linux-kernel - Re: [PATCH] sched: idle: Reenable sched tick for cpuidle request

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Date:   Thu, 9 Aug 2018 19:06:36 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] sched: idle: Reenable sched tick for cpuidle request

On Thu, Aug 9, 2018 at 7:04 PM,  <leo.yan@...aro.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@...aro.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.