linux-kernel - Re: [PATCH] mm, proc: collect percpu free pages into the free pages

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Message-ID: <87a5guh2fb.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Fri, 30 Aug 2024 15:53:28 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Wupeng Ma <mawupeng1@...wei.com>
Cc: <akpm@...ux-foundation.org>,  <mgorman@...hsingularity.net>,
  <mhocko@...e.com>,  <dmaluka@...omium.org>,  <liushixin2@...wei.com>,
  <wangkefeng.wang@...wei.com>,  <linux-mm@...ck.org>,
  <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, proc: collect percpu free pages into the free pages

Hi, Wupeng,

Wupeng Ma <mawupeng1@...wei.com> writes:

> From: Ma Wupeng <mawupeng1@...wei.com>
>
> The introduction of Per-CPU-Pageset (PCP) per zone aims to enhance the
> performance of the page allocator by enabling page allocation without
> requiring the zone lock. This kind of memory is free memory however is
> not included in Memfree or MemAvailable.
>
> With the support of higt-order pcp and pcp auto-tuning, the size of the
> pages in this list has become a matter of concern due to the following
> patches:
>
>   1. Introduction of Order 1~3 and PMD level PCP in commit 44042b449872
>   ("mm/page_alloc: allow high-order pages to be stored on the per-cpu
>   lists").
>   2. Introduction of PCP auto-tuning in commit 90b41691b988 ("mm: add
>   framework for PCP high auto-tuning").

With PCP auto-tuning, the idle pages in PCP will be freed to buddy after
some time (may be as long as tens seconds in some cases).

> Which lead to the total amount of the pcp can not be ignored just after
> booting without any real tasks for as the result show below:
>
> 		   w/o patch	  with patch	      diff	diff/total
> MemTotal:	525424652 kB	525424652 kB	      0 kB	        0%
> MemFree:	517030396 kB	520134136 kB	3103740 kB	      0.6%
> MemAvailable:	515837152 kB	518941080 kB	3103928 kB	      0.6%
>
> On a machine with 16 zones and 600+ CPUs, prior to these commits, the PCP
> list contained 274368 pages (1097M) immediately after booting. In the
> mainline, this number has increased to 3003M, marking a 173% increase.
>
> Since available memory is used by numerous services to determine memory
> pressure. A substantial PCP memory volume leads to an inaccurate estimation
> of available memory size, significantly impacting the service logic.
>
> Remove the useless CONFIG_HIGMEM in si_meminfo_node since it will always
> false in is_highmem_idx if config is not enabled.
>
> Signed-off-by: Ma Wupeng <mawupeng1@...wei.com>
> Signed-off-by: Liu Shixin <liushixin2@...wei.com>

This has been discussed before in the thread of the previous version,
better to refer to it and summarize it.

[1] https://lore.kernel.org/linux-mm/YwSGqtEICW5AlhWr@dhcp22.suse.cz/

--
Best Regards,
Huang, Ying