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

[Huang, Ying]

mawupeng <mawupeng1@xxxxxxxxxx> writes:

> On 2024/8/30 15:53, Huang, Ying wrote:
>> Hi, Wupeng,
>> 
>> Wupeng Ma <mawupeng1@xxxxxxxxxx> writes:
>> 
>>> From: Ma Wupeng <mawupeng1@xxxxxxxxxx>
>>>
>>> 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).
>
> Thank you for the detailed explanation regarding PCP auto-tuning. If the
> PCP pages are freed to the buddy after a certain period due to auto-tuning,
> it's possible that there is no direct association between PCP auto-tuning
> and the increase in the PCP count as indicated below, especially if no
> actual tasks have commenced after booting. The primary reason for the
> increase might be more orders and a surplus of CPUs.
>
>> 
>>> 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%
>
> We do the following experiments which make the pcp amount even bigger:
> 1. alloc 8G of memory in all of the 600+ cpus
> 2. kill all the above user tasks 
> 3. waiting for 36h
>
> the pcp amount 6161097(24644M) which 4.6% of the total 512G memory.
>
>
>>>
>>> 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@xxxxxxxxxx>
>>> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
>> 
>> 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@xxxxxxxxxxxxxx/
>
> As Michal Hocko mentioned in previous discussion:
>  1. If it is a real problem?
>  2. MemAvailable is documented as available without swapping, however
>     pcp need to drain reclaim.
>
> 1. 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.
> 2. MemAvailable here do seems wired. There is no reason to drain pcp to
> drop clean page cache As Michal Hocko already pointed in this post, drain
> clean page cache is much cheaper than drain remote pcp.Any idea on this?

Drain remote PCP may be not that expensive now after commit 4b23a68f9536
("mm/page_alloc: protect PCP lists with a spinlock").  No IPI is needed
to drain the remote PCP.

> [1] https://lore.kernel.org/linux-mm/ZWRYZmulV0B-Jv3k@tiehlicka/

--
Best Regards,
Huang, Ying