linux-kernel - Re: [PATCH] sched/pelt: Use rq_clock_task() for hw

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Message-ID: <ZqJafRf/fM/whI+6@chenyu5-mobl2>
Date: Thu, 25 Jul 2024 22:00:29 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Hongyan Xia <hongyan.xia2@....com>
CC: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann
	<dietmar.eggemann@....com>, Juri Lelli <juri.lelli@...hat.com>, Qais Yousef
	<qyousef@...alina.io>, Lukasz Luba <lukasz.luba@....com>,
	<linux-kernel@...r.kernel.org>, kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH] sched/pelt: Use rq_clock_task() for hw_pressure

Hi Hongyan,

On 2024-07-25 at 14:16:30 +0100, Hongyan Xia wrote:
> On 25/07/2024 12:42, Chen Yu wrote:
> > commit 97450eb90965 ("sched/pelt: Remove shift of thermal clock")
> > removed the decay_shift for hw_pressure. While looking at a related
> > bug report, it is found that this commit uses the sched_clock_task()
> > in sched_tick() while replaces the sched_clock_task() with rq_clock_pelt()
> > in __update_blocked_others(). This could bring inconsistence. One possible
> > scenario I can think of is in ___update_load_sum():
> > 
> > u64 delta = now - sa->last_update_time
> > 
> > 'now' could be calculated by rq_clock_pelt() from
> > __update_blocked_others(), and last_update_time was calculated by
> > rq_clock_task() previously from sched_tick(). Usually the former chases
> > after the latter, it cause a very large 'delta' and brings unexpected
> > behavior. Although this should not impact x86 platform in the bug report,
> > it should be fixed for other platforms.
> 
> I agree with this patch but I'm a bit confused here. May I know what you
> mean by 'should not impact x86 platform in the bug report'? But it closes a
> bug report on qemu x86_64, so it does have an impact?
>

It should not have any impact on x86_64. I added the bug link here because I checked
the code while looking at that report. But that report might be false positve,
or at least not caused by this logic introduced by this commit, because
CONFIG_SCHED_HW_PRESSURE was not even set in the kernel config[1]. Maybe I should
remove the 'reported-by' and 'closes' tags?

[1] https://download.01.org/0day-ci/archive/20240709/202407091527.bb0be229-lkp@intel.com/config-6.9.0-rc1-00051-g97450eb90965

> > -		  update_hw_load_avg(now, rq, hw_pressure) |
> > +		  update_hw_load_avg(rq_clock_task(rq), rq, hw_pressure) |
> >   		  update_irq_load_avg(rq, 0);
> >   	if (others_have_blocked(rq))
> 
> Reviewed-by: Hongyan Xia <hongyan.xia2@....com>

Thanks!

Best,
Chenyu