linux-kernel - Re: [PATCH V2 0/2] clockevents: Stop unused clockevent devices

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Date:	Thu, 7 May 2015 13:20:30 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Kevin Hilman <khilman@...aro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH V2 0/2] clockevents: Stop unused clockevent devices

On 3 April 2015 at 09:04, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> Hi,
>
> A clockevent device is used to service timers/hrtimers requests and the next
> event (when it should fire) is decided by the timer/hrtimer expiring next. When
> no timers/hrtimers are pending to be serviced, the expiry time is set to a
> special value: KTIME_MAX.
>
> This would normally happen with NO_HZ_{IDLE|FULL} in both LOWRES/HIGHRES modes.
>
> When 'expiry == KTIME_MAX', we either cancel the 'tick-sched' hrtimer
> (NOHZ_MODE_HIGHRES) or skip reprogramming clockevent device (NOHZ_MODE_LOWRES).
> But, the clockevent device is already reprogrammed from the tick-handler for
> next tick.
>
> As the clock event device is programmed in ONESHOT mode it will at least fire
> one more time (unnecessarily). Timers on few implementations (like
> arm_arch_timer, etc.) only support PERIODIC mode and their drivers emulate
> ONESHOT over that. Which means that on these platforms we will get spurious
> interrupts periodically (at last programmed interval rate, normally tick rate).
>
> In order to avoid spurious interrupts, the unused clockevent device should be
> stopped or its interrupts should be masked.
>
> Thomas suggested to add an optional state ONESHOT_STOPPED to solve this problem:
> lkml.org/lkml/2014/5/9/508.
>
> First patch implements the required infrastructure to start/stop clockevent
> device. The second patch stops/starts clockevent devices as and when required.
>
> It is also pushed here:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/vireshk/linux.git tick/oneshot-stopped
>
> @Preeti: I have applied your Reviewed-by only for the first commit as others
> have changed significantly. Feel free to give it again :)
>
> V1->V2:
> - Peter & Ingo suggested to to start/stop clockevent device from
>   tick_program_event() instead of the call sites that called
>   tick_program_event().

Hi Thomas,

Gentle reminder ping...

--
viresh
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