linux-kernel - Re: [patch V2 28/37] rseq: Switch to fast path processing on exit to user

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Message-ID: <87iki0zooo.ffs@tglx>
Date: Tue, 02 Sep 2025 20:36:55 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, LKML
 <linux-kernel@...r.kernel.org>
Cc: Jens Axboe <axboe@...nel.dk>, 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 V2 28/37] rseq: Switch to fast path processing on exit
 to user

On Wed, Aug 27 2025 at 09:45, Mathieu Desnoyers wrote:
> On 2025-08-26 11:40, Mathieu Desnoyers wrote:
>>>    RSEQ selftests      Before          After              Reduction
>>>
>>>    exit to user:       386281778          387373750
>>>    signal checks:       35661203                  0           100%
>>>    slowpath runs:      140542396 36.38%            100  0.00%    100%
>>>    fastpath runs:                         9509789  2.51%     N/A
>>>    id updates:         176203599 45.62%        9087994  2.35%     95%
>>>    cs checks:          175587856 45.46%        4728394  1.22%     98%
>>>      cs cleared:       172359544   98.16%    1319307   27.90%   99%
>>>      cs fixup:           3228312    1.84%    3409087   72.10%
>
> By the way, you should really not be using the entire rseq selftests
> as a representative workload for profiling the kernel rseq implementation.
>
> Those selftests include "loop injection", "yield injection", "kill
> injection" and "sleep injection" within the relevant userspace code
> paths, which really increase the likelihood of hitting stuff like
> "cs fixup" compared to anything that comes close to a realistic
> use-case. This is really useful for testing correctness, but not
> for profiling. For instance, the "loop injection" introduces busy
> loops within rseq critical sections to significantly increase the
> likelihood of hitting a cs fixup.
>
> Those specific selftests are really just "stress-tests" that don't
> represent any relevant workload.

True, they still tell how much useless work the kernel was doing, no?