From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Leo Yan <leo.yan@linaro.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
Daniel Lezcano <daniel.lezcano@linaro.org>,
Vincent Guittot <vincent.guittot@linaro.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux PM <linux-pm@vger.kernel.org>
Subject: Re: [PATCH] sched: idle: Reenable sched tick for cpuidle request
Date: Thu, 9 Aug 2018 19:06:36 +0200 [thread overview]
Message-ID: <CAJZ5v0h215dcOhESqu=E51kKX2TS9zUvz5hX5u2ogzP9OeVhsA@mail.gmail.com> (raw)
In-Reply-To: <20180809170449.GE14362@leoy-ThinkPad-X240s>
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.
next prev parent reply other threads:[~2018-08-09 17:06 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-09 5:47 [PATCH] sched: idle: Reenable sched tick for cpuidle request Leo Yan
2018-08-09 6:57 ` leo.yan
2018-08-09 10:45 ` Peter Zijlstra
2018-08-09 11:17 ` leo.yan
2018-08-09 12:05 ` Rafael J. Wysocki
2018-08-09 15:42 ` Rafael J. Wysocki
2018-08-09 16:29 ` leo.yan
2018-08-09 16:43 ` Rafael J. Wysocki
2018-08-09 17:04 ` leo.yan
2018-08-09 17:06 ` Rafael J. Wysocki [this message]
2018-08-09 21:31 ` Rafael J. Wysocki
2018-08-10 5:53 ` leo.yan
2018-08-10 7:24 ` Rafael J. Wysocki
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