Re: [RFC][PATCH 5/7] cpufreq / sched: UUF_IO flag to indicate iowait condition

[Steve Muckle <steve.muckle@linaro.org>]

On Wed, Aug 03, 2016 at 12:38:20AM +0200, Rafael J. Wysocki wrote:
> On Wed, Aug 3, 2016 at 12:02 AM, Steve Muckle <steve.muckle@linaro.org> wrote:
> > On Tue, Aug 02, 2016 at 03:37:02AM +0200, Rafael J. Wysocki wrote:
> >> On Tue, Aug 2, 2016 at 3:22 AM, Steve Muckle <steve.muckle@linaro.org> wrote:
> >> > On Mon, Aug 01, 2016 at 01:37:23AM +0200, Rafael J. Wysocki wrote:
> >> > ...
> >> >> For this purpose, define a new cpufreq_update_util() flag
> >> >> UUF_IO and modify enqueue_task_fair() to pass that flag to
> >> >> cpufreq_update_util() in the in_iowait case.  That generally
> >> >> requires cpufreq_update_util() to be called directly from there,
> >> >> because update_load_avg() is not likely to be invoked in that
> >> >> case.
> >> >
> >> > I didn't follow why the cpufreq hook won't likely be called if
> >> > in_iowait is set? AFAICS update_load_avg() gets called in the second loop
> >> > and calls update_cfs_rq_load_avg (triggers the hook).
> >>
> >> In practice it turns out that in the majority of cases when in_iowait
> >> is set the second loop will not run.
> >
> > My understanding of enqueue_task_fair() is that the first loop walks up
> > the portion of the sched_entity hierarchy that needs to be enqueued, and
> > the second loop updates the rest of the hierarchy that was already
> > enqueued.
> >
> > Even if the se corresponding to the root cfs_rq needs to be enqueued
> > (meaning the whole hierarchy is traversed in the first loop and the
> > second loop does nothing), enqueue_entity() on the root cfs_rq should
> > result in the cpufreq hook being called, via enqueue_entity() ->
> > enqueue_entity_load_avg() -> update_cfs_rq_load_avg().
> 
> But then it's rather difficult to pass the IO flag to this one, isn't it?
> 
> Essentially, the problem is to pass "IO" to cpufreq_update_util() when
> p->in_iowait is set.
> 
> If you can find a clever way to do it without adding an extra call
> site, that's fine by me, but in any case the extra
> cpufreq_update_util() invocation should not be too expensive.

I was under the impression that function pointer calls were more
expensive, and in the shared policy case there is a nontrivial amount of
code that is run in schedutil (including taking a spinlock) before we'd
see sugov_should_update_freq() return false and bail.

Agreed that getting knowledge of p->in_iowait down to the existing hook
is not easy. I spent some time fiddling with that. It seemed doable but
somewhat gross due to the required flag passing and modifications
to enqueue_entity, update_load_avg, etc. If it is decided that it is worth
pursuing I can keep working on it and post a draft.

But I also wonder if the hooks are in the best location.  They are
currently deep in the PELT code. This may make sense from a theoretical
standpoint, calling them whenever a root cfs_rq utilization changes, but
it also makes the hooks difficult to correlate (for policy purposes such
as this iowait change) with higher level logical events like a task
wakeup. Or load balance where we probably want to call the hook just
once after a load balance is complete.

This is also an issue for the remote wakeup case where I currently have
another invocation of the hook in check_preempt_curr(), so I can know if
preemption was triggered and skip a remote schedutil update in that case
to avoid a duplicate IPI.

It seems to me worth evaluating if a higher level set of hook locations
could be used. One possibility is higher up in CFS:
- enqueue_task_fair, dequeue_task_fair
- scheduler_tick
- active_load_balance_cpu_stop, load_balance

Though this wouldn't solve my issue with check_preempt_curr. That would
probably require going further up the stack to try_to_wake_up() etc. Not
yet sure what the other hook locations would be at that level.

thanks,
Steve