linux-kernel - Re: [PATCH 5/5] sched/fair: Rename set_next_buddy() to set_next

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Date: Tue, 9 Apr 2024 11:27:49 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 5/5] sched/fair: Rename set_next_buddy() to
 set_next_pick()

On Tue, Apr 09, 2024 at 10:32:59AM +0200, Ingo Molnar wrote:

> I don't think we want to nuke it - there's 3 users:
> 
>  - yield()
>  - CFS bandwidth
>  - wakeup
> 
> I think the yield() and CFS bandwidth ones are genuine, but non-working due 
> to NEXT_BUDDY at 0. Wakeup was the original intended NEXT_BUDDY logic, but 
> it got turned off due to some performance or latency considerations that 
> might or might not be valid & relevant today.
> 
> 2)
> 
> Even the task_hot() use of ->next isn't spurious: if a task has been marked 
> as run-next, then presumably the current task is descheduling and we should 
> probably not tear its ->next away in load-balancing.
> 
> 3)
> 
> Side note: a set rq->next should probably reduce a candidate runqueue's 
> weight both in periodic load-balancing and in idle-balancing, by rq->curr's 
> weight or so?
> 
> So what I think we should do is to keep ->next and fix all its intended 
> uses, and make it all unconditional by removing both NEXT_BUDDY and 
> CACHE_HOT_BUDDY. I can cook up a series if you agree in principle.

So yes on fixing those yield_to() and cfs_bandwidth thingies, but put
then under a new knob -- if the fix regresses we can simply flip it.

Ack on removing the current knobs, for them not having been changed in
forever.