Hi Rafael, On 04/05/17 16:29, Rafael J. Wysocki wrote: > On Wednesday, May 03, 2017 02:30:48 PM Juri Lelli wrote: > > Currently, sugov_next_freq_shared() uses last_freq_update_time as a > > reference to decide when to start considering CPU contributions as > > stale. > > > > However, since last_freq_update_time is set by the last CPU that issued > > a frequency transition, this might cause problems in certain cases. In > > practice, the detection of stale utilization values fails whenever the > > CPU with such values was the last to update the policy. For example (and > > please note again that the SCHED_CPUFREQ_RT flag is not the problem > > here, but only the detection of after how much time that flag has to be > > considered stale), suppose a policy with 2 CPUs: > > > > CPU0 | CPU1 > > | > > | RT task scheduled > > | SCHED_CPUFREQ_RT is set > > | CPU1->last_update = now > > | freq transition to max > > | last_freq_update_time = now > > | > > > > more than TICK_NSEC nsecs > > > > | > > a small CFS wakes up | > > CPU0->last_update = now1 | > > delta_ns(CPU0) < TICK_NSEC* | > > CPU0's util is considered | > > delta_ns(CPU1) = | > > last_freq_update_time - | > > CPU1->last_update = 0 | > > < TICK_NSEC | > > CPU1 is still considered | > > CPU1->SCHED_CPUFREQ_RT is set | > > we stay at max (until CPU1 | > > exits from idle) | > > > > * delta_ns is actually negative as now1 > last_freq_update_time > > > > While last_freq_update_time is a sensible reference for rate limiting, > > it doesn't seem to be useful for working around stale CPU states. > > > > Fix the problem by always considering now (time) as the reference for > > deciding when CPUs have stale contributions. > > > > Signed-off-by: Juri Lelli <juri.le...@arm.com> > > Cc: Rafael J. Wysocki <rafael.j.wyso...@intel.com> > > Cc: Viresh Kumar <viresh.ku...@linaro.org> > > OK > > I'll queue this up if there are no objections from the people in the CC. >
Thanks! Best, - Juri
