linux-kernel - Re: [patch V6 07/11] rseq: Implement time slice extension enforcement timer

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Message-ID: <87ecorbccp.ffs@tglx>
Date: Fri, 19 Dec 2025 00:26:46 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, "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>
Subject: Re: [patch V6 07/11] rseq: Implement time slice extension
 enforcement timer

On Thu, Dec 18 2025 at 16:05, Peter Zijlstra wrote:
> On Mon, Dec 15, 2025 at 05:52:22PM +0100, Thomas Gleixner wrote:
>
>> V5: Document the slice extension range - PeterZ
>
>> --- a/Documentation/admin-guide/sysctl/kernel.rst
>> +++ b/Documentation/admin-guide/sysctl/kernel.rst
>> @@ -1228,6 +1228,14 @@ reboot-cmd (SPARC only)
>>  ROM/Flash boot loader. Maybe to tell it what to do after
>>  rebooting. ???
>>  
>> +rseq_slice_extension_nsec
>> +=========================
>> +
>> +A task can request to delay its scheduling if it is in a critical section
>> +via the prctl(PR_RSEQ_SLICE_EXTENSION_SET) mechanism. This sets the maximum
>> +allowed extension in nanoseconds before scheduling of the task is enforced.
>> +Default value is 30000ns (30us). The possible range is 10000ns (10us) to
>> +50000ns (50us).
>
> The important bit: we're not going to increase these numbers. If
> anything, I would like the default to be 10us and taint the kernel if
> you up it.

Fine with me.

> I also think we want some tracing/tool to find the actual length of the
> extension used (min/avg/max etc.). That is the time between the kernel
> finding the extension bit set and arming the timer and the slice_yield()
> syscall.

I could probably integrate that easily into the RSEQ stats mechanism.

Thanks,

        tglx