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Message-ID: <CAKfTPtAnC4escfufhjJkqsYyhDPzpmQH4xYT4nj=EjCP7gzifg@mail.gmail.com>
Date: Mon, 29 Aug 2022 12:13:26 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
Ingo Molnar <mingo@...nel.org>,
Morten Rasmussen <morten.rasmussen@....com>,
Vincent Donnefort <vdonnefort@...gle.com>,
Quentin Perret <qperret@...gle.com>,
Patrick Bellasi <patrick.bellasi@...bug.net>,
Abhijeet Dharmapurikar <adharmap@...cinc.com>,
Jian-Min <Jian-Min.Liu@...iatek.com>,
Qais Yousef <qais.yousef@....com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] sched/pelt: Introduce PELT multiplier
On Mon, 29 Aug 2022 at 12:03, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Aug 29, 2022 at 10:08:13AM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 29, 2022 at 07:54:50AM +0200, Dietmar Eggemann wrote:
> > > From: Vincent Donnefort <vincent.donnefort@....com>
> > >
> > > The new sysctl sched_pelt_multiplier allows a user to set a clock
> > > multiplier to x2 or x4 (x1 being the default). This clock multiplier
> > > artificially speeds up PELT ramp up/down similarly to use a faster
> > > half-life than the default 32ms.
> > >
> > > - x1: 32ms half-life
> > > - x2: 16ms half-life
> > > - x4: 8ms half-life
> > >
> > > Internally, a new clock is created: rq->clock_task_mult. It sits in the
> > > clock hierarchy between rq->clock_task and rq->clock_pelt.
> > >
> > > Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
> > > Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> >
> > > +extern unsigned int sched_pelt_lshift;
> > > +
> > > +/*
> > > + * absolute time |1 |2 |3 |4 |5 |6 |
> > > + * @ mult = 1 --------****************--------****************-
> > > + * @ mult = 2 --------********----------------********---------
> > > + * @ mult = 4 --------****--------------------****-------------
> > > + * clock task mult
> > > + * @ mult = 2 | | |2 |3 | | | | |5 |6 | | |
> > > + * @ mult = 4 | | | | |2|3| | | | | | | | | | |5|6| | | | | | |
> > > + *
> > > + */
> > > +static inline void update_rq_clock_task_mult(struct rq *rq, s64 delta)
> > > +{
> > > + delta <<= READ_ONCE(sched_pelt_lshift);
> > > +
> > > + rq->clock_task_mult += delta;
> > > +
> > > + update_rq_clock_pelt(rq, delta);
> > > +}
> >
> > Hurmph... I'd almost go write you something like
> > static_call()/static_branch() but for immediates.
> >
> > That said; given there's only like 3 options, perhaps a few
> > static_branch() instances work just fine ?
>
> Also, I'm not at all sure about exposing that as an official sysctl.
Me too, I would even make it a boot time parameter so we can remove
the new clock_task_mult clock and left shift clock_taslk or the delta
before passing it to clock_pelt
>
> How about something like so?
>
> ---
> Subject: sched/pelt: Introduce PELT multiplier
> From: Vincent Donnefort <vincent.donnefort@....com>
> Date: Mon, 29 Aug 2022 07:54:50 +0200
>
> From: Vincent Donnefort <vincent.donnefort@....com>
>
> The new sysctl sched_pelt_multiplier allows a user to set a clock
> multiplier to x2 or x4 (x1 being the default). This clock multiplier
> artificially speeds up PELT ramp up/down similarly to use a faster
> half-life than the default 32ms.
>
> - x1: 32ms half-life
> - x2: 16ms half-life
> - x4: 8ms half-life
>
> Internally, a new clock is created: rq->clock_task_mult. It sits in the
> clock hierarchy between rq->clock_task and rq->clock_pelt.
>
> [peterz: Use sched_feat()]
> Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> kernel/sched/core.c | 2 +-
> kernel/sched/features.h | 3 +++
> kernel/sched/pelt.h | 45 +++++++++++++++++++++++++++++++++++++++++----
> kernel/sched/sched.h | 1 +
> 4 files changed, 46 insertions(+), 5 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -727,7 +727,7 @@ static void update_rq_clock_task(struct
> if ((irq_delta + steal) && sched_feat(NONTASK_CAPACITY))
> update_irq_load_avg(rq, irq_delta + steal);
> #endif
> - update_rq_clock_pelt(rq, delta);
> + update_rq_clock_task_mult(rq, delta);
> }
>
> void update_rq_clock(struct rq *rq)
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -101,3 +101,6 @@ SCHED_FEAT(LATENCY_WARN, false)
>
> SCHED_FEAT(ALT_PERIOD, true)
> SCHED_FEAT(BASE_SLICE, true)
> +
> +SCHED_FEAT(PELT_M2, false)
> +SCHED_FEAT(PELT_M4, false)
> --- a/kernel/sched/pelt.h
> +++ b/kernel/sched/pelt.h
> @@ -61,6 +61,14 @@ static inline void cfs_se_util_change(st
> WRITE_ONCE(avg->util_est.enqueued, enqueued);
> }
>
> +static inline u64 rq_clock_task_mult(struct rq *rq)
> +{
> + lockdep_assert_rq_held(rq);
> + assert_clock_updated(rq);
> +
> + return rq->clock_task_mult;
> +}
> +
> static inline u64 rq_clock_pelt(struct rq *rq)
> {
> lockdep_assert_rq_held(rq);
> @@ -72,7 +80,7 @@ static inline u64 rq_clock_pelt(struct r
> /* The rq is idle, we can sync to clock_task */
> static inline void _update_idle_rq_clock_pelt(struct rq *rq)
> {
> - rq->clock_pelt = rq_clock_task(rq);
> + rq->clock_pelt = rq_clock_task_mult(rq);
>
> u64_u32_store(rq->clock_idle, rq_clock(rq));
> /* Paired with smp_rmb in migrate_se_pelt_lag() */
> @@ -121,6 +129,30 @@ static inline void update_rq_clock_pelt(
> rq->clock_pelt += delta;
> }
>
> +extern unsigned int sched_pelt_lshift;
> +
> +/*
> + * absolute time |1 |2 |3 |4 |5 |6 |
> + * @ mult = 1 --------****************--------****************-
> + * @ mult = 2 --------********----------------********---------
> + * @ mult = 4 --------****--------------------****-------------
> + * clock task mult
> + * @ mult = 2 | | |2 |3 | | | | |5 |6 | | |
> + * @ mult = 4 | | | | |2|3| | | | | | | | | | |5|6| | | | | | |
> + *
> + */
> +static inline void update_rq_clock_task_mult(struct rq *rq, s64 delta)
> +{
> + if (sched_feat(PELT_M2))
> + delta *= 2;
> + if (sched_feat(PELT_M4))
> + delta *= 4;
> +
> + rq->clock_task_mult += delta;
> +
> + update_rq_clock_pelt(rq, delta);
> +}
> +
> /*
> * When rq becomes idle, we have to check if it has lost idle time
> * because it was fully busy. A rq is fully used when the /Sum util_sum
> @@ -147,7 +179,7 @@ static inline void update_idle_rq_clock_
> * rq's clock_task.
> */
> if (util_sum >= divider)
> - rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt;
> + rq->lost_idle_time += rq_clock_task_mult(rq) - rq->clock_pelt;
>
> _update_idle_rq_clock_pelt(rq);
> }
> @@ -218,13 +250,18 @@ update_irq_load_avg(struct rq *rq, u64 r
> return 0;
> }
>
> -static inline u64 rq_clock_pelt(struct rq *rq)
> +static inline u64 rq_clock_task_mult(struct rq *rq)
> {
> return rq_clock_task(rq);
> }
>
> +static inline u64 rq_clock_pelt(struct rq *rq)
> +{
> + return rq_clock_task_mult(rq);
> +}
> +
> static inline void
> -update_rq_clock_pelt(struct rq *rq, s64 delta) { }
> +update_rq_clock_task_mult(struct rq *rq, s64 delta) { }
>
> static inline void
> update_idle_rq_clock_pelt(struct rq *rq) { }
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1009,6 +1009,7 @@ struct rq {
> u64 clock;
> /* Ensure that all clocks are in the same cache line */
> u64 clock_task ____cacheline_aligned;
> + u64 clock_task_mult;
> u64 clock_pelt;
> unsigned long lost_idle_time;
> u64 clock_pelt_idle;
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