Commit fe7171d0 authored by K Prateek Nayak's avatar K Prateek Nayak Committed by Peter Zijlstra
Browse files

sched/fair: Simplify SIS_UTIL handling in select_idle_cpu() 



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: default avatarK Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarChen Yu <yu.c.chen@intel.com>
Reviewed-by: default avatarShrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://patch.msgid.link/20260312044434.1974-10-kprateek.nayak@amd.com
parent f1320a8d

kernel/sched/fair.c

+16 −11
Original line number Diff line number Diff line
@@ -7876,20 +7876,25 @@ 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;

		/*

		 * 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;