Re: [RFC PATCH v2 1/2] sched/fair: Introduce short duration task check

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Oct 23, 2022 at 11:32:31PM +0800, Chen Yu wrote:

> +	if (sched_feat(SIS_SHORT) && !prev->on_rq) {
> +		/*
> +		 * sum_exec_runtime has been updated in update_curr()
> +		 * because we reach here via dequeue.
> +		 */
> +		this_dur_avg = se->sum_exec_runtime - se->prev_sum_runtime_vol;
> +		/*
> +		 * Record the accumulated runtime when task voluntarily
> +		 * switches out. End of old duration period, a new period
> +		 * starts.
> +		 */
> +		se->prev_sum_runtime_vol = se->sum_exec_runtime;
> +
> +		last_dur_avg = se->dur_avg;
> +		delta = this_dur_avg - last_dur_avg;
> +		/* consider large change to avoid frequent update */
> +		if (abs(delta) >= sysctl_sched_min_granularity) {
> +			/*
> +			 * If it is the first time the task starts to
> +			 * record dur_avg, discard the initial value 0.
> +			 * Otherwise, calculate the EWMA.
> +			 */
> +			if (unlikely(!this_dur_avg))
> +				se->dur_avg = this_dur_avg;
> +			else
> +				update_avg(&se->dur_avg, this_dur_avg);
> +		}
> +	}

This seems excessively convoluted; what's wrong with something like:

	if (sched_feat(SIS_SHORT) && !prev->on_rq) {
		u64 this_dur = se->sum_exec_runtime - se->prev_sum_exec_runtime_vol;
		se->prev_sum_exec_runtime_vol = se->sum_exec_runtime;

		update_avg(&se->dur_avg, this_dur);
	}

All that other stuff just makes it unreadable and probably slower.