linux-kernel - Re: [PATCH 4/8] io_uring/io-wq: cache work->flags in variable

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAKPOu+-Mfx9q79nin7tGi1Rr4qGGY=y-2OhuP80U=7EtRpfBdg@mail.gmail.com>
Date: Wed, 29 Jan 2025 20:11:17 +0100
From: Max Kellermann <max.kellermann@...os.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: axboe@...nel.dk, io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] io_uring/io-wq: cache work->flags in variable

On Wed, Jan 29, 2025 at 7:56 PM Pavel Begunkov <asml.silence@...il.com> wrote:
> What architecture are you running? I don't get why the reads
> are expensive while it's relaxed and there shouldn't even be
> any contention. It doesn't even need to be atomics, we still
> should be able to convert int back to plain ints.

I measured on an AMD Epyc 9654P.
As you see in my numbers, around 40% of the CPU time was wasted on
spinlock contention. Dozens of io-wq threads are trampling on each
other's feet all the time.
I don't think this is about memory accesses being exceptionally
expensive; it's just about wringing every cycle from the code section
that's under the heavy-contention spinlock.