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

["Huang, Ying" <ying.huang@intel.com>]

Hi, Wupeng,

Wupeng Ma <mawupeng1@huawei.com> writes:

> From: Ma Wupeng <mawupeng1@huawei.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@huawei.com>
> Signed-off-by: Liu Shixin <liushixin2@huawei.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