The patch titled Subject: memcg: always call cond_resched() after fn() has been added to the -mm mm-hotfixes-unstable branch. Its filename is memcg-always-call-cond_resched-after-fn.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Breno Leitao leitao@debian.org Subject: memcg: always call cond_resched() after fn() Date: Fri, 23 May 2025 10:21:06 -0700

I am seeing soft lockup on certain machine types when a cgroup OOMs. This is happening because killing the process in certain machine might be very slow, which causes the soft lockup and RCU stalls. This happens usually when the cgroup has MANY processes and memory.oom.group is set.

Example I am seeing in real production:

[462012.244552] Memory cgroup out of memory: Killed process 3370438 (crosvm) .... .... [462037.318059] Memory cgroup out of memory: Killed process 4171372 (adb) .... [462037.348314] watchdog: BUG: soft lockup - CPU#64 stuck for 26s! [stat_manager-ag:1618982] ....

Quick look at why this is so slow, it seems to be related to serial flush for certain machine types. For all the crashes I saw, the target CPU was at console_flush_all().

In the case above, there are thousands of processes in the cgroup, and it is soft locking up before it reaches the 1024 limit in the code (which would call the cond_resched()). So, cond_resched() in 1024 blocks is not sufficient.

Remove the counter-based conditional rescheduling logic and call cond_resched() unconditionally after each task iteration, after fn() is called. This avoids the lockup independently of how slow fn() is.

Link: https://lkml.kernel.org/r/20250523-memcg_fix-v1-1-ad3eafb60477@debian.org Fixes: ade81479c7dd ("memcg: fix soft lockup in the OOM process") Signed-off-by: Breno Leitao leitao@debian.org Suggested-by: Rik van Riel riel@surriel.com Acked-by: Shakeel Butt shakeel.butt@linux.dev Cc: Michael van der Westhuizen rmikey@meta.com Cc: Usama Arif usamaarif642@gmail.com Cc: Pavel Begunkov asml.silence@gmail.com Cc: Chen Ridong chenridong@huawei.com Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Johannes Weiner hannes@cmpxchg.org Cc: Michal Hocko mhocko@kernel.org Cc: Michal Hocko mhocko@suse.com Cc: Muchun Song muchun.song@linux.dev Cc: Roman Gushchin roman.gushchin@linux.dev Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memcontrol.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-)

--- a/mm/memcontrol.c~memcg-always-call-cond_resched-after-fn +++ a/mm/memcontrol.c @@ -1168,7 +1168,6 @@ void mem_cgroup_scan_tasks(struct mem_cg { struct mem_cgroup *iter; int ret = 0; - int i = 0;

BUG_ON(mem_cgroup_is_root(memcg));

@@ -1178,10 +1177,9 @@ void mem_cgroup_scan_tasks(struct mem_cg

css_task_iter_start(&iter->css, CSS_TASK_ITER_PROCS, &it); while (!ret && (task = css_task_iter_next(&it))) { - /* Avoid potential softlockup warning */ - if ((++i & 1023) == 0) - cond_resched(); ret = fn(task, arg); + /* Avoid potential softlockup warning */ + cond_resched(); } css_task_iter_end(&it); if (ret) { _

Patches currently in -mm which might be from leitao@debian.org are

memcg-always-call-cond_resched-after-fn.patch