Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Mon, 10 Apr 2017 23:13:08 +0200 | Subject | Re: [RFC/RFT][PATCH 2/2] cpufreq: schedutil: Utilization aggregation |
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On Mon, Apr 10, 2017 at 1:26 PM, Juri Lelli <juri.lelli@arm.com> wrote: > Hi Rafael,
Hi,
> thanks for this set. I'll give it a try (together with your previous > patch) in the next few days. > > A question below. > > On 10/04/17 02:11, Rafael J. Wysocki wrote: >> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com> >> >> Due to the limitation of the rate of frequency changes the schedutil >> governor only estimates the CPU utilization entirely when it is about >> to update the frequency for the corresponding cpufreq policy. As a >> result, the intermediate utilization values are discarded by it, >> but that is not appropriate in general (like, for example, when >> tasks migrate from one CPU to another or exit, in which cases the >> utilization measured by PELT may change abruptly between frequency >> updates). >> >> For this reason, modify schedutil to estimate CPU utilization >> completely whenever it is invoked for the given CPU and store the >> maximum encountered value of it as input for subsequent new frequency >> computations. This way the new frequency is always based on the >> maximum utilization value seen by the governor after the previous >> frequency update which effectively prevents intermittent utilization >> variations from causing it to be reduced unnecessarily. >> >> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> >> --- > > [...] > >> -static void sugov_get_util(unsigned long *util, unsigned long *max) >> +static void sugov_get_util(struct sugov_cpu *sg_cpu, unsigned int flags) >> { >> + unsigned long cfs_util, cfs_max; >> struct rq *rq = this_rq(); >> - unsigned long cfs_max; >> >> - cfs_max = arch_scale_cpu_capacity(NULL, smp_processor_id()); >> + sg_cpu->flags |= flags & SCHED_CPUFREQ_RT_DL; >> + if (sg_cpu->flags & SCHED_CPUFREQ_RT_DL) >> + return; >> > > IIUC, with this you also keep track of any RT/DL tasks that woke up > during the last throttling period, and react accordingly as soon a > triggering event happens after the throttling period elapses.
Right (that's the idea at least).
> Given that for RT (and still for DL as well) the next event is a > periodic tick, couldn't happen that the required frequency transition > for an RT task, that unfortunately woke up before the end of a throttling > period, gets delayed of a tick interval (at least 4ms on ARM)?
No, that won't be an entire tick unless it wakes up exactly at the update time AFAICS.
> Don't we need to treat such wake up events (RT/DL) in a special way and > maybe set a timer to fire and process them as soon as the current > throttling period elapses? Might be a patch on top of this I guess.
Setting a timer won't be a good idea at all, as it would need to be a deferrable one and Thomas would not like that (I'm sure).
We could in principle add some special casing around that, like for example pass flags to sugov_should_update_freq() and opportunistically ignore freq_update_delay_ns if SCHED_CPUFREQ_RT_DL is set in there, but that would lead to extra overhead on systems where frequency updates happen in-context.
Also the case looks somewhat corner to me to be honest.
Thanks, Rafael
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