linux-kernel - Re: [PATCH 2/2] mm, page_alloc: use unlikely() in task

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Message-ID: <20200622085808.GP3183@techsingularity.net>
Date:   Mon, 22 Jun 2020 09:58:08 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Hugh Dickins <hughd@...gle.com>, akpm@...ux-foundation.org,
        alex.shi@...ux.alibaba.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, liwang@...hat.com
Subject: Re: [PATCH 2/2] mm, page_alloc: use unlikely() in task_capc()

On Wed, Jun 17, 2020 at 11:55:07AM +0200, Vlastimil Babka wrote:
> <SNIP>
>
> It makes no difference, at least on my gcc10 which seems to be smart enough to
> do the right thing. But yeah, your suggestion is more readable and precise and
> maybe can work better with a less smart compiler. Thanks.
> 
> ----8<----
> From 615eea6f6abe288ffb708aa0d1bdfbeaf30a4cbd Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@...e.cz>
> Date: Tue, 16 Jun 2020 10:14:47 +0200
> Subject: [PATCH] mm, page_alloc: use unlikely() in task_capc()
> 
> Hugh noted that task_capc() could use unlikely(), as most of the time there is
> no capture in progress and we are in page freeing hot path. Indeed adding
> unlikely() produces assembly that better matches the assumption and moves
> all the tests away from the hot path.
> 
> I have also noticed that we don't need to test for cc->direct_compaction as the
> only place we set current->task_capture is compact_zone_order() which also
> always sets cc->direct_compaction true.
> 
> Suggested-by: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> Acked-by: Hugh Dickins <hughd@...glecom>

Acked-by: Mel Gorman <mgorman@...hsingularity.net>

-- 
Mel Gorman
SUSE Labs