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sched/fair: Check CPU capacity before comparing group types during load balance

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update_sd_pick_busiest() may incorrectly select a fully_busy group as the
busiest group when its per-CPU capacity exceeds that of the destination
CPU. This happens because the type of busiest group is initialized to
group_has_spare and allows the fully_busy group to win the type comparison.

update_sd_pick_busiest() should not choose a candidate scheduling group
with at most one runnable task if its per-CPU capacity is greater than that
of the destination CPU. Such a check already exists, but it is done too
late: after the type comparison, preventing a subsequent fully_busy group
of equal per-CPU capacity from being correctly selected.

Move this check to occur before comparing group types.

Fixes: 0b0695f2b3 ("sched/fair: Rework load_balance()")
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: Christian Loehle <christian.loehle@arm.com>
Tested-by: Andrea Righi <arighi@nvidia.com>
Link: https://patch.msgid.link/20260720-rneri-fix-cas-clusters-v6-3-bb500bf4afd4@linux.intel.com
This commit is contained in:
Ricardo Neri 2026-07-20 19:43:19 -07:00 committed by Peter Zijlstra
parent 6060d61d13
commit 50b101f6e5
1 changed files with 11 additions and 11 deletions
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22
kernel/sched/fair.c
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@ -12054,6 +12054,17 @@ static bool update_sd_pick_busiest(struct lb_env *env,
sds->local_stat.group_type != group_has_spare))
return false;
/*
* Candidate sg has no more than one task per CPU and has higher
* per-CPU capacity. Migrating tasks to less capable CPUs may harm
* throughput. Maximize throughput, power/energy consequences are not
* considered.
*/
if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
(sgs->group_type <= group_fully_busy) &&
(capacity_greater(sg->sgc->min_capacity, capacity_of(env->dst_cpu))))
return false;
if (sgs->group_type > busiest->group_type)
return true;
@ -12160,17 +12171,6 @@ static bool update_sd_pick_busiest(struct lb_env *env,
break;
}
/*
* Candidate sg has no more than one task per CPU and has higher
* per-CPU capacity. Migrating tasks to less capable CPUs may harm
* throughput. Maximize throughput, power/energy consequences are not
* considered.
*/
if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
(sgs->group_type <= group_fully_busy) &&
(capacity_greater(sg->sgc->min_capacity, capacity_of(env->dst_cpu))))
return false;
return true;
}