Commit 6ee43047 authored by Chen Ridong's avatar Chen Ridong Committed by Tejun Heo
Browse files

cpuset: Remove unnecessary checks in rebuild_sched_domains_locked 



Commit 406100f3 ("cpuset: fix race between hotplug work and later CPU
offline") added a check for empty effective_cpus in partitions for cgroup
v2. However, this check did not account for remote partitions, which were
introduced later.

After commit 2125c003 ("cgroup/cpuset: Make cpuset hotplug processing
synchronous"), cpuset hotplug handling is now synchronous. This eliminates
the race condition with subsequent CPU offline operations that the original
check aimed to fix.

Instead of extending the check to support remote partitions, this patch
removes all the redundant effective_cpus check. Additionally, it adds a
check and warning to verify that all generated sched domains consist of
active CPUs, preventing partition_sched_domains from being invoked with
offline CPUs.

Signed-off-by: default avatarChen Ridong <chenridong@huawei.com>
Reviewed-by: default avatarWaiman Long <longman@redhat.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 82d7e59e

kernel/cgroup/cpuset.c

+15 −35
Original line number Diff line number Diff line
@@ -1103,52 +1103,32 @@ void dl_rebuild_rd_accounting(void) 
 */

void rebuild_sched_domains_locked(void)

{

	struct cgroup_subsys_state *pos_css;

	struct sched_domain_attr *attr;

	cpumask_var_t *doms;

	struct cpuset *cs;

	int ndoms;

	int i;



	lockdep_assert_cpus_held();

	lockdep_assert_held(&cpuset_mutex);

	force_sd_rebuild = false;



	/* Generate domain masks and attrs */

	ndoms = generate_sched_domains(&doms, &attr);



	/*

	 * If we have raced with CPU hotplug, return early to avoid

	 * passing doms with offlined cpu to partition_sched_domains().

	 * Anyways, cpuset_handle_hotplug() will rebuild sched domains.

	* cpuset_hotplug_workfn is invoked synchronously now, thus this

	* function should not race with CPU hotplug. And the effective CPUs

	* must not include any offline CPUs. Passing an offline CPU in the

	* doms to partition_sched_domains() will trigger a kernel panic.

	*

	 * With no CPUs in any subpartitions, top_cpuset's effective CPUs

	 * should be the same as the active CPUs, so checking only top_cpuset

	 * is enough to detect racing CPU offlines.

	* We perform a final check here: if the doms contains any

	* offline CPUs, a warning is emitted and we return directly to

	* prevent the panic.

	*/

	if (cpumask_empty(subpartitions_cpus) &&

	    !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))

	for (i = 0; i < ndoms; ++i) {

		if (WARN_ON_ONCE(!cpumask_subset(doms[i], cpu_active_mask)))

			return;



	/*

	 * With subpartition CPUs, however, the effective CPUs of a partition

	 * root should be only a subset of the active CPUs.  Since a CPU in any

	 * partition root could be offlined, all must be checked.

	 */

	if (!cpumask_empty(subpartitions_cpus)) {

		rcu_read_lock();

		cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {

			if (!is_partition_valid(cs)) {

				pos_css = css_rightmost_descendant(pos_css);

				continue;

			}

			if (!cpumask_subset(cs->effective_cpus,

					    cpu_active_mask)) {

				rcu_read_unlock();

				return;

			}

	}

		rcu_read_unlock();

	}



	/* Generate domain masks and attrs */

	ndoms = generate_sched_domains(&doms, &attr);



	/* Have scheduler rebuild the domains */

	partition_sched_domains(ndoms, doms, attr);