Date: Mon, 12 Jun 2017 08:29:18 +0200
From: Michal Hocko <mhocko@kernel.org>
To: David Rientjes <rientjes@google.com>
Cc: Matthew Wilcox <willy@infradead.org>, Vlastimil Babka <vbabka@suse.cz>,
        Larry Finger <Larry.Finger@lwfinger.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org
Subject: Re: Sleeping BUG in khugepaged for i586
Message-ID: <20170612062918.GA4145@dhcp22.suse.cz>
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 <20170608170557.GA8118@bombadil.infradead.org>
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On Sun 11-06-17 16:28:11, David Rientjes wrote:
> On Sat, 10 Jun 2017, Michal Hocko wrote:
> 
> > > > I would just pull the cond_resched out of __collapse_huge_page_copy
> > > > right after pte_unmap. But I am not really sure why this cond_resched is
> > > > really needed because the changelog of the patch which adds is is quite
> > > > terse on details.
> > > 
> > > I'm not sure what could possibly be added to the changelog.  We have 
> > > encountered need_resched warnings during the iteration.
> > 
> > Well, the part the changelog is not really clear about is whether the
> > HPAGE_PMD_NR loops itself is the source of the stall. This would be
> > quite surprising because doing 512 iterations taking up to 20+s sounds
> > way to much.
> 
> I have no idea where you come up with 20+ seconds.

OK, I misread your report as a soft lockup.

> These are not soft lockups, these are need_resched warnings.  We monitor 
> how long need_resched has been set and when a thread takes an excessive 
> amount of time to reschedule after it has been set.  A loop of 512 pages 
> with ptl contention and doing {clear,copy}_user_highpage() shows that 
> need_resched can sit without scheduling for an excessive amount of time.

How much is excessive here?
-- 
Michal Hocko
SUSE Labs