Re: [RFCv7 PATCH 03/10] sched: scheduler-driven cpu frequency selection

[Rafael J. Wysocki]

On Tue, Mar 1, 2016 at 3:31 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Sun, Feb 28, 2016 at 03:26:21AM +0100, Rafael J. Wysocki wrote:
>
>> > > That said I'm unconvinced about the approach still.
>> > >
>> > > Having more RT threads in a system that already is under RT pressure seems like
>> > > a recipe for trouble.  Moreover, it's likely that those new RT threads will
>> > > disturb the system's normal operation somehow even without the RT pressure and
>> > > have you investigated that?
>> >
>> > Sorry I'm not sure what you mean by disturb normal operation.
>>
>> That would introduce a number of extra RT threads that would be woken up quite
>> often and on a regular basis, so there would be some extra RT noise in the
>> system, especially on systems with one CPU per cpufreq policy and many CPUs.
>>
>> That's not present ATM and surely need not be completely transparent.
>
> Having RT tasks should not be a problem. You can always set their
> priority such that they do not interfere with an actual RT workload.
>
>> > Generally speaking, increasing the capacity of a heavily loaded system
>> > seems to me to be something that should run urgently, so that the system
>> > can potentially get itself out of trouble and meet the workload's needs.
>> >
>> > > Also having them per policy may be overkill and
>> > > binding them to policy CPUs only is not necessary.
>> > >
>> > > Overall, it looks like a dynamic pool of threads that may run on every CPU
>> > > might be a better approach, but that would almost duplicate the workqueues
>> > > subsystem, so is it really worth it?
>> > >
>> > > And is the problem actually visible in practice?  I have no record of any reports
>> > > mentioning it, although theoretically it's been there forever, so had it been
>> > > real, someone would have noticed it and complained about it IMO.
>> >
>> > While I don't have a test case drawn up to provide it seems like it'd be
>> > easy to create one. More importantly the interactive governor in Android
>> > uses this same kind of model, starting a frequency change thread and
>> > making it RT. Android is particularly sensitive to latency in frequency
>> > response. So that's likely one big reason why you're not hearing about
>> > this issue - some folks have already worked around it.
>>
>> OK, so Android is the reason. :-)
>>
>> Fair enough.  I still think that care is needed here, though.
>
> So I sort of see the point of having per-cpu RT kthread tasks to effect
> the OPP change for CFS. And here I would indeed suggest just having a
> task per cpu, moving tasks about is too complex and generates even more
> noise.
>
> But the problem of having RT tasks is that you should run RT tasks at
> max OPP.
>
> Now you can probably fudge things and not account these RT tasks to the
> RT 'workload' since you know their characteristics etc.. But its going
> to be ugly I suspect.

Good point.