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Message-ID: <0f3cfff3-0df4-3cb7-95cb-ea378517e13b@efficios.com>
Date: Sat, 30 Sep 2023 07:45:38 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Chen Yu <yu.c.chen@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Juri Lelli <juri.lelli@...hat.com>,
Swapnil Sapkal <Swapnil.Sapkal@....com>,
Aaron Lu <aaron.lu@...el.com>, Tim Chen <tim.c.chen@...el.com>,
K Prateek Nayak <kprateek.nayak@....com>,
"Gautham R . Shenoy" <gautham.shenoy@....com>, x86@...nel.org
Subject: Re: [RFC PATCH] sched/fair: Bias runqueue selection towards almost
idle prev CPU
On 9/30/23 03:11, Chen Yu wrote:
> Hi Mathieu,
>
> On 2023-09-29 at 14:33:50 -0400, Mathieu Desnoyers wrote:
>> Introduce the WAKEUP_BIAS_PREV_IDLE scheduler feature. It biases
>> select_task_rq towards the previous CPU if it was almost idle
>> (avg_load <= 0.1%).
>
> Yes, this is a promising direction IMO. One question is that,
> can cfs_rq->avg.load_avg be used for percentage comparison?
> If I understand correctly, load_avg reflects that more than
> 1 tasks could have been running this runqueue, and the
> load_avg is the direct proportion to the load_weight of that
> cfs_rq. Besides, LOAD_AVG_MAX seems to not be the max value
> that load_avg can reach, it is the sum of
> 1024 * (y + y^1 + y^2 ... )
>
> For example,
> taskset -c 1 nice -n -20 stress -c 1
> cat /sys/kernel/debug/sched/debug | grep 'cfs_rq\[1\]' -A 12 | grep "\.load_avg"
> .load_avg : 88763
> .load_avg : 1024
>
> 88763 is higher than LOAD_AVG_MAX=47742
I would have expected the load_avg to be limited to LOAD_AVG_MAX
somehow, but it appears that it does not happen in practice.
That being said, if the cutoff is really at 0.1% or 0.2% of the real
max, does it really matter ?
> Maybe the util_avg can be used for precentage comparison I suppose?
[...]
> Or
> return cpu_util_without(cpu_rq(cpu), p) * 1000 <= capacity_orig_of(cpu) ?
Unfortunately using util_avg does not seem to work based on my testing.
Even at utilization thresholds at 0.1%, 1% and 10%.
Based on comments in fair.c:
* CPU utilization is the sum of running time of runnable tasks plus the
* recent utilization of currently non-runnable tasks on that CPU.
I think we don't want to include currently non-runnable tasks in the
statistics we use, because we are trying to figure out if the cpu is a
idle-enough target based on the tasks which are currently running, for
the purpose of runqueue selection when waking up a task which is
considered at that point in time a non-runnable task on that cpu, and
which is about to become runnable again.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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