10s load for a big CPU:
> Before patch: ~19.3s
> After patch: ~18s (-6.7%)
> 
> Similar issue reported at:
> https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/
> 
> v1:
> https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@arm.com/
> v2:
> https://lore.kernel.org/all/20231124153323.3202444-1-pierre.gondois@arm.com/
> 
> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---

Thanks Pierre! I think this is a good candidate to stable. It is likely to help
some folks shipping with phantom domains on impacted stable kernels.


Cheers

--
Qais Yousef

[tip: sched/core] sched/fair: Use all little CPUs for CPU-bound workloads

[tip-bot2 for Pierre Gondois]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     3af7524b14198f5159a86692d57a9f28ec9375ce
Gitweb:        https://git.kernel.org/tip/3af7524b14198f5159a86692d57a9f28ec9375ce
Author:        Pierre Gondois <pierre.gondois@arm.com>
AuthorDate:    Wed, 06 Dec 2023 10:00:43 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sat, 23 Dec 2023 16:06:36 +01:00

sched/fair: Use all little CPUs for CPU-bound workloads

Running N CPU-bound tasks on an N CPUs platform:

- with asymmetric CPU capacity

- not being a DynamIq system (i.e. having a PKG level sched domain
  without the SD_SHARE_PKG_RESOURCES flag set)

.. might result in a task placement where two tasks run on a big CPU
and none on a little CPU. This placement could be more optimal by
using all CPUs.

Testing platform:

  Juno-r2:
    - 2 big CPUs (1-2), maximum capacity of 1024
    - 4 little CPUs (0,3-5), maximum capacity of 383

Testing workload ([1]):

  Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks
  is affine to a CPU, except for:

    - one little CPU which is left idle.
    - one big CPU which has 2 tasks affine.

  After the 100ms (step 2), remove the cpumask affinity.

Behavior before the patch:

  During step 2, the load balancer running from the idle CPU tags sched
  domains as:

  - little CPUs: 'group_has_spare'. Cf. group_has_capacity() and
    group_is_overloaded(), 3 CPU-bound tasks run on a 4 CPUs
    sched-domain, and the idle CPU provides enough spare capacity
    regarding the imbalance_pct

  - big CPUs: 'group_overloaded'. Indeed, 3 tasks run on a 2 CPUs
    sched-domain, so the following path is used:

      group_is_overloaded()
      \-if (sgs->sum_nr_running <= sgs->group_weight) return true;

    The following path which would change the migration type to
    'migrate_task' is not taken:

      calculate_imbalance()
      \-if (env->idle != CPU_NOT_IDLE && env->imbalance == 0)

    as the local group has some spare capacity, so the imbalance
    is not 0.

  The migration type requested is 'migrate_util' and the busiest
  runqueue is the big CPU's runqueue having 2 tasks (each having a
  utilization of 512). The idle little CPU cannot pull one of these
  task as its capacity is too small for the task. The following path
  is used:

   detach_tasks()
   \-case migrate_util:
     \-if (util > env->imbalance) goto next;

After the patch:

As the number of failed balancing attempts grows (with
'nr_balance_failed'), progressively make it easier to migrate
a big task to the idling little CPU. A similar mechanism is
used for the 'migrate_load' migration type.

Improvement:

Running the testing workload [1] with the step 2 representing
a ~10s load for a big CPU:

  Before patch: ~19.3s
  After patch:  ~18s (-6.7%)

Similar issue reported at:

  https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Acked-by: Qais Yousef <qyousef@layalina.io>
Link: https://lore.kernel.org/r/20231206090043.634697-1-pierre.gondois@arm.com
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 823dd76..1d561b5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9071,7 +9071,7 @@ static int detach_tasks(struct lb_env *env)
 		case migrate_util:
 			util = task_util_est(p);
 
-			if (util > env->imbalance)
+			if (shr_bound(util, env->sd->nr_balance_failed) > env->imbalance)
 				goto next;
 
 			env->imbalance -= util;

Re: [PATCH v3] sched/fair: Use all little CPUs for CPU-bound workload

[Pierre Gondois]

Hello stable folk,

This patch was merged as:
   commit 3af7524b1419 ("sched/fair: Use all little CPUs for CPU-bound workloads")
into 6.7, improving the following:
   commit 0b0695f2b34a ("sched/fair: Rework load_balance()")

Would it be possible to port it to the 6.1 stable branch ?
The patch should apply cleanly by cherry-picking onto v6.1.94,

Regards,
Pierre


On 12/6/23 10:00, Pierre Gondois wrote:
> Running n CPU-bound tasks on an n CPUs platform:
> - with asymmetric CPU capacity
> - not being a DynamIq system (i.e. having a PKG level sched domain
>    without the SD_SHARE_PKG_RESOURCES flag set)
> might result in a task placement where two tasks run on a big CPU
> and none on a little CPU. This placement could be more optimal by
> using all CPUs.
> 
> Testing platform:
> Juno-r2:
> - 2 big CPUs (1-2), maximum capacity of 1024
> - 4 little CPUs (0,3-5), maximum capacity of 383
> 
> Testing workload ([1]):
> Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks
> is affine to a CPU, except for:
> - one little CPU which is left idle.
> - one big CPU which has 2 tasks affine.
> After the 100ms (step 2), remove the cpumask affinity.
> 
> Before patch:
> During step 2, the load balancer running from the idle CPU tags sched
> domains as:
> - little CPUs: 'group_has_spare'. Cf. group_has_capacity() and
>    group_is_overloaded(), 3 CPU-bound tasks run on a 4 CPUs
>    sched-domain, and the idle CPU provides enough spare capacity
>    regarding the imbalance_pct
> - big CPUs: 'group_overloaded'. Indeed, 3 tasks run on a 2 CPUs
>    sched-domain, so the following path is used:
>    group_is_overloaded()
>    \-if (sgs->sum_nr_running <= sgs->group_weight) return true;
> 
>    The following path which would change the migration type to
>    'migrate_task' is not taken:
>    calculate_imbalance()
>    \-if (env->idle != CPU_NOT_IDLE && env->imbalance == 0)
>    as the local group has some spare capacity, so the imbalance
>    is not 0.
> 
> The migration type requested is 'migrate_util' and the busiest
> runqueue is the big CPU's runqueue having 2 tasks (each having a
> utilization of 512). The idle little CPU cannot pull one of these
> task as its capacity is too small for the task. The following path
> is used:
> detach_tasks()
> \-case migrate_util:
>    \-if (util > env->imbalance) goto next;
> 
> After patch:
> As the number of failed balancing attempts grows (with
> 'nr_balance_failed'), progressively make it easier to migrate
> a big task to the idling little CPU. A similar mechanism is
> used for the 'migrate_load' migration type.
> 
> Improvement:
> Running the testing workload [1] with the step 2 representing
> a ~10s load for a big CPU:
> Before patch: ~19.3s
> After patch: ~18s (-6.7%)
> 
> Similar issue reported at:
> https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/
> 
> v1:
> https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@arm.com/
> v2:
> https://lore.kernel.org/all/20231124153323.3202444-1-pierre.gondois@arm.com/
> 
> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---
> 
> Notes:
>      v2:
>      - Used Vincent's approach.
>      v3:
>      - Updated commit message.
>      - Added Reviewed-by tags
> 
>   kernel/sched/fair.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d7a3c63a2171..9481b8cff31b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9060,7 +9060,7 @@ static int detach_tasks(struct lb_env *env)
>   		case migrate_util:
>   			util = task_util_est(p);
>   
> -			if (util > env->imbalance)
> +			if (shr_bound(util, env->sd->nr_balance_failed) > env->imbalance)
>   				goto next;
>   
>   			env->imbalance -= util;

Re: [PATCH v3] sched/fair: Use all little CPUs for CPU-bound workload

[Pierre Gondois]

Hello Sasha,
Would it be possible to pick this patch for the 6.1 stable branch ?
Or is there something I should do for this purpose ?

Regards,
Pierre

On 6/25/24 15:25, Pierre Gondois wrote:
> Hello stable folk,
> 
> This patch was merged as:
>     commit 3af7524b1419 ("sched/fair: Use all little CPUs for CPU-bound workloads")
> into 6.7, improving the following:
>     commit 0b0695f2b34a ("sched/fair: Rework load_balance()")
> 
> Would it be possible to port it to the 6.1 stable branch ?
> The patch should apply cleanly by cherry-picking onto v6.1.94,
> 
> Regards,
> Pierre
> 
> 
> On 12/6/23 10:00, Pierre Gondois wrote:
>> Running n CPU-bound tasks on an n CPUs platform:
>> - with asymmetric CPU capacity
>> - not being a DynamIq system (i.e. having a PKG level sched domain
>>     without the SD_SHARE_PKG_RESOURCES flag set)
>> might result in a task placement where two tasks run on a big CPU
>> and none on a little CPU. This placement could be more optimal by
>> using all CPUs.
>>
>> Testing platform:
>> Juno-r2:
>> - 2 big CPUs (1-2), maximum capacity of 1024
>> - 4 little CPUs (0,3-5), maximum capacity of 383
>>
>> Testing workload ([1]):
>> Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks
>> is affine to a CPU, except for:
>> - one little CPU which is left idle.
>> - one big CPU which has 2 tasks affine.
>> After the 100ms (step 2), remove the cpumask affinity.
>>
>> Before patch:
>> During step 2, the load balancer running from the idle CPU tags sched
>> domains as:
>> - little CPUs: 'group_has_spare'. Cf. group_has_capacity() and
>>     group_is_overloaded(), 3 CPU-bound tasks run on a 4 CPUs
>>     sched-domain, and the idle CPU provides enough spare capacity
>>     regarding the imbalance_pct
>> - big CPUs: 'group_overloaded'. Indeed, 3 tasks run on a 2 CPUs
>>     sched-domain, so the following path is used:
>>     group_is_overloaded()
>>     \-if (sgs->sum_nr_running <= sgs->group_weight) return true;
>>
>>     The following path which would change the migration type to
>>     'migrate_task' is not taken:
>>     calculate_imbalance()
>>     \-if (env->idle != CPU_NOT_IDLE && env->imbalance == 0)
>>     as the local group has some spare capacity, so the imbalance
>>     is not 0.
>>
>> The migration type requested is 'migrate_util' and the busiest
>> runqueue is the big CPU's runqueue having 2 tasks (each having a
>> utilization of 512). The idle little CPU cannot pull one of these
>> task as its capacity is too small for the task. The following path
>> is used:
>> detach_tasks()
>> \-case migrate_util:
>>     \-if (util > env->imbalance) goto next;
>>
>> After patch:
>> As the number of failed balancing attempts grows (with
>> 'nr_balance_failed'), progressively make it easier to migrate
>> a big task to the idling little CPU. A similar mechanism is
>> used for the 'migrate_load' migration type.
>>
>> Improvement:
>> Running the testing workload [1] with the step 2 representing
>> a ~10s load for a big CPU:
>> Before patch: ~19.3s
>> After patch: ~18s (-6.7%)
>>
>> Similar issue reported at:
>> https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/
>>
>> v1:
>> https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@arm.com/
>> v2:
>> https://lore.kernel.org/all/20231124153323.3202444-1-pierre.gondois@arm.com/
>>
>> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
>> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
>> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
>> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
>> ---
>>
>> Notes:
>>       v2:
>>       - Used Vincent's approach.
>>       v3:
>>       - Updated commit message.
>>       - Added Reviewed-by tags
>>
>>    kernel/sched/fair.c | 2 +-
>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index d7a3c63a2171..9481b8cff31b 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -9060,7 +9060,7 @@ static int detach_tasks(struct lb_env *env)
>>    		case migrate_util:
>>    			util = task_util_est(p);
>>    
>> -			if (util > env->imbalance)
>> +			if (shr_bound(util, env->sd->nr_balance_failed) > env->imbalance)
>>    				goto next;
>>    
>>    			env->imbalance -= util;
> 

Re: [PATCH v3] sched/fair: Use all little CPUs for CPU-bound workload

[Greg KH]

On Mon, Jul 29, 2024 at 11:50:40AM +0200, Pierre Gondois wrote:
> Hello Sasha,
> Would it be possible to pick this patch for the 6.1 stable branch ?
> Or is there something I should do for this purpose ?
> 
> Regards,
> Pierre
> 
> On 6/25/24 15:25, Pierre Gondois wrote:
> > Hello stable folk,
> > 
> > This patch was merged as:
> >     commit 3af7524b1419 ("sched/fair: Use all little CPUs for CPU-bound workloads")
> > into 6.7, improving the following:
> >     commit 0b0695f2b34a ("sched/fair: Rework load_balance()")
> > 
> > Would it be possible to port it to the 6.1 stable branch ?
> > The patch should apply cleanly by cherry-picking onto v6.1.94,

You also forgot 5.10.y and 5.15.y which need it, I've queued it up for
those as well now, thanks.

greg k-h