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Message-ID: <170334778858.398.14876709177297398482.tip-bot2@tip-bot2>
Date: Sat, 23 Dec 2023 16:09:48 -0000
From: "tip-bot2 for Pierre Gondois" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Vincent Guittot <vincent.guittot@...aro.org>,
Pierre Gondois <pierre.gondois@....com>, Ingo Molnar <mingo@...nel.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Qais Yousef <qyousef@...alina.io>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject:
[tip: sched/core] sched/fair: Use all little CPUs for CPU-bound workloads
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@....com>
AuthorDate: Wed, 06 Dec 2023 10:00:43 +01:00
Committer: Ingo Molnar <mingo@...nel.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@...aro.org>
Signed-off-by: Pierre Gondois <pierre.gondois@....com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
Acked-by: Qais Yousef <qyousef@...alina.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;
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