linux-kernel - Re: [RFC PATCH 1/1] sched/pelt: Introduce PELT multiplier

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Message-ID: <Ywxz7cuR+/+1CFMR@hirez.programming.kicks-ass.net>
Date:   Mon, 29 Aug 2022 10:08:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.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, 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 ?