Re: [PATCH v6 08/16] sched/cpufreq: uclamp: Add utilization clamping for FAIR tasks

Peter Zijlstra
2019-01-22T17:13:14Z

On Tue, Jan 15, 2019 at 10:15:05AM +0000, Patrick Bellasi wrote:
> @@ -342,11 +350,24 @@ static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
> return;
> sg_cpu->iowait_boost_pending = true;
>
> + /*
> + * Boost FAIR tasks only up to the CPU clamped utilization.
> + *
> + * Since DL tasks have a much more advanced bandwidth control, it's
> + * safe to assume that IO boost does not apply to those tasks.

I'm not buying that argument. IO-boost isn't related to b/w management.

IO-boot is more about compensating for hidden dependencies, and those
don't get less hidden for using a different scheduling class.

Now, arguably DL should not be doing IO in the first place, but that's a
whole different discussion.

> + * Instead, since RT tasks are not utilization clamped, we don't want
> + * to apply clamping on IO boost while there is blocked RT
> + * utilization.
> + */
> + max_boost = sg_cpu->iowait_boost_max;
> + if (!cpu_util_rt(cpu_rq(sg_cpu->cpu)))
> + max_boost = uclamp_util(cpu_rq(sg_cpu->cpu), max_boost);
> +
> /* Double the boost at each request */
> if (sg_cpu->iowait_boost) {
> sg_cpu->iowait_boost <<= 1;
> - if (sg_cpu->iowait_boost > sg_cpu->iowait_boost_max)
> - sg_cpu->iowait_boost = sg_cpu->iowait_boost_max;
> + if (sg_cpu->iowait_boost > max_boost)
> + sg_cpu->iowait_boost = max_boost;
> return;
> }

Hurmph... so I'm not sold on this bit.