kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
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;
--
2.25.1On 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
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>
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>
>
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.
[...]
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 >
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