Re: [PATCH v1] memcg: fix soft lockup in the OOM process

[Michal Hocko <mhocko@suse.com>]

On Wed 18-12-24 17:00:38, Chen Ridong wrote:
> 
> 
> On 2024/12/18 15:56, Michal Hocko wrote:
> > On Wed 18-12-24 15:44:34, Chen Ridong wrote:
> >>
> >>
> >> On 2024/12/17 20:54, Michal Hocko wrote:
> >>> On Tue 17-12-24 12:18:28, Chen Ridong wrote:
> >>> [...]
> >>>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> >>>> index 1c485beb0b93..14260381cccc 100644
> >>>> --- a/mm/oom_kill.c
> >>>> +++ b/mm/oom_kill.c
> >>>> @@ -390,6 +390,7 @@ static int dump_task(struct task_struct *p, void *arg)
> >>>>  	if (!is_memcg_oom(oc) && !oom_cpuset_eligible(p, oc))
> >>>>  		return 0;
> >>>>  
> >>>> +	cond_resched();
> >>>>  	task = find_lock_task_mm(p);
> >>>>  	if (!task) {
> >>>>  		/*
> >>>
> >>> This is called from RCU read lock for the global OOM killer path and I
> >>> do not think you can schedule there. I do not remember specifics of task
> >>> traversal for crgoup path but I guess that you might need to silence the
> >>> soft lockup detector instead or come up with a different iteration
> >>> scheme.
> >>
> >> Thank you, Michal.
> >>
> >> I made a mistake. I added cond_resched in the mem_cgroup_scan_tasks
> >> function below the fn, but after reconsideration, it may cause
> >> unnecessary scheduling for other callers of mem_cgroup_scan_tasks.
> >> Therefore, I moved it into the dump_task function. However, I missed the
> >> RCU lock from the global OOM.
> >>
> >> I think we can use touch_nmi_watchdog in place of cond_resched, which
> >> can silence the soft lockup detector. Do you think that is acceptable?
> > 
> > It is certainly a way to go. Not the best one at that though. Maybe we
> > need different solution for the global and for the memcg OOMs. During
> > the global OOM we rarely care about latency as the whole system is
> > likely to struggle. Memcg ooms are much more likely. Having that many
> > tasks in a memcg certainly requires a further partitioning so if
> > configured properly the OOM latency shouldn't be visible much. But I am
> > wondering whether the cgroup task iteration could use cond_resched while
> > the global one would touch_nmi_watchdog for every N iterations. I might
> > be missing something but I do not see any locking required outside of
> > css_task_iter_*.
> 
> Do you mean like that:

I've had something like this (untested) in mind
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7b3503d12aaf..37abc94abd2e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1167,10 +1167,14 @@ void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
 	for_each_mem_cgroup_tree(iter, memcg) {
 		struct css_task_iter it;
 		struct task_struct *task;
+		unsigned int i = 0
 
 		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 % 1000)
+				cond_resched();
+		}
 		css_task_iter_end(&it);
 		if (ret) {
 			mem_cgroup_iter_break(memcg, iter);
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 1c485beb0b93..3bf2304ed20c 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -430,10 +430,14 @@ static void dump_tasks(struct oom_control *oc)
 		mem_cgroup_scan_tasks(oc->memcg, dump_task, oc);
 	else {
 		struct task_struct *p;
+		unsigned int i = 0;
 
 		rcu_read_lock();
-		for_each_process(p)
+		for_each_process(p) {
+			if (++i % 1000)
+				touch_softlockup_watchdog();
 			dump_task(p, oc);
+		}
 		rcu_read_unlock();
 	}
 }
-- 
Michal Hocko
SUSE Labs