Messages in this thread | | | Date | Fri, 10 Aug 2018 01:04:50 +0800 | From | leo.yan@linaro ... | Subject | Re: [PATCH] sched: idle: Reenable sched tick for cpuidle request |
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On Thu, Aug 09, 2018 at 06:43:55PM +0200, Rafael J. Wysocki wrote: > On Thu, Aug 9, 2018 at 6:29 PM, <leo.yan@linaro.org> wrote: > > On Thu, Aug 09, 2018 at 05:42:30PM +0200, Rafael J. Wysocki wrote: > > > > [...] > > > >> >> This issue can be easily reproduce with the case on Arm Hikey board: use > >> >> CPU0 to send IPI to CPU7, CPU7 receives the IPI and in the callback > >> >> function it start a hrtimer with 4ms, so the 4ms timer delta value can > >> >> let 'menu' governor to choose deepest state in the next entering idle > >> >> time. From then on, CPU7 restarts hrtimer with 1ms interval for total > >> >> 10 times, so this can utilize the typical pattern in 'menu' governor to > >> >> have prediction for 1ms duration, finally idle governor is easily to > >> >> select a shallow state, on Hikey board it usually is to select CPU off > >> >> state. From then on, CPU7 stays in this shallow state for long time > >> >> until there have other interrupts on it. > >> > > >> > And which means that the above-mentioned code misses this case. > >> > >> And I don't really understand how this happens. :-/ > >> > >> If menu sees that the tick has been stopped, it sets > >> data->predicted_us to the minimum of TICK_USEC and > >> ktime_to_us(delta_next) and the latency requirements comes from PM QoS > >> (no interactivity boost). Thus the only case when it will say "do not > >> stop the tick" is when delta_next is below the tick period length, but > >> that's OK, because it means that there is a timer pending that much > >> time away, so it doesn't make sense to select a deeper idle state > >> then. > >> > >> If there is a short-interval timer pending every time we go idle, it > >> doesn't matter that the tick is stopped really, because the other > >> timer will wake the CPU up anyway. > >> > >> Have I missed anything? > > > > Yeah, you miss one case is if there haven't anymore timer event, for this > > case the ktime_to_us(delta_next) is a quite large value and > > data->predicted_us will be to set TICK_USEC; if HZ=1000 then TICK_USEC is > > 1000us, on Hikey board if data->predicted_us is 1000us then it's easily > > to set shallow state (C1) rather than C2. Unfortunately, this is the > > last time the CPU can predict idle state before it will stay in idle > > for long period. > > Fair enough, but in that case the governor will want the tick to be > stopped, because expected_interval is TICK_USEC then, so I'm not sure > how the patch helps?
Correct, I might introduce confusion at here and I mentioned in another email I have one prerequisite patch [1]: "cpuidle: menu: Correct the criteria for stopping tick", if without this dependency patch, the idle governor will always stop the tick even it selects one shallow state.
Sorry when I sent patchs with [1], I didn't send to linux-pm mailing list, do you want me to send these patches to linux-pm?
[...]
Thanks, Leo Yan
[1] https://lkml.org/lkml/2018/8/7/407
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