This patch adds driver callback for fast_switch and below observations on schedutil governor are done with this patch.
In POWER8 there is a regression observed with schedutil compared to ondemand. With schedutil the frequency is not ramping down and is mostly stuck at max frequency during idle . This is because of the watchdog timer, an RT task which is fired every 4 seconds which results in requesting max frequency. In a completely idle system, when there are no processes running apart from few short running housekeeping tasks (like watchdog) the system is stuck at max frequency due to 'cpufreq_trigger_update()' static inline void cpufreq_trigger_update(u64 time) { cpufreq_update_util(time, ULONG_MAX, 0); } If there is no noise apart from the watchdog timer the cpu is held at max frequency for no good reason. On a 16 core system I can see an increase in 20% idle power with schedutil compared to ondemand governor. Below is the trace with 'sched:sched_switch' and 'power:cpu_frequency' events. Here the watchdog timer that runs for a very small period is requesting Pmax and this gets triggered regularly. <idle>-0 19059.992912: sched_switch: prev_comm=swapper/16 prev_state=R ==> next_comm=watchdog/16 watchdog/16-107 19059.992914: cpu_frequency: state=4322000 cpu_id=16 watchdog/16-107 19059.992915: sched_switch: prev_comm=watchdog/16 prev_state=S ==> next_comm=swapper/16 However adding a cpufreq hook in pick_next_task_idle() to decrease the frequency helped to reduce the problem. static inline void cpufreq_trigger_idle(u64 time) { cpufreq_update_util(time, 0, 1); } This might not be the right fix for the problem, however this thread is reporting the other short-comings of cpufreq_trigger_update(). Shilpasri G Bhat (1): cpufreq: powernv: Add fast_switch callback drivers/cpufreq/powernv-cpufreq.c | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) -- 1.9.3 _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev