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SubjectRe: [patch] mm, oom: prevent soft lockup on memcg oom for UP systems
On Thu, 12 Mar 2020, Tetsuo Handa wrote:

> > On Thu, 12 Mar 2020, Tetsuo Handa wrote:
> > > > If you have an alternate patch to try, we can test it. But since this
> > > > cond_resched() is needed anyway, I'm not sure it will change the result.
> > >
> > > schedule_timeout_killable(1) is an alternate patch to try; I don't think
> > > that this cond_resched() is needed anyway.
> > >
> >
> > You are suggesting schedule_timeout_killable(1) in shrink_node_memcgs()?
> >
>
> Andrew Morton also mentioned whether cond_resched() in shrink_node_memcgs()
> is enough. But like you mentioned,
>

It passes our testing because this is where the allocator is looping while
the victim is trying to exit if only it could be scheduled.

> you can try re-adding sleep outside of oom_lock:
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d09776cd6e10..3aee7e0eca4e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1576,6 +1576,7 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
> */
> ret = should_force_charge() || out_of_memory(&oc);
> mutex_unlock(&oom_lock);
> + schedule_timeout_killable(1);
> return ret;
> }
>

If current was process chosen for oom kill, this would actually induce the
problem, not fix it.

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3c4eb750a199..e80158049651 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3797,7 +3797,6 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
> */
> if (!mutex_trylock(&oom_lock)) {
> *did_some_progress = 1;
> - schedule_timeout_uninterruptible(1);
> return NULL;
> }
>
> @@ -4590,6 +4589,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>
> /* Retry as long as the OOM killer is making progress */
> if (did_some_progress) {
> + schedule_timeout_uninterruptible(1);
> no_progress_loops = 0;
> goto retry;
> }
>
> By the way, will you share the reproducer (and how to use the reproducer) ?
>

On an UP kernel with swap disabled, you limit a memcg to 100MB and start
three processes that each fault 40MB attached to it. Same reproducer as
the "mm, oom: make a last minute check to prevent unnecessary memcg oom
kills" patch except in that case there are two cores.

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