linux-kernel - Re: [PATCH] sched/fair: Dequeue sched_delayed tasks when waking to a busy CPU

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Message-ID: <20241112142302.GI6497@noisy.programming.kicks-ass.net>
Date: Tue, 12 Nov 2024 15:23:02 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Phil Auld <pauld@...hat.com>
Cc: Mike Galbraith <efault@....de>, mingo@...hat.com, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, linux-kernel@...r.kernel.org,
	kprateek.nayak@....com, wuyun.abel@...edance.com,
	youssefesmat@...omium.org, tglx@...utronix.de
Subject: Re: [PATCH] sched/fair: Dequeue sched_delayed tasks when waking to a
 busy CPU

On Tue, Nov 12, 2024 at 07:41:17AM -0500, Phil Auld wrote:
> On Tue, Nov 12, 2024 at 08:05:04AM +0100 Mike Galbraith wrote:
> > On Fri, 2024-11-08 at 01:24 +0100, Mike Galbraith wrote:
> > > On Thu, 2024-11-07 at 15:09 +0100, Peter Zijlstra wrote:
> > > > On Thu, Nov 07, 2024 at 03:02:36PM +0100, Mike Galbraith wrote:
> > > > > On Thu, 2024-11-07 at 10:46 +0100, Mike Galbraith wrote:
> > > > > > On Thu, 2024-11-07 at 05:03 +0100, Mike Galbraith wrote:
> > > > > > > 
> > > > > > > I built that patch out of curiosity, and yeah, set_next_task_fair()
> > > > > > > finding a cfs_rq->curr ends play time pretty quickly.
> > > > > > 
> > > > > > The below improved uptime, and trace_printk() says it's doing the
> > > > > > intended, so I suppose I'll add a feature and see what falls out.
> > > > > 
> > > > > From netperf, I got.. number tabulation practice.  Three runs of each
> > > > > test with and without produced nothing but variance/noise.
> > > > 
> > > > Make it go away then.
> > > > 
> > > > If you could write a Changelog for you inspired bit and stick my cleaned
> > > > up version under it, I'd be much obliged.
> > > 
> > > Salut, much obliged for eyeball relief.
> > 
> > Unfortunate change log place holder below aside, I think this patch may
> > need to be yanked as trading one not readily repeatable regression for
> > at least one that definitely is, and likely multiple others.
> > 
> > (adds knob)
> >
> 
> Yes, I ws just coming here to reply. I have the results from the first
> version of the patch (I don't think the later one fundemtally changed
> enough that it will matter but those results are still pending).
> 
> Not entirely surprisingly we've traded a ~10% rand write regression for
> 5-10% rand read regression. This makes sense to me since the reads are
> more likely to be synchronous and thus be more buddy-like and benefit
> from flipping back and forth on the same cpu.  

OK, so I'm going to make this commit disappear.