linux-kernel - Re: [PATCH 15/28] mm, page_alloc: Move might_sleep

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Message-ID: <20160426145006.GD2858@techsingularity.net>
Date:	Tue, 26 Apr 2016 15:50:06 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 15/28] mm, page_alloc: Move might_sleep_if check to the
 allocator slowpath

On Tue, Apr 26, 2016 at 03:41:22PM +0200, Vlastimil Babka wrote:
> On 04/15/2016 11:07 AM, Mel Gorman wrote:
> >There is a debugging check for callers that specify __GFP_DIRECT_RECLAIM
> >from a context that cannot sleep. Triggering this is almost certainly
> >a bug but it's also overhead in the fast path.
> 
> For CONFIG_DEBUG_ATOMIC_SLEEP, enabling is asking for the overhead. But for
> CONFIG_PREEMPT_VOLUNTARY which turns it into _cond_resched(), I guess it's
> not.
> 

Either way, it struck me as odd. It does depend on the config and it's
marginal so if there is a problem then I can drop it.

> >Move the check to the slow
> >path. It'll be harder to trigger as it'll only be checked when watermarks
> >are depleted but it'll also only be checked in a path that can sleep.
> 
> Hmm what about zone_reclaim_mode=1, should the check be also duplicated to
> that part of get_page_from_freelist()?
> 

zone_reclaim has a !gfpflags_allow_blocking() check, does not call
cond_resched() before that check so it does not fall into an accidental
sleep path. I'm not seeing why the check is necessary there.

-- 
Mel Gorman
SUSE Labs