linux-kernel - Re: [PATCH v3] sched/pelt: Use rq_clock_task() for hw

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Message-ID: <CAKfTPtD4Vg8rjWVAp=WnaCoMXkee6y2Vd7LT2FyMMtHGtORa5w@mail.gmail.com>
Date: Mon, 5 Aug 2024 12:56:09 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Chen Yu <yu.c.chen@...el.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Juri Lelli <juri.lelli@...hat.com>, 
	Hongyan Xia <hongyan.xia2@....com>, Qais Yousef <qyousef@...alina.io>, 
	Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] sched/pelt: Use rq_clock_task() for hw_pressure

Sorry for the late reply on this

On Mon, 29 Jul 2024 at 09:05, Chen Yu <yu.c.chen@...el.com> wrote:
>
> commit 97450eb90965 ("sched/pelt: Remove shift of thermal clock")
> removed the decay_shift for hw_pressure. This commit uses the
> sched_clock_task() in sched_tick() while it replaces the
> sched_clock_task() with rq_clock_pelt() in __update_blocked_others().

Good catch, it should be sched_clock_task() everywhere

> 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.
>
> Fix this by using rq_clock_task() inside update_hw_load_avg(),
> because:

No, don't call the rq_clock_task() inside update_hw_load_avg(), keep
it outside. V2 was the correct solution
Nack for this v3. Please come back on v2

> 1. hw_pressure doesn't care about invariance.
> 2. avoid any inconsistence in the future.
>
> Fixes: 97450eb90965 ("sched/pelt: Remove shift of thermal clock")
> Suggested-by: Qais Yousef <qyousef@...alina.io>
> Reviewed-by: Hongyan Xia <hongyan.xia2@....com>
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> ---
> v2>v3:
>   call rq_clock_task() inside update_hw_load_avg() to avoid any
>   inconsistence in the future. (Qais Yousef)
>   Added comments around update_hw_load_avg(). (Qais Yousef)
> v1->v2:
>   Added Hongyan's Reviewed-by tag.
>   Removed the Reported-by/Closes tags because they are not related
>   to this fix.(Hongyan Xia)
> ---
>  kernel/sched/core.c | 2 +-
>  kernel/sched/fair.c | 2 +-
>  kernel/sched/pelt.c | 5 +++--
>  kernel/sched/pelt.h | 6 +++---
>  4 files changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a9f655025607..011d447e065f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5462,7 +5462,7 @@ void sched_tick(void)
>
>         update_rq_clock(rq);
>         hw_pressure = arch_scale_hw_pressure(cpu_of(rq));
> -       update_hw_load_avg(rq_clock_task(rq), rq, hw_pressure);
> +       update_hw_load_avg(rq, hw_pressure);
>         curr->sched_class->task_tick(rq, curr, 0);
>         if (sched_feat(LATENCY_WARN))
>                 resched_latency = cpu_resched_latency(rq);
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9057584ec06d..193ac2c702d9 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, hw_pressure) |
>                   update_irq_load_avg(rq, 0);
>
>         if (others_have_blocked(rq))
> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> index fa52906a4478..06a5fa85327c 100644
> --- a/kernel/sched/pelt.c
> +++ b/kernel/sched/pelt.c
> @@ -400,9 +400,10 @@ int update_dl_rq_load_avg(u64 now, struct rq *rq, int running)
>   *                     capped capacity a cpu due to a HW event.
>   */
>
> -int update_hw_load_avg(u64 now, struct rq *rq, u64 capacity)
> +int update_hw_load_avg(struct rq *rq, u64 capacity)
>  {
> -       if (___update_load_sum(now, &rq->avg_hw,
> +       /* hw_pressure doesn't care about invariance */
> +       if (___update_load_sum(rq_clock_task(rq), &rq->avg_hw,
>                                capacity,
>                                capacity,
>                                capacity)) {
> diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h
> index 2150062949d4..261172c29f8f 100644
> --- a/kernel/sched/pelt.h
> +++ b/kernel/sched/pelt.h
> @@ -8,7 +8,7 @@ int update_rt_rq_load_avg(u64 now, struct rq *rq, int running);
>  int update_dl_rq_load_avg(u64 now, struct rq *rq, int running);
>
>  #ifdef CONFIG_SCHED_HW_PRESSURE
> -int update_hw_load_avg(u64 now, struct rq *rq, u64 capacity);
> +int update_hw_load_avg(struct rq *rq, u64 capacity);
>
>  static inline u64 hw_load_avg(struct rq *rq)
>  {
> @@ -16,7 +16,7 @@ static inline u64 hw_load_avg(struct rq *rq)
>  }
>  #else
>  static inline int
> -update_hw_load_avg(u64 now, struct rq *rq, u64 capacity)
> +update_hw_load_avg(struct rq *rq, u64 capacity)
>  {
>         return 0;
>  }
> @@ -202,7 +202,7 @@ update_dl_rq_load_avg(u64 now, struct rq *rq, int running)
>  }
>
>  static inline int
> -update_hw_load_avg(u64 now, struct rq *rq, u64 capacity)
> +update_hw_load_avg(struct rq *rq, u64 capacity)
>  {
>         return 0;
>  }
> --
> 2.25.1
>