linux-kernel - Re: [RFC][PATCH 1/2] sched: Extended scheduler time slice

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Message-ID: <20250204111119.10ee37c8@gandalf.local.home>
Date: Tue, 4 Feb 2025 11:11:19 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Thomas
 Gleixner <tglx@...utronix.de>, Ankur Arora <ankur.a.arora@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-mm@...ck.org,
 x86@...nel.org, akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
 dave.hansen@...ux.intel.com, hpa@...or.com, juri.lelli@...hat.com,
 vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
 jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
 boris.ostrovsky@...cle.com, konrad.wilk@...cle.com, jgross@...e.com,
 andrew.cooper3@...rix.com, Joel Fernandes <joel@...lfernandes.org>, Vineeth
 Pillai <vineethrp@...gle.com>, Suleiman Souhlal <suleiman@...gle.com>, Ingo
 Molnar <mingo@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Clark Williams
 <clark.williams@...il.com>, bigeasy@...utronix.de, daniel.wagner@...e.com,
 joseph.salisbury@...cle.com, broonie@...il.com
Subject: Re: [RFC][PATCH 1/2] sched: Extended scheduler time slice

On Tue, 4 Feb 2025 16:30:53 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> If you go back and reread that initial thread, you'll find the 50us is
> below the scheduling latency that random test box already had.
> 
> I'm sure more modern systems will have a lower number, and slower
> systems will have a larger number, but we got to pick a number :/
> 
> I'm fine with making it 20us. Or whatever. Its just a stupid number.
> 
> But yes. If we're going to be doing this, there is absolutely no reason
> not to allow DEADLINE/FIFO threads the same. Misbehaving FIFO is already
> a problem, and we can make DL-CBS enforcement punch through it if we
> have to.
> 
> And less retries on the RSEQ for FIFO can equally improve performance.
> 
> There is no difference between a 'malicious/broken' userspace consuming
> the entire window in userspace (50us, 20us whatever it will be) and
> doing a system call which we know will cause similar delays because it
> does in-kernel locking.

This is where we will disagree for the reasons I explained in my second
email. This feature affects other tasks. And no, making it 20us doesn't
make it better. Because from what I get from you, if we implement this, it
will be available for all preemption methods (including PREEMPT_RT), where
we do have less than 50us latency, and and even a 20us will break those
applications.

This was supposed to be only a hint to the kernel, not a complete feature
that is hard coded and will override how other tasks behave. As system
calls themselves can make how things are scheduled depending on the
preemption method, I didn't want to add something that will change how
things are scheduled that ignores the preemption method that was chosen.

-- Steve