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

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Message-ID: <95ff75cacab4720bbbecd54e881bb94d97087b45.camel@gmx.de>
Date: Thu, 14 Nov 2024 12:07:03 +0100
From: Mike Galbraith <efault@....de>
To: Phil Auld <pauld@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>, 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, 2024-11-12 at 17:15 +0100, Mike Galbraith wrote:
> On Tue, 2024-11-12 at 10:41 -0500, Phil Auld wrote:
> > On Tue, Nov 12, 2024 at 03:23:38PM +0100 Mike Galbraith wrote:
> >
> > >
> > > We don't however have to let sched_delayed block SIS though.  Rendering
> > > them transparent in idle_cpu() did NOT wreck the progression, so
> > > maaaybe could help your regression.
> > >
> >
> > You mean something like:
> >
> > if (rq->nr_running > rq->h_nr_delayed)
> >        return 0;
> >
> > in idle_cpu() instead of the straight rq->nr_running check?
>
> Yeah, close enough.

The below is all you need.

Watching blockage rate during part of a netperf scaling run without, a
bit over 2/sec was the highest it got, but with, that drops to the same
zero as turning off the feature, so... relevance highly unlikely but
not quite impossible?

---
 kernel/sched/fair.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9454,11 +9454,15 @@ int can_migrate_task(struct task_struct

 	/*
 	 * We do not migrate tasks that are:
+	 * 0) not runnable (not useful here/now, but are annoying), or
 	 * 1) throttled_lb_pair, or
 	 * 2) cannot be migrated to this CPU due to cpus_ptr, or
 	 * 3) running (obviously), or
 	 * 4) are cache-hot on their current CPU.
 	 */
+	if (p->se.sched_delayed)
+		return 0;
+
 	if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
 		return 0;