linux-kernel - Re: [PATCH v2 1/2] mm/pageblock: mitigation cmpxchg false sharing in pageblock flags

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Message-ID: <8ec2a4b0-9e51-abf9-fa7a-29989d3f1fac@arm.com>
Date:   Wed, 19 Aug 2020 13:25:38 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Alex Shi <alex.shi@...ux.alibaba.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/pageblock: mitigation cmpxchg false sharing in
 pageblock flags



On 08/19/2020 11:17 AM, Alex Shi wrote:
> pageblock_flags is used as long, since every pageblock_flags is just 4
> bits, 'long' size will include 8(32bit machine) or 16 pageblocks' flags,
> that flag setting has to sync in cmpxchg with 7 or 15 other pageblock
> flags. It would cause long waiting for sync.
> 
> If we could change the pageblock_flags variable as char, we could use
> char size cmpxchg, which just sync up with 2 pageblock flags. it could
> relief much false sharing in cmpxchg.

Do you have numbers demonstrating claimed performance improvement
after this change ?