linux-kernel - Re: Sleeping BUG in khugepaged for i586

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Message-ID: <20170610080941.GA12347@dhcp22.suse.cz>
Date:   Sat, 10 Jun 2017 10:09:42 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Larry Finger <Larry.Finger@...inger.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: Sleeping BUG in khugepaged for i586

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