Re: [BUG] schedutil governor produces regular max freq spikes because of lockup detector watchdog threads

[Lucas Stach <l.stach@pengutronix.de>]

Am Dienstag, den 09.01.2018, 16:43 +0200 schrieb Leonard Crestez:
> On Tue, 2018-01-09 at 02:17 +0100, Rafael J. Wysocki wrote:
> > On Mon, Jan 8, 2018 at 4:51 PM, Leonard Crestez  wrote:
> > > On Mon, 2018-01-08 at 15:14 +0000, Patrick Bellasi wrote:
> > > > On 08-Jan 15:20, Leonard Crestez wrote:
> > > > > On Mon, 2018-01-08 at 09:31 +0530, Viresh Kumar wrote:
> > > > > > On 05-01-18, 23:18, Rafael J. Wysocki wrote:
> > > > > > > On Fri, Jan 5, 2018 at 9:37 PM, Leonard Crestez  wrote:
> > > > > > > > When using the schedutil governor together with the softlockup detector
> > > > > > > > all CPUs go to their maximum frequency on a regular basis. This seems
> > > > > > > > to be because the watchdog creates a RT thread on each CPU and this
> > > > > > > > causes regular kicks with:
> > > > > > > > 
> > > > > > > >     cpufreq_update_this_cpu(rq, SCHED_CPUFREQ_RT);
> > > > > > > > 
> > > > > > > > The schedutil governor responds to this by immediately setting the
> > > > > > > > maximum cpu frequency, this is very undesirable.
> > > > > > > > 
> > > > > > > > The issue can be fixed by this patch from android:
> > > > > > > > 
> > > > > > > > The patch stalled in a long discussion about how it's difficult for
> > > > > > > > cpufreq to deal with RT and how some RT users might just disable
> > > > > > > > cpufreq. It is indeed hard but if the system experiences regular power
> > > > > > > > kicks from a common debug feature they will end up disabling schedutil
> > > > > > > > instead.
> > > > > > > Patrick has a series of patches dealing with this problem area AFAICS,
> > > > > > > but we are currently integrating material from Juri related to
> > > > > > > deadline tasks.
> > > > > > I am not sure if Patrick's patches would solve this problem at all as
> > > > > > we still go to max for RT and the RT task is created from the
> > > > > > softlockup detector somehow.
> > > > > I assume you're talking about the series starting with
> > > > > "[PATCH v3 0/6] cpufreq: schedutil: fixes for flags updates"
> > > > > 
> > > > > I checked and they have no effect on this particular issue (not
> > > > > surprising).
> > > > 
> > > > Yeah, that series was addressing the same issue but for one specific
> > > > RT thread: the one used by schedutil to change the frequency.
> > > > For all other RT threads the intended behavior was still to got
> > > > to max... moreover those patches has been superseded by a different
> > > > solution which has been recently proposed by Peter:
> > > > 
> > > >    20171220155625.lopjlsbvss3qgb4d@hirez.programming.kicks-ass.net
> > > > 
> > > > As Viresh and Rafael suggested, we should eventually consider a
> > > > different scheduling class and/or execution context for the watchdog.
> > > > Maybe a generalization of Juri's proposed SCHED_FLAG_SUGOV flag for
> > > > DL tasks can be useful:
> > > > 
> > > >    20171204102325.5110-4-juri.lelli@redhat.com
> > > > 
> > > > Although that solution is already considered "gross" and thus perhaps
> > > > it does not make sense to keep adding special DL tasks.
> > > > 
> > > > Another possible alternative to "tag an RT task" as being special, is
> > > > to use an API similar to the one proposed by the util_clamp RFC:
> > > > 
> > > >    20170824180857.32103-1-patrick.bellasi@arm.com
> > > > 
> > > > which would allow to define what's the maximum utilization which can
> > > > be required by a properly configured RT task.
> > > Marking the watchdog as somehow "not important for performance" would
> > > probably work, I guess it will take a while to get a stable solution.
> > > 
> > > BTW, in the current version it seems the kick happens *after* the RT
> > > task executes. It seems very likely that cpufreq will go back down
> > > before a RT executes again, so how does it help? Unless most of the
> > > workload is RT. But even in that case aren't you better off with
> > > regular scaling since schedutil will notice utilization is high anyway?
> > > 
> > > Scaling freq up first would make more sense except such operations can
> > > have very high latencies anyway.
> > I guess what happens is that it takes time to switch the frequency and
> > the RT task gives the CPU away before the frequency actually changes.
> 
> What I am saying is that as far as I can tell when cpufreq_update_util
> is called when the task has already executed and is been switched out.
> My tests are not very elaborate but based on some ftracing it seems to
> me that the current behavior is for cpufreq spikes to always trail RT
> activity. Like this:

On i.MX switching the CPU frequency involves both a regulator and PLL
reconfiguration. Both actions have really long latencies (giving the
CPU away to other processes while waiting to finish), so the frequency
switch only happens after the sort-lived watchdog RT process has
already completed its work.

This behavior is probably less bad for regular RT tasks that actually
use a bit more CPU when running, but it's completely nonsensical for
the lightweight watchdog thread.

Regards,
Lucas