Re: [PATCH v2 8/8] sched/pelt: Introduce PELT multiplier

[Qais Yousef <qyousef@layalina.io>]

On 01/30/24 18:38, Vincent Guittot wrote:
> Le vendredi 08 déc. 2023 à 00:23:42 (+0000), Qais Yousef a écrit :
> > From: Vincent Donnefort <vincent.donnefort@arm.com>
> > 
> > The new sched_pelt_multiplier boot param 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.
> > 
> > The param is set as read only and can only be changed at boot time via
> > 
> > 	kernel.sched_pelt_multiplier=[1, 2, 4]
> > 
> > PELT has a big impact on the overall system response and reactiveness to
> > change. Smaller PELT HF means it'll require less time to reach the
> > maximum performance point of the system when the system become fully
> > busy; and equally shorter time to go back to lowest performance point
> > when the system goes back to idle.
> > 
> > This faster reaction impacts both dvfs response and migration time
> > between clusters in HMP system.
> > 
> > Smaller PELT values are expected to give better performance at the cost
> > of more power. Under powered systems can particularly benefit from
> > smaller values. Powerful systems can still benefit from smaller values
> > if they want to be tuned towards perf more and power is not the major
> > concern for them.
> > 
> > This combined with respone_time_ms from schedutil should give the user
> > and sysadmin a deterministic way to control the triangular power, perf
> > and thermals for their system. The default response_time_ms will half
> > as PELT HF halves.
> > 
> > Update approximate_{util_avg, runtime}() to take into account the PELT
> > HALFLIFE multiplier.
> > 
> > Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
> > Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > [Converted from sysctl to boot param and updated commit message]
> > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > ---
> >  kernel/sched/core.c  |  2 +-
> >  kernel/sched/pelt.c  | 52 ++++++++++++++++++++++++++++++++++++++++++--
> >  kernel/sched/pelt.h  | 42 +++++++++++++++++++++++++++++++----
> >  kernel/sched/sched.h |  1 +
> >  4 files changed, 90 insertions(+), 7 deletions(-)
> > 
> 
> ...
> 
> > +__read_mostly unsigned int sched_pelt_lshift;
> > +static unsigned int sched_pelt_multiplier = 1;
> > +
> > +static int set_sched_pelt_multiplier(const char *val, const struct kernel_param *kp)
> > +{
> > +	int ret;
> > +
> > +	ret = param_set_int(val, kp);
> > +	if (ret)
> > +		goto error;
> > +
> > +	switch (sched_pelt_multiplier)  {
> > +	case 1:
> > +		fallthrough;
> > +	case 2:
> > +		fallthrough;
> > +	case 4:
> > +		WRITE_ONCE(sched_pelt_lshift,
> > +			   sched_pelt_multiplier >> 1);
> > +		break;
> > +	default:
> > +		ret = -EINVAL;
> > +		goto error;
> > +	}
> > +
> > +	return 0;
> > +
> > +error:
> > +	sched_pelt_multiplier = 1;
> > +	return ret;
> > +}
> > +
> > +static const struct kernel_param_ops sched_pelt_multiplier_ops = {
> > +	.set = set_sched_pelt_multiplier,
> > +	.get = param_get_int,
> > +};
> > +
> > +#ifdef MODULE_PARAM_PREFIX
> > +#undef MODULE_PARAM_PREFIX
> > +#endif
> > +/* XXX: should we use sched as prefix? */
> > +#define MODULE_PARAM_PREFIX "kernel."
> > +module_param_cb(sched_pelt_multiplier, &sched_pelt_multiplier_ops, &sched_pelt_multiplier, 0444);
> > +MODULE_PARM_DESC(sched_pelt_multiplier, "PELT HALFLIFE helps control the responsiveness of the system.");
> > +MODULE_PARM_DESC(sched_pelt_multiplier, "Accepted value: 1 32ms PELT HALIFE - roughly 200ms to go from 0 to max performance point (default).");
> > +MODULE_PARM_DESC(sched_pelt_multiplier, "                2 16ms PELT HALIFE - roughly 100ms to go from 0 to max performance point.");
> > +MODULE_PARM_DESC(sched_pelt_multiplier, "                4  8ms PELT HALIFE - roughly  50ms to go from 0 to max performance point.");
> > +
> >  /*
> >   * Approximate the new util_avg value assuming an entity has continued to run
> >   * for @delta us.
> 
> ...
> 
> > +
> >  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) { }
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index bbece0eb053a..a7c89c623250 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1029,6 +1029,7 @@ struct rq {
> >  	u64			clock;
> >  	/* Ensure that all clocks are in the same cache line */
> >  	u64			clock_task ____cacheline_aligned;
> > +	u64			clock_task_mult;
> 
> I'm not sure that we want yet another clock and this doesn't apply for irq_avg.
> 
> What about the below is simpler and I think cover all cases ?

Looks better, yes. I'll change to this and if no issues come up I'll add your
signed-off-by if that's okay with you for the next version.


Thanks!

--
Qais Yousef

> 
> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> index f951c44f1d52..5cdd147b7abe 100644
> --- a/kernel/sched/pelt.c
> +++ b/kernel/sched/pelt.c
> @@ -180,6 +180,7 @@ static __always_inline int
>  ___update_load_sum(u64 now, struct sched_avg *sa,
>  		  unsigned long load, unsigned long runnable, int running)
>  {
> +	int time_shift;
>  	u64 delta;
> 
>  	delta = now - sa->last_update_time;
> @@ -195,12 +196,17 @@ ___update_load_sum(u64 now, struct sched_avg *sa,
>  	/*
>  	 * Use 1024ns as the unit of measurement since it's a reasonable
>  	 * approximation of 1us and fast to compute.
> +	 * On top of this, we can change the half-time period from the default
> +	 * 32ms to a shorter value. This is equivalent to left shifting the
> +	 * time.
> +	 * Merge both right and left shifts in one single right shift
>  	 */
> -	delta >>= 10;
> +	time_shift = 10 - sched_pelt_lshift;
> +	delta >>= time_shift;
>  	if (!delta)
>  		return 0;
> 
> -	sa->last_update_time += delta << 10;
> +	sa->last_update_time += delta << time_shift;
> 
>  	/*
>  	 * running is a subset of runnable (weight) so running can't be set if
> 
> 
> 
> >  	u64			clock_pelt;
> >  	unsigned long		lost_idle_time;
> >  	u64			clock_pelt_idle;
> > -- 
> > 2.34.1
> >