Re: [PATCH v3] memcg: fix soft lockup in the OOM process

[Michal Hocko <mhocko@suse.com>]

On Mon 13-01-25 19:45:46, Andrew Morton wrote:
> On Mon, 13 Jan 2025 14:51:55 +0800 Chen Ridong <chenridong@huaweicloud.com> wrote:
> 
> > 
> > 
> > On 2025/1/6 16:45, Vlastimil Babka wrote:
> > > On 12/24/24 03:52, Chen Ridong wrote:
> > >> From: Chen Ridong <chenridong@huawei.com>
> > > 
> > > +CC RCU
> > > 
> > >> 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.
> > >>
> >
> > ...
> >
> > >> @@ -430,10 +431,15 @@ static void dump_tasks(struct oom_control *oc)
> > >>  		mem_cgroup_scan_tasks(oc->memcg, dump_task, oc);
> > >>  	else {
> > >>  		struct task_struct *p;
> > >> +		int i = 0;
> > >>  
> > >>  		rcu_read_lock();
> > >> -		for_each_process(p)
> > >> +		for_each_process(p) {
> > >> +			/* Avoid potential softlockup warning */
> > >> +			if ((++i & 1023) == 0)
> > >> +				touch_softlockup_watchdog();
> > > 
> > > This might suppress the soft lockup, but won't a rcu stall still be detected?
> > 
> > Yes, rcu stall was still detected.
> > For global OOM, system is likely to struggle, do we have to do some
> > works to suppress RCU detete?
> 
> rcu_cpu_stall_reset()?

Do we really care about those? The code to iterate over all processes
under RCU is there (basically) since ever and yet we do not seem to have
many reports of stalls? Chen's situation is specific to memcg OOM and
touching the global case was mostly for consistency reasons.
-- 
Michal Hocko
SUSE Labs