Commit 3514309e authored by Chuyi Zhou's avatar Chuyi Zhou Committed by Tejun Heo
Browse files

cpuset: Defer flushing of the cpuset_migrate_mm_wq to task_work 

Now in cpuset_attach(), we need to synchronously wait for
flush_workqueue to complete. The execution time of flushing
cpuset_migrate_mm_wq depends on the amount of mm migration initiated by
cpusets at that time. When the cpuset.mems of a cgroup occupying a large
amount of memory is modified, it may trigger extensive mm migration,
causing cpuset_attach() to block on flush_workqueue for an extended period.
This could be dangerous because cpuset_attach() is within the critical
section of cgroup_mutex, which may ultimately cause all cgroup-related
operations in the system to be blocked.

This patch attempts to defer the flush_workqueue() operation until
returning to userspace using the task_work which is originally proposed by
tejun[1], so that flush happens after cgroup_mutex is dropped. That way we
maintain the operation synchronicity while avoiding bothering anyone else.

[1]: https://lore.kernel.org/cgroups/ZgMFPMjZRZCsq9Q-@slm.duckdns.org/T/#m117f606fa24f66f0823a60f211b36f24bd9e1883



Originally-by: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarChuyi Zhou <zhouchuyi@bytedance.com>
Reviewed-by: default avatarWaiman Long <longman@redhat.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent c0fb16ef

kernel/cgroup/cpuset.c

+24 −5
Original line number Diff line number Diff line
@@ -40,6 +40,7 @@ 
#include <linux/sched/isolation.h>

#include <linux/wait.h>

#include <linux/workqueue.h>

#include <linux/task_work.h>



DEFINE_STATIC_KEY_FALSE(cpusets_pre_enable_key);

DEFINE_STATIC_KEY_FALSE(cpusets_enabled_key);
@@ -2619,9 +2620,24 @@ static void cpuset_migrate_mm(struct mm_struct *mm, const nodemask_t *from, 
	}

}



static void cpuset_post_attach(void)

static void flush_migrate_mm_task_workfn(struct callback_head *head)

{

	flush_workqueue(cpuset_migrate_mm_wq);

	kfree(head);

}



static void schedule_flush_migrate_mm(void)

{

	struct callback_head *flush_cb;



	flush_cb = kzalloc(sizeof(struct callback_head), GFP_KERNEL);

	if (!flush_cb)

		return;



	init_task_work(flush_cb, flush_migrate_mm_task_workfn);



	if (task_work_add(current, flush_cb, TWA_RESUME))

		kfree(flush_cb);

}



/*
@@ -3178,6 +3194,7 @@ static void cpuset_attach(struct cgroup_taskset *tset) 
	struct cpuset *cs;

	struct cpuset *oldcs = cpuset_attach_old_cs;

	bool cpus_updated, mems_updated;

	bool queue_task_work = false;



	cgroup_taskset_first(tset, &css);

	cs = css_cs(css);
@@ -3228,15 +3245,18 @@ static void cpuset_attach(struct cgroup_taskset *tset) 
			 * @old_mems_allowed is the right nodesets that we

			 * migrate mm from.

			 */

			if (is_memory_migrate(cs))

			if (is_memory_migrate(cs)) {

				cpuset_migrate_mm(mm, &oldcs->old_mems_allowed,

						  &cpuset_attach_nodemask_to);

			else

				queue_task_work = true;

			} else

				mmput(mm);

		}

	}



out:

	if (queue_task_work)

		schedule_flush_migrate_mm();

	cs->old_mems_allowed = cpuset_attach_nodemask_to;



	if (cs->nr_migrate_dl_tasks) {
@@ -3292,7 +3312,7 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file *of, 
out_unlock:

	cpuset_full_unlock();

	if (of_cft(of)->private == FILE_MEMLIST)

		flush_workqueue(cpuset_migrate_mm_wq);

		schedule_flush_migrate_mm();

	return retval ?: nbytes;

}
@@ -3739,7 +3759,6 @@ struct cgroup_subsys cpuset_cgrp_subsys = { 
	.can_attach	= cpuset_can_attach,

	.cancel_attach	= cpuset_cancel_attach,

	.attach		= cpuset_attach,

	.post_attach	= cpuset_post_attach,

	.bind		= cpuset_bind,

	.can_fork	= cpuset_can_fork,

	.cancel_fork	= cpuset_cancel_fork,