linux-kernel - Re: [patch V6 01/11] rseq: Add fields and constants for time slice extension

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Message-ID: <lhu1pjlhobj.fsf@oldenburg.str.redhat.com>
Date: Mon, 19 Jan 2026 11:46:56 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,  Thomas Gleixner
 <tglx@...nel.org>,  LKML <linux-kernel@...r.kernel.org>,  "Paul E.
 McKenney" <paulmck@...nel.org>,  Boqun Feng <boqun.feng@...il.com>,
  Jonathan Corbet <corbet@....net>,  Prakash Sangappa
 <prakash.sangappa@...cle.com>,  Madadi Vineeth Reddy
 <vineethr@...ux.ibm.com>,  K Prateek Nayak <kprateek.nayak@....com>,
  Steven Rostedt <rostedt@...dmis.org>,  Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>,  Arnd Bergmann <arnd@...db.de>,
  linux-arch@...r.kernel.org,  Randy Dunlap <rdunlap@...radead.org>,  Ron
 Geva <rongevarg@...il.com>,  Waiman Long <longman@...hat.com>,
  "carlos@...hat.com" <carlos@...hat.com>,  Michael Jeanson
 <mjeanson@...icios.com>
Subject: Re: [patch V6 01/11] rseq: Add fields and constants for time slice
 extension

* Peter Zijlstra:

> On Sat, Jan 17, 2026 at 05:16:16PM +0100, Mathieu Desnoyers wrote:
>
>> My main concern is about the overhead of added system calls at thread
>> creation. I recall that doing an additional rseq system call at thread
>> creation was analyzed thoroughly for performance regressions at the
>> libc level. I would not want to start requiring libc to issue a
>> handful of additional prctl system calls per thread creation for no good
>> reason.
>
> A wee something like so?
>
> That would allow registering rseq with RSEQ_FLAG_SLICE_EXT_DEFAULT_ON
> set and if all the stars align, it will then have it on at the end.

I think this would work for glibc because it will only show up in
__rseq_flags if we set the flag on process startup, and then all threads
would get it.  It doesn't matter that __rseq_flags is not per-thread.

Thanks,
Florian