linux-kernel - Re: [RFC PATCH] mm: Drain PCP during direct reclaim

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Message-ID: <ba26fba0-f623-4857-8a2c-6c7d0967287e@amd.com>
Date: Wed, 11 Jun 2025 13:25:55 +0530
From: Raghavendra K T <raghavendra.kt@....com>
To: Wupeng Ma <mawupeng1@...wei.com>, akpm@...ux-foundation.org,
 vbabka@...e.cz
Cc: surenb@...gle.com, jackmanb@...gle.com, hannes@...xchg.org,
 ziy@...dia.com, wangkefeng.wang@...wei.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Nikhil Dhama <nikhil.dhama@....com>,
 "Huang, Ying" <ying.huang@...ux.alibaba.com>,
 "Rao, Bharata Bhasker" <bharata@....com>
Subject: Re: [RFC PATCH] mm: Drain PCP during direct reclaim

++
On 6/6/2025 12:29 PM, Wupeng Ma wrote:
> Memory retained in Per-CPU Pages (PCP) caches can prevent hugepage
> allocations from succeeding despite sufficient free system memory. This
> occurs because:
> 1. Hugepage allocations don't actively trigger PCP draining
> 2. Direct reclaim path fails to trigger drain_all_pages() when:
>     a) All zone pages are free/hugetlb (!did_some_progress)
>     b) Compaction skips due to costly order watermarks (COMPACT_SKIPPED)
> 
> Reproduction:
>    - Alloc page and free the page via put_page to release to pcp
>    - Observe hugepage reservation failure
> 
> Solution:
>    Actively drain PCP during direct reclaim for memory allocations.
>    This increases page allocation success rate by making stranded pages
>    available to any order allocations.
> 
> Verification:
>    This issue can be reproduce easily in zone movable with the following
>    step:
> 
> w/o this patch
>    # numactl -m 2 dd if=/dev/urandom of=/dev/shm/testfile bs=4k count=64
>    # rm -f /dev/shm/testfile
>    # sync
>    # echo 3 > /proc/sys/vm/drop_caches
>    # echo 2048 > /sys/devices/system/node/node2/hugepages/hugepages-2048kB/nr_hugepages
>    # cat /sys/devices/system/node/node2/hugepages/hugepages-2048kB/nr_hugepages
>      2029
> 
> w/ this patch
>    # numactl -m 2 dd if=/dev/urandom of=/dev/shm/testfile bs=4k count=64
>    # rm -f /dev/shm/testfile
>    # sync
>    # echo 3 > /proc/sys/vm/drop_caches
>    # echo 2048 > /sys/devices/system/node/node2/hugepages/hugepages-2048kB/nr_hugepages
>    # cat /sys/devices/system/node/node2/hugepages/hugepages-2048kB/nr_hugepages
>      2047
> 

Hello Wupeng Ma,

Can you also post results of iperf/netperf for this patch in future?

Thanks and Regards
- Raghu