linux-kernel - Re: [PATCH] mm: Skip the reserved bootmem for compaction

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Message-ID: <5fmd2poyeas2pomho2io5zgmqd26vnxd77czmlhqn5tapy4fv4@5tdcqamzgyj5>
Date: Wed, 4 Sep 2024 12:13:56 +0100
From: Mel Gorman <mgorman@...hsingularity.net>
To: Rong Qianfeng <rongqianfeng@...o.com>
Cc: vbabka@...e.cz, Andrew Morton <akpm@...ux-foundation.org>, 
	Mike Rapoport <rppt@...nel.org>, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
	Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: [PATCH] mm: Skip the reserved bootmem for compaction

On Mon, Sep 02, 2024 at 08:24:43PM +0800, Rong Qianfeng wrote:
> Reserved pages are basically non-lru pages. This kind of memory can't be
> used as migration sources and targets, skip it can bring some performance
> benefits.
> 
> Because some drivers may also use PG_reserved, we just set PB_migrate_skip
> for those clustered reserved bootmem during memory initialization.
> 
> Signed-off-by: Rong Qianfeng <rongqianfeng@...o.com>

I'm not convinced the savings due to skipping a few pages during the scan
would justify the additional code. There would have to be a large number
of reserved pages scattered throughout the zone to make a difference and
even that situation would be a big surprise. I'm not even sure this can be
explicitly tested unless you artifically create reserved pages throughout the
zone, which would not be convincing, or know if a driver that exhibits such
behaviour in which case my first question is -- what is that driver doing?!?

-- 
Mel Gorman
SUSE Labs