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

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Message-ID: <20200817095821.GA29756@L-31X9LVDL-1304.local>
Date:   Mon, 17 Aug 2020 17:58:21 +0800
From:   Wei Yang <richard.weiyang@...ux.alibaba.com>
To:     Alex Shi <alex.shi@...ux.alibaba.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 1/2] mm/pageblock: mitigation cmpxchg false sharing in
 pageblock flags

On Sun, Aug 16, 2020 at 11:47:56AM +0800, 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.
>

If my understanding is correct, CPU reads data in the size of cacheline.
Define a variable a char or other, doesn't help on false sharing. 

Correct me, if not.

-- 
Wei Yang
Help you, Help me