linux-kernel - Re: [patch V4 00/36] rseq: Optimize exit to user space

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Message-ID: <87bjnitl2j.ffs@tglx>
Date: Wed, 10 Sep 2025 16:54:12 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Jens Axboe <axboe@...nel.dk>, LKML <linux-kernel@...r.kernel.org>
Cc: Michael Jeanson <mjeanson@...icios.com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng
 <boqun.feng@...il.com>, Paolo Bonzini <pbonzini@...hat.com>, Sean
 Christopherson <seanjc@...gle.com>, Wei Liu <wei.liu@...nel.org>, Dexuan
 Cui <decui@...rosoft.com>, x86@...nel.org, Arnd Bergmann <arnd@...db.de>,
 Heiko Carstens <hca@...ux.ibm.com>, Christian Borntraeger
 <borntraeger@...ux.ibm.com>, Sven Schnelle <svens@...ux.ibm.com>, Huacai
 Chen <chenhuacai@...nel.org>, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>
Subject: Re: [patch V4 00/36] rseq: Optimize exit to user space

On Wed, Sep 10 2025 at 07:55, Jens Axboe wrote:
> On 9/8/25 3:31 PM, Thomas Gleixner wrote:
>> For your convenience all of it is also available as a conglomerate from
>> git:
>> 
>>     git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git rseq/perf
>
> I used this branch for some quick testing. Since I last looked at the
> rseq performance overhead, glibc must have improved a few things. FWIW,
> box is running libc 2.41 at the moment. Test box is on debian unstable,
> so gets frequent updates. In any case, for one of my usual kernel
> overhead runs of checking running a basic IOPS based test, I see the
> following on the stock (-rc5 + 6.18 targeted changes) kernel running
> that test:
>
> +    0.89%  io_uring  [kernel.kallsyms]  [k] __get_user_8
> +    0.58%  io_uring  [kernel.kallsyms]  [k] __put_user_8
> +    1.13%  io_uring  [kernel.kallsyms]  [k] __rseq_handle_notify_resume
>
> which is about 2.6% of purely rseq related overhead. Pulling in the
> above branch and running the exact same test, all of the above are gone
> and perusing the profile has nothing jump out at me in terms of shifting
> those cycles to other bookkeeping.
>
> So yes, this work does make a very noticeable difference!

I would have been really surprised if not :)

Thanks a lot for testing!

       tglx