linux-kernel - Re: [PATCH v7] sched: Consolidate cpufreq updates

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Message-ID: <6e4a4605-f6c5-4948-ac38-c4ddf4990754@arm.com>
Date: Wed, 11 Sep 2024 21:34:19 +0100
From: Christian Loehle <christian.loehle@....com>
To: Qais Yousef <qyousef@...alina.io>, "Rafael J. Wysocki"
 <rafael@...nel.org>, Viresh Kumar <viresh.kumar@...aro.org>,
 Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Juri Lelli <juri.lelli@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>, Ben Segall
 <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
 Valentin Schneider <vschneid@...hat.com>, Hongyan Xia
 <hongyan.xia2@....com>, John Stultz <jstultz@...gle.com>,
 linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7] sched: Consolidate cpufreq updates

On 7/28/24 19:45, Qais Yousef wrote:
> Improve the interaction with cpufreq governors by making the
> cpufreq_update_util() calls more intentional.
> 
> At the moment we send them when load is updated for CFS, bandwidth for
> DL and at enqueue/dequeue for RT. But this can lead to too many updates
> sent in a short period of time and potentially be ignored at a critical
> moment due to the rate_limit_us in schedutil.
> 
> For example, simultaneous task enqueue on the CPU where 2nd task is
> bigger and requires higher freq. The trigger to cpufreq_update_util() by
> the first task will lead to dropping the 2nd request until tick. Or
> another CPU in the same policy triggers a freq update shortly after.
> 
> Updates at enqueue for RT are not strictly required. Though they do help
> to reduce the delay for switching the frequency and the potential
> observation of lower frequency during this delay. But current logic
> doesn't intentionally (at least to my understanding) try to speed up the
> request.
> 
> To help reduce the amount of cpufreq updates and make them more
> purposeful, consolidate them into these locations:
> 
> 1. context_switch()
> 2. task_tick_fair()
> 3. sched_balance_update_blocked_averages()
> 4. on sched_setscheduler() syscall that changes policy or uclamp values
> 5. on check_preempt_wakeup_fair() if wakeup preemption failed
> 6. on __add_running_bw() to guarantee DL bandwidth requirements.
> 

Actually now reading that code again reminded me, there is another
iowait boost change for intel_pstate.
intel_pstate has either intel_pstate_update_util() or
intel_pstate_update_util_hwp().
Both have
	if (smp_processor_id() != cpu->cpu)
		return;
Now since we move that update from enqueue to context_switch() that will
always be false.
I don't think that was deliberate but rather to simplify intel_pstate
synchronization, although !mcq device IO won't be boosted which you
could argue is good.
Just wanted to mention that, doesn't have to be a bad, but surely some
behavior change.