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cpuset: Defer flushing of the cpuset_migrate_mm_wq to task_work

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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: Tejun Heo <tj@kernel.org>
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Chuyi Zhou 2025-09-04 15:45:04 +08:00 committed by Tejun Heo
parent c0fb16ef88
commit 3514309e03
1 changed files with 24 additions and 5 deletions
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29
kernel/cgroup/cpuset.c
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@ -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,