linux-kernel - Re: [PATCH] mm: batch unlink_file_vma calls in free_pgd

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Date: Sun, 19 May 2024 00:31:20 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: akpm@...ux-foundation.org, Liam.Howlett@...cle.com, vbabka@...e.cz,
	lstoakes@...il.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm: batch unlink_file_vma calls in free_pgd_range

On Sat, May 18, 2024 at 08:20:05AM +0200, Mateusz Guzik wrote:
> Execs of dynamically linked binaries at 20-ish cores are bottlenecked on
> the i_mmap_rwsem semaphore, while the biggest singular contributor is
> free_pgd_range inducing the lock acquire back-to-back for all
> consecutive mappings of a given file.
> 
> Tracing the count of said acquires while building the kernel shows:
> [1, 2)     799579 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [2, 3)          0 |                                                    |
> [3, 4)       3009 |                                                    |
> [4, 5)       3009 |                                                    |
> [5, 6)     326442 |@@@@@@@@@@@@@@@@@@@@@                               |

This makes sense.  A snippet of /proc/self/maps:

7f0a44725000-7f0a4474b000 r--p 00000000 fe:01 100663437                  /usr/lib/x86_64-linux-gnu/libc.so.6
7f0a4474b000-7f0a448a0000 r-xp 00026000 fe:01 100663437                  /usr/lib/x86_64-linux-gnu/libc.so.6
7f0a448a0000-7f0a448f4000 r--p 0017b000 fe:01 100663437                  /usr/lib/x86_64-linux-gnu/libc.so.6
7f0a448f4000-7f0a448f8000 r--p 001cf000 fe:01 100663437                  /usr/lib/x86_64-linux-gnu/libc.so.6
7f0a448f8000-7f0a448fa000 rw-p 001d3000 fe:01 100663437                  /usr/lib/x86_64-linux-gnu/libc.so.6

so we frequently have the same file mmaped five times in a row.

> The lock remains the main bottleneck, I have not looked at other spots
> yet.

You're not the first to report high contention on this lock.
https://lore.kernel.org/all/20240202093407.12536-1-JonasZhou-oc@zhaoxin.com/
for example.

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b6bdaa18b9e9..443d0c55df80 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h

I do object to this going into mm.h.  mm/internal.h would be better.

I haven't reviewed the patch in depth, but I don't have a problem with
the idea.  I think it's only a stopgap and we really do need a better
data structure than this.