linux-kernel - Re: [PATCH] mm, slub: Use prefetchw instead of prefetch

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Message-ID: <20211011073239.GA63896@kvm.asia-northeast3-a.c.our-ratio-313919.internal>
Date:   Mon, 11 Oct 2021 07:32:39 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Christoph Lameter <cl@...two.de>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm, slub: Use prefetchw instead of prefetch

On Mon, Oct 11, 2021 at 09:20:36AM +0200, Christoph Lameter wrote:
> On Fri, 8 Oct 2021, Hyeonggon Yoo wrote:
> 
> > It's certain that an object will be not only read, but also
> > written after allocation.
> 
> get_freepointer is used in multiple code path not only in allocation. It
> is for example used when scanning through a freelist.
> 
> With this change all objects get needlessly dirtied and the cross cpu
> cache contention is increased.

I didn't touch get_freepointer and there's
only one caller of prefetch_freepointer.

My change was not adding additional prefetch on get_freepointer,
but changing existing prefetch into prefetchw.

The prefetch was introcued by commit 0ad9500e16fe ("slub: prefetch next
freelist pointer in slab_alloc()") that you ACKed in 2011.

Do you think removeing existing prefetch is better than changing it
from prefetch to prefetchw?