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

[Michal Hocko <mhocko@suse.com>]

On Fri 20-12-24 14:47:34, Andrew Morton wrote:
> 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?

Yes, I do not think this needs to pretend to be a more generic mechanism
to prevent soft lockups. The number of iterations highly depends on the
operation itself.

> 
> >  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.

This is a cold path used during OOM. While we can make it more optimal I
doubt it matters in practice so we should aim at readbility. I do not
mind either way, I just wanted to note that this is not performance
sensitive.

> 
> Perhaps a simple
> 
> 		/* Avoid potential softlockup warning */
> 		if ((++i & 1023) == 0)
> 
> at both sites will suffice.  Opinions might vary...
> 

-- 
Michal Hocko
SUSE Labs