linux-kernel - Re: [PATCH] workqueue: warn if memory reclaim tries to flush !WQ_MEM

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Date:	Thu, 3 Dec 2015 17:04:06 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ulrich Obergfell <uobergfe@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH] workqueue: warn if memory reclaim tries to flush
 !WQ_MEM_RECLAIM workqueue

Hello, Peter.

On Thu, Dec 03, 2015 at 10:09:11PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 03, 2015 at 03:56:32PM -0500, Tejun Heo wrote:
> > So, if I'm not mistaken, those are all marking tasks which can be
> > depended upon during memory reclaim and we do want to catch them all.
> 
> Up to a point yes, these are things that want to be reliable during
> reclaim, but lacking memory reserves and usage bounds (which we
> discussed last at lsf/mm) these are just wanna-be.

Hmmm... even if buggy in that they can't guarantee forward-progress
even with access to the emergency pool, I think it makes sense to warn
them about creating an extra dependency which doesn't have access to
the emergency pool.

> > PF_MEMALLOC shouldn't depend on something which require memory to be
> > reclaimed to guarantee forward progress.
> 
> PF_MEMALLOC basically avoids reclaim for any memory allocation while its
> set.

So, the assumption is that they're already on the reclaim path and
thus shouldn't recurse into it again.

> The thing is, even if your workqueue has WQ_MEM_RECLAIM set, it will not
> hit the mayday button until you're completely full flat out of memory.

It's more trigger-happy than that.  It's timer based.  If new worker
can't be created for a certain amount of time for whatever reason,
it'll summon the rescuer.

> At which point you're probably boned anyway, because, as per the above,
> all that code assumes there's _some_ memory to be had.

Not really.  PF_MEMALLOC tasks have access to the emergency pool,
creating new workers doesn't, so this really is creating a dependency
which is qualitatively different.

> One solution is to always fail maybe_create_worker() when PF_MEMALLOC is
> set, thus always hitting the mayday button.

I'm not following.  When PF_MEMALLOC is set where?

Thanks.

-- 
tejun
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