linux-kernel - Re: [PATCH 1/2] mm,thp,shmem: limit shmem THP alloc gfp

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Message-ID: <20201112105258.GZ12240@dhcp22.suse.cz>
Date:   Thu, 12 Nov 2020 11:52:58 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Rik van Riel <riel@...riel.com>
Cc:     hughd@...gle.com, xuyu@...ux.alibaba.com,
        akpm@...ux-foundation.org, mgorman@...e.de, aarcange@...hat.com,
        willy@...radead.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, linux-mm@...ck.org, vbabka@...e.cz
Subject: Re: [PATCH 1/2] mm,thp,shmem: limit shmem THP alloc gfp_mask

On Thu 05-11-20 14:15:07, Rik van Riel wrote:
> The allocation flags of anonymous transparent huge pages can be controlled
> through the files in /sys/kernel/mm/transparent_hugepage/defrag, which can
> help the system from getting bogged down in the page reclaim and compaction
> code when many THPs are getting allocated simultaneously.
> 
> However, the gfp_mask for shmem THP allocations were not limited by those
> configuration settings, and some workloads ended up with all CPUs stuck
> on the LRU lock in the page reclaim code, trying to allocate dozens of
> THPs simultaneously.
> 
> This patch applies the same configurated limitation of THPs to shmem
> hugepage allocations, to prevent that from happening.

I believe you should also exaplain why we want to control defrag by the
global knob while the enable logic is per mount.

> This way a THP defrag setting of "never" or "defer+madvise" will result
> in quick allocation failures without direct reclaim when no 2MB free
> pages are available.
> 
> With this patch applied, THP allocations for tmpfs will be a little
> more aggressive than today for files mmapped with MADV_HUGEPAGE,
> and a little less aggressive for files that are not mmapped or
> mapped without that flag.

This begs some numbers. A little is rather bad unit of performance. I do
agree that unifying those makes sense in general though.

> Signed-off-by: Rik van Riel <riel@...riel.com>
> ---
>  include/linux/gfp.h | 2 ++
>  mm/huge_memory.c    | 6 +++---
>  mm/shmem.c          | 8 +++++---
>  3 files changed, 10 insertions(+), 6 deletions(-)
-- 
Michal Hocko
SUSE Labs