linux-kernel - Re: [PATCH v4 1/4] mm/pageblock: mitigation cmpxchg false sharing in pageblock flags

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Date:   Thu, 3 Sep 2020 10:19:34 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alex Shi <alex.shi@...ux.alibaba.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/4] mm/pageblock: mitigation cmpxchg false sharing in
 pageblock flags

On 03.09.20 09:01, 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 the false sharing in cmpxchg.
> 
> Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org

Could you please

1. Send a cover letter and indicate the changees between versions. I
cannot find any in my mailbox or on -mm - is there any? (also, is there
a patch 4 ?)

2. Report proper performance numbers as requested, especially, over
multiple runs. This should go into patch 1/2. Are they buried somewhere?

3. Clarify what patch 3 does: Do we actually waste 8*sizeof(long) where
we only need 4 bits?

Also, breaking stuff in patch 1 and fixing it in patch 3 is not
acceptable. This breaks git bisect. Skimming over the patches I think
this is the case.

I am not convinced yet that we need and want this. As Alex mentioned, we
touch pageblock flags while holding the zone lock already in most cases
-  and as Mel mentiones, updates should be rare.

-- 
Thanks,

David / dhildenb