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sched/fair: Simplify SIS_UTIL handling in select_idle_cpu()

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Use the "sd_llc" passed to select_idle_cpu() to obtain the
"sd_llc_shared" instead of dereferencing the per-CPU variable.

Since "sd->shared" is always reclaimed at the same time as "sd" via
call_rcu() and update_top_cache_domain() always ensures a valid
"sd->shared" assignment when "sd_llc" is present, "sd_llc->shared" can
always be dereferenced without needing an additional check.

While at it move the cpumask_and() operation after the SIS_UTIL bailout
check to avoid unnecessarily computing the cpumask.

Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://patch.msgid.link/20260312044434.1974-10-kprateek.nayak@amd.com
This commit is contained in:
K Prateek Nayak
2026-03-12 04:44:34 +00:00
committed by Peter Zijlstra
parent f1320a8dd8
commit fe7171d0d5
1 changed files with 16 additions and 11 deletions
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27
kernel/sched/fair.c
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@@ -7876,21 +7876,26 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
{
struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
int i, cpu, idle_cpu = -1, nr = INT_MAX;
struct sched_domain_shared *sd_share;
cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
if (sched_feat(SIS_UTIL)) {
sd_share = rcu_dereference_all(per_cpu(sd_llc_shared, target));
if (sd_share) {
/* because !--nr is the condition to stop scan */
nr = READ_ONCE(sd_share->nr_idle_scan) + 1;
/* overloaded LLC is unlikely to have idle cpu/core */
if (nr == 1)
return -1;
}
/*
* Increment because !--nr is the condition to stop scan.
*
* Since "sd" is "sd_llc" for target CPU dereferenced in the
* caller, it is safe to directly dereference "sd->shared".
* Topology bits always ensure it assigned for "sd_llc" abd it
* cannot disappear as long as we have a RCU protected
* reference to one the associated "sd" here.
*/
nr = READ_ONCE(sd->shared->nr_idle_scan) + 1;
/* overloaded LLC is unlikely to have idle cpu/core */
if (nr == 1)
return -1;
}
if (!cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr))
return -1;
if (static_branch_unlikely(&sched_cluster_active)) {
struct sched_group *sg = sd->groups;