linux-kernel - Re: [RFC 3/4] mm, page_alloc: move cpuset seqcount checking to slowpath

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Message-ID: <20170118095549.GM7015@dhcp22.suse.cz>
Date:   Wed, 18 Jan 2017 10:55:50 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Ganapatrao Kulkarni <gpkulkarni@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC 3/4] mm, page_alloc: move cpuset seqcount checking to
 slowpath

On Wed 18-01-17 10:48:55, Vlastimil Babka wrote:
> On 01/18/2017 10:40 AM, Michal Hocko wrote:
> > On Tue 17-01-17 23:16:09, Vlastimil Babka wrote:
> > > This is a preparation for the following patch to make review simpler. While
> > > the primary motivation is a bug fix, this could also save some cycles in the
> > > fast path.
> > 
> > I cannot say I would be happy about this patch :/ The code is still very
> > confusing and subtle. I really think we should get rid of
> > synchronization with the concurrent cpuset/mempolicy updates instead.
> > Have you considered that instead?
> 
> Not so thoroughly yet, but I already suspect it would be intrusive for
> stable. We could make copies of nodemask and mems_allowed and protect just
> the copying with seqcount, but that would mean overhead and stack space.
> Also we might try revert 682a3385e773 ("mm, page_alloc: inline the fast path
> of the zonelist iterator") ...

If reverting that patch makes the problem go away and it is applicable
for the stable I would rather go that way for stable and take a deep
breath and rethink the whole cpuset and nodemask manipulation in the
allocation path for a better long term solution.

-- 
Michal Hocko
SUSE Labs