[PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload

[PATCH v2] 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 having SD_SHARE_PKG_RESOURCES flag set at the DIE
  sched domain level (i.e. not DynamIQ systems)
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'. Indeed, 3 CPU-bound tasks run on a
  4 CPUs sched-domain, and the idle CPU provides enough spare
  capacity.
- 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/

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Pierre Gondois <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 df348aa55d3c..53c18fd23ae7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8907,7 +8907,7 @@ 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 v2] sched/fair: Use all little CPUs for CPU-bound workload

[Vincent Guittot]

On Fri, 24 Nov 2023 at 16:33, Pierre Gondois <pierre.gondois@arm.com> wrote:
>
> Running n CPU-bound tasks on an n CPUs platform:
> - with asymmetric CPU capacity
> - not having SD_SHARE_PKG_RESOURCES flag set at the DIE
>   sched domain level (i.e. not DynamIQ systems)

Nit:  SD_SHARE_PKG_RESOURCES is never set at the DIE level. In case of
DynamIQ systems, all CPUs are in the same MC level which has
SD_SHARE_PKG_RESOURCES flag

> 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'. Indeed, 3 CPU-bound tasks run on a
>   4 CPUs sched-domain, and the idle CPU provides enough spare
>   capacity.
> - 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/
>
> 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>

> ---
>  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 df348aa55d3c..53c18fd23ae7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8907,7 +8907,7 @@ 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 v2] sched/fair: Use all little CPUs for CPU-bound workload

[Dietmar Eggemann]

On 24/11/2023 16:33, Pierre Gondois wrote:
> Running n CPU-bound tasks on an n CPUs platform:
> - with asymmetric CPU capacity
> - not having SD_SHARE_PKG_RESOURCES flag set at the DIE

nit: DIE is now called PKG on tip sched/core.

f577cd57bfaa - sched/topology: Rename 'DIE' domain to 'PKG' (2023-10-12
Peter Zijlstra)

>   sched domain level (i.e. not DynamIQ systems)
> 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'. Indeed, 3 CPU-bound tasks run on a
>   4 CPUs sched-domain, and the idle CPU provides enough spare
>   capacity.

What is meant by 'idle CPU provides enough spare capacity? I thought the
task (util_avg ~ 512_ does not fit on the sched group [1,3-5] when we
consider util_avg/capacity (383)

The calculated imbalance of ~350 is too small for the task-size and
that's why we need the 'shr_bound(util, env->sd->nr_balance_failed)' to
let the task load-balance if nr_balance_failed = 2?

[...]

> 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/
> 
> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>

Even though this cures only classical big.LITTLE it might have a
positive effect on today's Arm DynamIQ Android systems with Phantom SDs
when running benchmarks like Geekbench.

[...]

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

Re: [PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload

[Pierre Gondois]

On 11/29/23 09:55, Dietmar Eggemann wrote:
> On 24/11/2023 16:33, Pierre Gondois wrote:
>> Running n CPU-bound tasks on an n CPUs platform:
>> - with asymmetric CPU capacity
>> - not having SD_SHARE_PKG_RESOURCES flag set at the DIE
> 
> nit: DIE is now called PKG on tip sched/core.
> 
> f577cd57bfaa - sched/topology: Rename 'DIE' domain to 'PKG' (2023-10-12
> Peter Zijlstra)
> 
>>    sched domain level (i.e. not DynamIQ systems)
>> 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'. Indeed, 3 CPU-bound tasks run on a
>>    4 CPUs sched-domain, and the idle CPU provides enough spare
>>    capacity.
> 
> What is meant by 'idle CPU provides enough spare capacity? I thought the
> task (util_avg ~ 512_ does not fit on the sched group [1,3-5] when we
> consider util_avg/capacity (383)

Right, I meant that when evaluating the 'group_type', there is enough spare
capacity when summing the utilization of CPUs in the the MC sched domain:

---
group_has_capacity()
{
	[...]
	if ((sgs->group_capacity * 100) >
			(sgs->group_util * imbalance_pct))
		return true;
	[...]
}
---

> 
> The calculated imbalance of ~350 is too small for the task-size and
> that's why we need the 'shr_bound(util, env->sd->nr_balance_failed)' to
> let the task load-balance if nr_balance_failed = 2?

Yes exact, the tasks are too big and cannot fit this imbalance value
(representing the available spare capacity in the little CPUs in this case).

'shr_bound(...)' allows to progressively reduce the size of the tasks and
allow migrations after having tried to balance 'nr_balance_failed' times.


> 
> [...]
> 
>> 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/
>>
>> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
>> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
> 
> Even though this cures only classical big.LITTLE it might have a
> positive effect on today's Arm DynamIQ Android systems with Phantom SDs
> when running benchmarks like Geekbench.
> 
> [...]
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> 

Re: [PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload

[Dietmar Eggemann]

On 29/11/2023 11:48, Pierre Gondois wrote:
> 
> 
> On 11/29/23 09:55, Dietmar Eggemann wrote:
>> On 24/11/2023 16:33, Pierre Gondois wrote:

[...]

>>> 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'. Indeed, 3 CPU-bound tasks run on a
>>>    4 CPUs sched-domain, and the idle CPU provides enough spare
>>>    capacity.
>>
>> What is meant by 'idle CPU provides enough spare capacity? I thought the
>> task (util_avg ~ 512_ does not fit on the sched group [1,3-5] when we
>> consider util_avg/capacity (383)
> 
> Right, I meant that when evaluating the 'group_type', there is enough spare
> capacity when summing the utilization of CPUs in the the MC sched domain:
> 
> ---
> group_has_capacity()
> {
>     [...]
>     if ((sgs->group_capacity * 100) >
>             (sgs->group_util * imbalance_pct))
>         return true;
>     [...]
> }

I see. But doesn't group_has_capacity() already return true because of
`if (sgs->sum_nr_running < sgs->group_weight)` in this case (3 < 4)?

Maybe you can make this clearer when sending the v3 with the Reviewed-By
tags?

> ---
> 
>>
>> The calculated imbalance of ~350 is too small for the task-size and
>> that's why we need the 'shr_bound(util, env->sd->nr_balance_failed)' to
>> let the task load-balance if nr_balance_failed = 2?
> 
> Yes exact, the tasks are too big and cannot fit this imbalance value
> (representing the available spare capacity in the little CPUs in this
> case).
> 
> 'shr_bound(...)' allows to progressively reduce the size of the tasks and
> allow migrations after having tried to balance 'nr_balance_failed' times.

[...]

Re: [PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload

[Qais Yousef]

On 11/24/23 16:33, Pierre Gondois wrote:
> Running n CPU-bound tasks on an n CPUs platform:
> - with asymmetric CPU capacity
> - not having SD_SHARE_PKG_RESOURCES flag set at the DIE
>   sched domain level (i.e. not DynamIQ systems)
> 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'. Indeed, 3 CPU-bound tasks run on a
>   4 CPUs sched-domain, and the idle CPU provides enough spare
>   capacity.
> - 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/
> 
> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Pierre Gondois <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 df348aa55d3c..53c18fd23ae7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8907,7 +8907,7 @@ 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)

FWIW, this did help but not entirely when I tried it in the past

	https://lore.kernel.org/lkml/20230718161829.ws3vn3ufnod6kpxh@airbuntu/

And nit, the comment in that posted diff was useful.

No objections anyway but I suspect there's room for further improvement if
somebody still cares.


Cheers

--
Qais Yousef