linux-kernel - Re: [PATCH] sched/fair: Untangle NEXT_BUDDY and pick_next

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Message-ID: <20241129114504.GF35539@noisy.programming.kicks-ass.net>
Date: Fri, 29 Nov 2024 12:45:04 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Adam Li <adamli@...amperecomputing.com>
Cc: K Prateek Nayak <kprateek.nayak@....com>, 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,
	patches@...erecomputing.com, cl@...ux.com, christian.loehle@....com,
	vineethr@...ux.ibm.com
Subject: Re: [PATCH] sched/fair: Untangle NEXT_BUDDY and pick_next_task()

On Fri, Nov 29, 2024 at 06:37:06PM +0800, Adam Li wrote:
> On 11/29/2024 6:18 PM, Peter Zijlstra wrote:
> > On Fri, Nov 29, 2024 at 11:15:41AM +0100, Peter Zijlstra wrote:
> >> On Fri, Nov 29, 2024 at 10:55:00AM +0100, Peter Zijlstra wrote:
> >>
> >>> Anyway..  I'm sure I started a patch series cleaning up the whole next
> >>> buddy thing months ago (there's more problems here), but I can't seem to
> >>> find it in a hurry :/
> >>
> >> There was this..
> > 
> > And this I think.
> > 
> > Adam, what was the reason you were enabling NEXT_BUDDY in the first
> > place?
> > 
> Hi Peter,
> 
> I am tuning Specjbb critical-jOPS, which is latency sensitive.

There is a lot to latency, sometimes it's best to not preempt. I think
Prateek has found a fair number of workloads where SCHED_BATCH has been
helpful.

> NEXT_BUDDY affects schedule latency so I tried to enable NEXT_BUDDY.
> However Specjbb critical-jOPS drops with NEXT_BUDDY enabled (after my patch fixing panic).

Yes, picking outside of the EEVDF policy can make worse decisions for
latency.

The yield_to_task() can help performance for KVM (the only user AFAIK
-- oh DMA fences seem to also use it these days).

And the CGROUP_BUDDY thing can sometimes help when using cgroups.

But the wakeup thing is very situational -- it's disabled for a reason.
Unfortunately it seems to also have disabled the other users, which
wasn't intended.
	
> I will test your new NEXT_BUDDY patches.

We still need Prateek's fix. That ensures a delayed task will ever end
up being ->next.