Re: [PATCH v5 00/10] track CPU utilization
From: Vincent Guittot
Date: Wed Jun 06 2018 - 06:03:02 EST
On 6 June 2018 at 11:59, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> On 6 June 2018 at 11:44, Quentin Perret <quentin.perret@xxxxxxx> wrote:
>> On Tuesday 05 Jun 2018 at 16:18:09 (+0200), Peter Zijlstra wrote:
>>> > As you mentioned, scale_rt_capacity give the remaining capacity for
>>> > cfs and it will behave like cfs util_avg now that it uses PELT. So as
>>> > long as cfs util_avg < scale_rt_capacity(we probably need a margin)
>>> > we keep using dl bandwidth + cfs util_avg + rt util_avg for selecting
>>> > OPP because we have remaining spare capacity but if cfs util_avg ==
>>> > scale_rt_capacity, we make sure to use max OPP.
>>> Good point, when cfs-util < cfs-cap then there is idle time and the util
>>> number is 'right', when cfs-util == cfs-cap we're overcommitted and
>>> should go max.
>>> Since the util and cap values are aligned that should track nicely.
I have re run my tests and and the results seem to be ok so far.
I'm going to clean up a bit the code used for the test and sent a new
version of the proposal
>> So Vincent proposed to have a margin between cfs util and cfs cap to be
>> sure there is a little bit of idle time. This is _exactly_ what the
>> overutilized flag in EAS does. That would actually make a lot of sense
>> to use that flag in schedutil. The idea is basically to say, if there
>> isn't enough idle time on all CPUs, the util signal are kinda wrong, so
>> let's not make any decisions (task placement or OPP selection) based on
>> that. If overutilized, go to max freq. Does that make sense ?
> Yes it's similar to the overutilized except that
> - this is done per cpu and whereas overutilization is for the whole system
> - the test is done at every freq update and not only during some cfs
> event and it uses the last up to date value and not a periodically
> updated snapshot of the value
> - this is done also without EAS
> Then for the margin, it has to be discussed if it is really needed or not