Date: Sat, 10 Jun 2017 10:09:42 +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: <20170610080941.GA12347@dhcp22.suse.cz>
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On Fri 09-06-17 15:38:44, David Rientjes wrote:
> On Thu, 8 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. So is it possible that we are missing a cond_resched
somewhere up the __collapse_huge_page_copy call path? Or do we really do
something stupidly expensive here?

> We fix these 
> because need_resched warnings suppress future warnings of the same type 
> for issues that are more important.

Sure thing. I do care about soft lockups as well.

> I can fix the i386 issue but removing the cond_resched() entirely isn't 
> really suitable.

I am not calling for a complete removal. I just do not yet see what is
the source of the long processing of the the loop.
-- 
Michal Hocko
SUSE Labs