Re: [PATCH 1/2] sched/fair: move cpufreq hook to update_cfs_rq_load_avg()

[Steve Muckle]

On 03/28/2016 11:30 AM, Dietmar Eggemann wrote:
> On 03/28/2016 06:34 PM, Steve Muckle wrote:
>> Hi Dietmar,
>>
>> On 03/28/2016 05:02 AM, Dietmar Eggemann wrote:
>>> Hi Steve,
>>>
>>> these patches fall into the bucket of 'optimization of updating the
>>> value only if the root cfs_rq util has changed' as discussed in '[PATCH
>>> 5/8] sched/cpufreq: pass sched class into cpufreq_update_util' of Mike
>>> T's current series '[PATCH 0/8] schedutil enhancements', right?
>>
>> I would say just the second patch is an optimization. The first and
>> third patches cover additional paths in CFS where the hook should be
>> called but currently is not, which I think is a correctness issue.
> 
> Not disagreeing here but I don't know if this level of accuracy is
> really needed. I mean we currently miss updates in
> enqueue_task_fair()->enqueue_entity()->enqueue_entity_load_avg() and
> idle_balance()/rebalance_domains()->update_blocked_averages() but there
> are plenty of call sides of update_load_avg(se, ...) with
> '&rq_of(cfs_rq_of(se))->cfs == cfs_rq_of(se)'.
>
> The question for me is does schedutil work better with this new, more
> accurate signal? IMO, not receiving a bunch of consecutive
> cpufreq_update_util's w/ the same 'util' value is probably a good thing,
> unless we see the interaction with RT/DL class as mentioned by Sai. Here
> an agreement on the design for the 'capacity vote aggregation from
> CFS/RT/DL' would help to clarify.

Without covering all the paths where CFS utilization changes it's
possible to have to wait up to a tick to act on some changes, since the
tick is the only guaranteed regularly-occurring instance of the hook.
That's an unacceptable amount of latency IMO...

thanks,
Steve