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

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Message-ID: <ZqJmZdZtwQ4+A5hj@chenyu5-mobl2>
Date: Thu, 25 Jul 2024 22:51:17 +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

On 2024-07-25 at 15:18:50 +0100, Hongyan Xia wrote:
> On 25/07/2024 15:00, Chen Yu wrote:
> > 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
> > 
> 
> Yeah, it might be a good idea to remove the link to avoid confusion, like
> you said HW pressure is not compiled in.
>

OK, will do and send a new version.
 
> Even if there is pressure support, before your patch the big 'delta' should
> only result in a HW pressure value that decays more than it should, and
> should not be able to block tasks like in that bug report, so it's very
> likely that it's unrelated.

Yes, for x86_64, the rq->avg_hw.load_avg is even 0 as it does not have a chance to
get accumulated.

thanks,
Chenyu