Re: schedutil issue with serial workloads

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 05, 2020 at 06:51:12PM +0200, Rafael J. Wysocki wrote:
> On 6/4/2020 11:29 PM, Alexander Monakov wrote:

> > this is a question/bugreport about behavior of schedutil on serial workloads
> > such as rsync, or './configure', or 'make install'. These workloads are
> > such that there's no single task that takes a substantial portion of CPU
> > time, but at any moment there's at least one runnable task, and overall
> > the workload is compute-bound. To run the workload efficiently, cpufreq
> > governor should select a high frequency.
> > 
> > Assume the system is idle except for the workload in question.
> > 
> > Sadly, schedutil will select the lowest frequency, unless the workload is
> > confined to one core with taskset (in which case it will select the
> > highest frequency, correctly though somewhat paradoxically).
> 
> That's because the CPU utilization generated by the workload on all CPUs is
> small.
> 
> Confining it to one CPU causes the utilization of this one to grow and so
> schedutil selects a higher frequency for it.

My initial question was why doesn't io-boosting fix this up, but a quick
look at our pipe code shows me that it doesn't seem to use
io_schedule().

That is currently our only means to express 'someone is waiting on us'
to which we then say 'lets hurry up a bit'.

Because, as you've found, if the tasks do not queue up, there is nothing
to push the frequency up.