linux-kernel - Re: [v4 PATCH 2/2] mm: swap: use smp_mb__after

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Message-ID: <20200319140305.GC187654@cmpxchg.org>
Date:   Thu, 19 Mar 2020 10:03:05 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     shakeelb@...gle.com, vbabka@...e.cz, willy@...radead.org,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v4 PATCH 2/2] mm: swap: use smp_mb__after_atomic() to order LRU
 bit set

On Wed, Mar 18, 2020 at 11:02:21AM +0800, Yang Shi wrote:
> Memory barrier is needed after setting LRU bit, but smp_mb() is too
> strong.  Some architectures, i.e. x86, imply memory barrier with atomic
> operations, so replacing it with smp_mb__after_atomic() sounds better,
> which is nop on strong ordered machines, and full memory barriers on
> others.  With this change the vm-scalability cases would perform better
> on x86, I saw total 6% improvement with this patch and previous inline
> fix.
> 
> The test data (lru-file-readtwice throughput) against v5.6-rc4:
> 	mainline	w/ inline fix	w/ both (adding this)
> 	150MB		154MB		159MB
> 
> Fixes: 9c4e6b1a7027 ("mm, mlock, vmscan: no more skipping pagevecs")
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> Reviewed-and-Tested-by: Shakeel Butt <shakeelb@...gle.com>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>

Acked-by: Johannes Weiner <hannes@...xchg.org>