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

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Date:   Wed, 19 Aug 2020 10:04:51 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Matthew Wilcox <willy@...radead.org>
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 19.08.20 09:55, Anshuman Khandual wrote:
> 
> 
> 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 ?
> 

I asked for that in v1 and there are no performance numbers to justify
the change. IMHO, that will be required to consider this for inclusion,
otherwise it's just code churn resulting in an (although minimal)
additional memory consumption.

-- 
Thanks,

David / dhildenb