linux-kernel - Re: [RFC PATCH] mm/page_alloc.c: Micro-optimisation Remove unnecessary branch

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Date:   Sun, 8 Mar 2020 04:50:07 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     mateusznosek0@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm/page_alloc.c: Micro-optimisation Remove
 unnecessary branch

On Sat, Mar 07, 2020 at 03:15:42PM -0800, Andrew Morton wrote:
> On Sat,  7 Mar 2020 23:53:35 +0100 mateusznosek0@...il.com wrote:
> > -	if (unlikely(ac.nodemask != nodemask))
> > -		ac.nodemask = nodemask;
> > +	ac.nodemask = nodemask;
> 
> This will now unconditionally dirty the ac.nodemask cacheline, which
> means that cacheline will need to be written back.  If it is truly
> unlikely that the write was needed then the thinking goes that the
> test-and-branch is worthwhile, by saving on memory traffic.
> 
> At least, I assume that's why the code is the way it is.

The line immediately before this hunk is:

        ac.spread_dirty_pages = false;

ac is on-stack and is only 32 bytes.  I don't see a reason not to do this.

Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>