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

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Message-ID: <7b984dde-78c5-2efc-daef-bcdcc51fc9cb@suse.cz>
Date:   Wed, 18 Jan 2017 10:48:55 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
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 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") ...