Re: [PATCH v10 5/9] sched/fair: Take into account latency priority at wakeup

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jan 13, 2023 at 03:12:30PM +0100, Vincent Guittot wrote:

> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 6c61bde49152..38decae3e156 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -568,6 +568,8 @@ struct sched_entity {
>  	/* cached value of my_q->h_nr_running */
>  	unsigned long			runnable_weight;
>  #endif
> +	/* preemption offset in ns */
> +	long				latency_offset;

I wonder about the type here; does it make sense to have it depend on
the bitness; that is if s32 is big enough on 32bit then surely it is so
too on 64bit, and if not, then it should be unconditionally s64.


> +static void set_latency_offset(struct task_struct *p)
> +{
> +	long weight = sched_latency_to_weight[p->latency_prio];
> +	s64 offset;
> +
> +	offset = weight * get_sleep_latency(false);
> +	offset = div_s64(offset, NICE_LATENCY_WEIGHT_MAX);
> +	p->se.latency_offset = (long)offset;
> +}

> +/*
> + * latency weight for wakeup preemption
> + */
> +const int sched_latency_to_weight[40] = {
> + /* -20 */     -1024,     -973,     -922,      -870,      -819,
> + /* -15 */      -768,     -717,     -666,      -614,      -563,
> + /* -10 */      -512,     -461,     -410,      -358,      -307,
> + /*  -5 */      -256,     -205,     -154,      -102,       -51,
> + /*   0 */         0,       51,      102,       154,       205,
> + /*   5 */       256,      307,      358,       410,       461,
> + /*  10 */       512,      563,      614,       666,       717,
> + /*  15 */       768,      819,      870,       922,       973,
> +};

I'm slightly confused by this table, isn't that simply the linear
function?

Isn't all that the same as:

	se->se.latency_offset = get_sleep_latency * nice / (NICE_LATENCY_WIDTH/2);

? The reason we have prio_to_weight[] is because it's an exponential,
which is a bit more cumbersome to calculate, but surely we can do a
linear function at runtime.