Re: [PATCH] mm,page_alloc: softlockup on warn_alloc on

[Johannes Weiner <hannes@cmpxchg.org>]

On Sat, Sep 16, 2017 at 12:23:53AM +0900, Tetsuo Handa wrote:
> Johannes Weiner wrote:
> > How can we figure out if there is a bug here? Can we time the calls to
> > __alloc_pages_direct_reclaim() and __alloc_pages_direct_compact() and
> > drill down from there? Print out the number of times we have retried?
> > We're counting no_progress_loops, but we are also very much interested
> > in progress_loops that didn't result in a successful allocation. Too
> > many of those and I think we want to OOM kill as per above.
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index bec5e96f3b88..01736596389a 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3830,6 +3830,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >  			"page allocation stalls for %ums, order:%u",
> >  			jiffies_to_msecs(jiffies-alloc_start), order);
> >  		stall_timeout += 10 * HZ;
> > +		goto oom;
> >  	}
> >  
> >  	/* Avoid recursion of direct reclaim */
> > @@ -3882,6 +3883,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >  	if (read_mems_allowed_retry(cpuset_mems_cookie))
> >  		goto retry_cpuset;
> >  
> > +oom:
> >  	/* Reclaim has failed us, start killing things */
> >  	page = __alloc_pages_may_oom(gfp_mask, order, ac, &did_some_progress);
> >  	if (page)
> > 
> 
> According to my stress tests, it is mutex_trylock() in __alloc_pages_may_oom()
> that causes warn_alloc() to be called for so many times. The comment
> 
> 	/*
> 	 * Acquire the oom lock.  If that fails, somebody else is
> 	 * making progress for us.
> 	 */
> 
> is true only if the owner of oom_lock can call out_of_memory() and is __GFP_FS
> allocation. Consider a situation where there are 1 GFP_KERNEL allocating thread
> and 99 GFP_NOFS/GFP_NOIO allocating threads contending the oom_lock. How likely
> the OOM killer is invoked? It is very unlikely because GFP_KERNEL allocating thread
> likely fails to grab oom_lock because GFP_NOFS/GFP_NOIO allocating threads is
> grabing oom_lock. And GFP_KERNEL allocating thread yields CPU time for
> GFP_NOFS/GFP_NOIO allocating threads to waste pointlessly.
> s/!mutex_trylock(&oom_lock)/mutex_lock_killable()/ significantly improves
> this situation for my stress tests. How is your case?

Interesting analysis, that definitely sounds plausible.

It just started happening to us in production and I haven't isolated
it yet. If you already have a reproducer, that's excellent.

The synchronization has worked this way for a long time (trylock
failure assuming progress, but the order/NOFS/zone bailouts from
actually OOM-killing inside the locked section). We should really fix
*that* rather than serializing warn_alloc().

For GFP_NOFS, it seems to go back to 9879de7373fc ("mm: page_alloc:
embed OOM killing naturally into allocation slowpath"). Before that we
didn't use to call __alloc_pages_may_oom() for NOFS allocations. So I
still wonder why this only now appears to be causing problems.

In any case, converting that trylock to a sleeping lock in this case
makes sense to me. Nobody is blocking under this lock (except that one
schedule_timeout_killable(1) after dispatching a victim) and it's not
obvious to me why we'd need that level of concurrency under OOM.

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