Re: [PATCH v2] memcg: fix soft lockup in the OOM process

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 20 Dec 2024 10:31:23 +0000 Chen Ridong <chenridong@huaweicloud.com> wrote:

> From: Chen Ridong <chenridong@huawei.com>
> 
> A soft lockup issue was found in the product with about 56,000 tasks were
> in the OOM cgroup, it was traversing them when the soft lockup was
> triggered.
> 
> ...
>
> This is because thousands of processes are in the OOM cgroup, it takes a
> long time to traverse all of them. As a result, this lead to soft lockup
> in the OOM process.
> 
> To fix this issue, call 'cond_resched' in the 'mem_cgroup_scan_tasks'
> function per 1000 iterations. For global OOM, call
> 'touch_softlockup_watchdog' per 1000 iterations to avoid this issue.
> 
> ...
>
> --- a/include/linux/oom.h
> +++ b/include/linux/oom.h
> @@ -14,6 +14,13 @@ struct notifier_block;
>  struct mem_cgroup;
>  struct task_struct;
>  
> +/* When it traverses for long time,  to prevent softlockup, call
> + * cond_resched/touch_softlockup_watchdog very 1000 iterations.
> + * The 1000 value  is not exactly right, it's used to mitigate the overhead
> + * of cond_resched/touch_softlockup_watchdog.
> + */
> +#define SOFTLOCKUP_PREVENTION_LIMIT 1000

If this is to have potentially kernel-wide scope, its name should
identify which subsystem it belongs to.  Maybe OOM_KILL_RESCHED or
something.

But I'm not sure that this really needs to exist.  Are the two usage
sites particularly related?

>  enum oom_constraint {
>  	CONSTRAINT_NONE,
>  	CONSTRAINT_CPUSET,
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5c373d275e7a..f4c12d6e7b37 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1161,6 +1161,7 @@ void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
>  {
>  	struct mem_cgroup *iter;
>  	int ret = 0;
> +	int i = 0;
>  
>  	BUG_ON(mem_cgroup_is_root(memcg));
>  
> @@ -1169,8 +1170,11 @@ void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
>  		struct task_struct *task;
>  
>  		css_task_iter_start(&iter->css, CSS_TASK_ITER_PROCS, &it);
> -		while (!ret && (task = css_task_iter_next(&it)))
> +		while (!ret && (task = css_task_iter_next(&it))) {
>  			ret = fn(task, arg);
> +			if (++i % SOFTLOCKUP_PREVENTION_LIMIT)

And a modulus operation is somewhat expensive.

Perhaps a simple

		/* Avoid potential softlockup warning */
		if ((++i & 1023) == 0)

at both sites will suffice.  Opinions might vary...