Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Thu, 9 Aug 2018 19:06:36 +0200 | Subject | Re: [PATCH] sched: idle: Reenable sched tick for cpuidle request |
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On Thu, Aug 9, 2018 at 7:04 PM, <leo.yan@linaro.org> wrote: > 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?
Please do.
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