linux-kernel - Re: [GIT PULL] Scheduler changes for v6.8

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Date: Thu, 11 Jan 2024 09:11:49 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Juri Lelli <juri.lelli@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>
Subject: Re: [GIT PULL] Scheduler changes for v6.8

Le mercredi 10 janv. 2024 à 14:57:14 (-0800), Linus Torvalds a écrit :
> On Wed, 10 Jan 2024 at 14:41, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > It's one of these two:
> >
> >   f12560779f9d sched/cpufreq: Rework iowait boost
> >   9c0b4bb7f630 sched/cpufreq: Rework schedutil governor performance estimation
> >
> > one more boot to go, then I'll try to revert whichever causes my
> > machine to perform horribly much worse.
> 
> I guess it should come as no surprise that the result is
> 
>    9c0b4bb7f6303c9c4e2e34984c46f5a86478f84d is the first bad commit
> 
> but to revert cleanly I will have to revert all of
> 
>       b3edde44e5d4 ("cpufreq/schedutil: Use a fixed reference frequency")
>       f12560779f9d ("sched/cpufreq: Rework iowait boost")
>       9c0b4bb7f630 ("sched/cpufreq: Rework schedutil governor
> performance estimation")
> 
> This is on a 32-core (64-thread) AMD Ryzen Threadripper 3970X, fwiw.

Could you confirm that cpufreq governor is schedutil and the driver is
amd-pstate on your system ?

Also I'm interested by the output of the amd_pstate to confirm that it uses the
adjust_perf callback

I suppose that you don't use uclamp feature and amd doesn't use EAS so that let
the change of the min parameter of adjust_perf which was probably always 0
unless you use deadline scheduler and which now takes into account irq pressure. 

Could you try the patch below which restores the previous min value ?

---
 kernel/sched/cpufreq_schedutil.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 95c3c097083e..3fe8ac6ce9cc 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -194,10 +194,11 @@ unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual,
 static void sugov_get_util(struct sugov_cpu *sg_cpu, unsigned long boost)
 {
 	unsigned long min, max, util = cpu_util_cfs_boost(sg_cpu->cpu);
+	struct rq *rq = cpu_rq(sg_cpu->cpu);

 	util = effective_cpu_util(sg_cpu->cpu, util, &min, &max);
 	util = max(util, boost);
-	sg_cpu->bw_min = min;
+	sg_cpu->bw_min = cpu_bw_dl(rq);
 	sg_cpu->util = sugov_effective_cpu_perf(sg_cpu->cpu, util, min, max);
 }

@@ -442,7 +443,7 @@ static void sugov_update_single_perf(struct update_util_data *hook, u64 time,
 	    sugov_cpu_is_busy(sg_cpu) && sg_cpu->util < prev_util)
 		sg_cpu->util = prev_util;

-	cpufreq_driver_adjust_perf(sg_cpu->cpu, sg_cpu->bw_min,
+	cpufreq_driver_adjust_perf(sg_cpu->cpu, map_util_perf(sg_cpu->bw_min),
 				   sg_cpu->util, max_cap);

 	sg_cpu->sg_policy->last_freq_update_time = time;
--
2.34.1


> 
> I'll keep that revert in my private test-tree for now (so that I have
> a working machine again), but I'll move it to my main branch soon
> unless somebody has a quick fix for this problem.
> 
>                 Linus