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

Patrick Bellasi
2019-01-23T14:24:55Z

On 23-Jan 10:52, Peter Zijlstra wrote:
> On Tue, Jan 22, 2019 at 06:18:31PM +0000, Patrick Bellasi wrote:
> > On 22-Jan 18:13, Peter Zijlstra wrote:
> > > On Tue, Jan 15, 2019 at 10:15:05AM +0000, Patrick Bellasi wrote:

[...]

> > If a task is not clamped we execute it at its required utilization or
> > even max frequency in case of wakeup from IO.
> >
> > When a task is util_max clamped instead, we are saying that we don't
> > care to run it above the specified clamp value and, if possible, we
> > should run it below that capacity level.
> >
> > If that's the case, why this clamping hints should not be enforced on
> > IO wakeups too?
> >
> > At the end it's still a user-space decision, we basically allow
> > userspace to defined what's the max IO boost they like to get.
>
> Because it is the wrong knob for it.
>
> Ideally we'd extend the IO-wait state to include the device-busy state
> at the time of sleep. At the very least double state io_schedule() state
> space from 1 to 2 bits, where we not only indicate: yes this is an
> IO-sleep, but also can indicate device saturation. When the device is
> saturated, we don't need to boost further.
>
> (this binary state will ofcourse cause oscilations where we drop the
> freq, drop device saturation, then ramp the freq, regain device
> saturation etc..)
>
> However, doing this is going to require fairly massive surgery on our
> whole IO stack.
>
> Also; how big of a problem is 'supriouos' boosting really? Joel tried to
> introduce a boost_max tunable, but the grandual boosting thing was good
> enough at the time.

Ok then, I'll drop the clamp on IOBoost... you right and moreover we
can always investigate for a better solution in the future with a
real use-case on hand.

Cheers.

--
#include <best/regards.h>

Patrick Bellasi