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Message-ID: <20240805222515.wshfsl3x4khc6ytg@airbuntu>
Date: Mon, 5 Aug 2024 23:25:15 +0100
From: Qais Yousef <qyousef@...alina.io>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Chen Yu <yu.c.chen@...el.com>, 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>,
Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] sched/pelt: Use rq_clock_task() for hw_pressure
On 08/05/24 12:56, Vincent Guittot wrote:
> 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
Curious, why this is not correct?
> 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
> >
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