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

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Date: Sun, 14 Jan 2024 23:37:28 +0000
From: Qais Yousef <qyousef@...alina.io>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Wyes Karny <wkarny@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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

On 01/14/24 19:58, Qais Yousef wrote:

> > This is not correct because you will have to wait to reach full
> > utilization at the current OPP possibly the lowest OPP before moving
> > directly to max OPP
> 
> Isn't this already the case? The ratio (util+headroom/max) will be less than
> 1 until util is 80% (with 25% headroom). And for all values <= 80% * max, we
> will request a frequency smaller than/equal policy->cur, no?
> 
> ie:
> 
> 	util = 600
> 	max = 1024
> 
> 	freq = 1.25 * 600 * policy->cur / 1024 = 0.73 * policy->cur
> 
> (util+headroom/max) must be greater than 1 for us to start going above
> policy->cur - which seems to have been working by accident IIUC.
> 
> So yes my proposal is incorrect, but it seems the conversion is not right to me
> now.
> 
> I could reproduce the problem now (thanks Wyes!). I have 3 freqs on my system
> 
> 2.2GHz, 2.8GHz and 3.8GHz
> 
> which (I believe) translates into capacities
> 
> ~592, ~754, 1024
> 
> which means we should pick 2.8GHz as soon as util * 1.25 > 592; which
> translates into util = ~473.
> 
> But what I see is that we go to 2.8GHz when we jump from 650 to 680 (see
> attached picture), which is what you'd expect since we apply two headrooms now,
> which means the ratio (util+headroom/max) will be greater than 1 after go above
> this value
> 
> 	1024 * 0.8 * 0.8 = ~655
> 
> So I think the math makes sense logically, but we're missing some other
> correction factor.
> 
> When I re-enable CPPC I see for the same test that we go into 3.8GHz straight
> away. My test is simple busyloop via
> 
> 	cat /dev/zero > /dev/null
> 
> I see the CPU util_avg is at 523 at fork. I expected us to run to 2.8GHz here
> to be honest, but I am not sure if util_cfs_boost() and util_est() are maybe
> causing us to be slightly above 523 and that's why we start with max freq.
> 
> Or I've done the math wrong :-) But the two don't behave the same for the same
> kernel with and without CPPC.

I think the relationship should be:

	freq = util * f_curr / cap_curr

(patch below)

with that I see (almost) the expected behavior (picture attached). We go to
2.8GHz when we are above 500. But the move to 3.8GHz is a bit earlier at 581
(instead of 754 * 0.8 = 603). Not sure why. With 25% headroom 581 is 726. So
it's a tad too early.


diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 95c3c097083e..155f96a44fa0 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -123,7 +123,8 @@ static void sugov_deferred_update(struct sugov_policy *sg_policy)
  * Return: the reference CPU frequency to compute a capacity.
  */
 static __always_inline
-unsigned long get_capacity_ref_freq(struct cpufreq_policy *policy)
+unsigned long get_capacity_ref_freq(struct cpufreq_policy *policy,
+                                   unsigned long *max)
 {
        unsigned int freq = arch_scale_freq_ref(policy->cpu);
 
@@ -133,6 +134,9 @@ unsigned long get_capacity_ref_freq(struct cpufreq_policy *policy)
        if (arch_scale_freq_invariant())
                return policy->cpuinfo.max_freq;
 
+       if (max)
+               *max = policy->cur * (*max) / policy->cpuinfo.max_freq;
+
        return policy->cur;
 }
 
@@ -164,7 +168,7 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
        struct cpufreq_policy *policy = sg_policy->policy;
        unsigned int freq;
 
-       freq = get_capacity_ref_freq(policy);
+       freq = get_capacity_ref_freq(policy, &max);
        freq = map_util_freq(util, freq, max);
 
        if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)

Download attachment "cppc_freq_fix2.png" of type "image/png" (30853 bytes)