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

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Message-ID: <38a631cf-1f6d-4d68-887a-12c7c5808ebf@arm.com>
Date: Thu, 25 Jul 2024 14:16:30 +0100
From: Hongyan Xia <hongyan.xia2@....com>
To: Chen Yu <yu.c.chen@...el.com>, 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>
Cc: 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 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?

> 
> Fixes: 97450eb90965 ("sched/pelt: Remove shift of thermal clock")
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/oe-lkp/202407091527.bb0be229-lkp@intel.com
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> ---
>   kernel/sched/fair.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9057584ec06d..cfd4755954fd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9362,7 +9362,7 @@ static bool __update_blocked_others(struct rq *rq, bool *done)
>   
>   	decayed = update_rt_rq_load_avg(now, rq, curr_class == &rt_sched_class) |
>   		  update_dl_rq_load_avg(now, rq, curr_class == &dl_sched_class) |
> -		  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>