linux-kernel - Re: [PATCH 1/3] clockevents: Introduce CLOCK_EVT_STATE_ONESHOT

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Date:	Mon, 06 Apr 2015 14:26:43 -0700
From:	Kevin Hilman <khilman@...nel.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 1/3] clockevents: Introduce CLOCK_EVT_STATE_ONESHOT_STOPPED state

Viresh Kumar <viresh.kumar@...aro.org> writes:

> When no timers/hrtimers are pending, the expiry time is set to a special value:
> 'KTIME_MAX'. This normally happens 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 clockevent device should be stopped
> or its interrupts should be masked.
>
> A simple (yet hacky) solution to get this fixed could be: update
> hrtimer_force_reprogram() to always reprogram clockevent device and update
> clockevent drivers to STOP generating events (or delay it to max time) when
> 'expires' is set to KTIME_MAX. But the drawback here is that every clockevent
> driver has to be hacked for this particular case and its very easy for new ones
> to miss this.
>
> However, Thomas suggested to add an optional state ONESHOT_STOPPED to solve this
> problem: lkml.org/lkml/2014/5/9/508.
>
> This patch adds support for ONESHOT_STOPPED state in clockevents core. It will
> only be available to drivers that implement the state-specific callbacks instead
> of the legacy ->set_mode() callback.
>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>

Reviewed-by: Kevin Hilman <khilman@...aro.org>

with a minor nit...

[...]

> diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
> index 73689df1e4b8..04f6c3433f8e 100644
> --- a/kernel/time/clockevents.c
> +++ b/kernel/time/clockevents.c
> @@ -138,6 +138,17 @@ static int __clockevents_set_state(struct clock_event_device *dev,
>  			return -ENOSYS;
>  		return dev->set_state_oneshot(dev);
>  
> +	case CLOCK_EVT_STATE_ONESHOT_STOPPED:
> +		/* Core internal bug */

This comment is not useful at all (nor are all the other ones already in
this file.)  IMO, the comment should say something like:
"ONESHOT_STOPPED is only valid when currently in the ONESHOT state." or
something similar.


> +		if (WARN_ONCE(dev->state != CLOCK_EVT_STATE_ONESHOT,
> +			      "Current state: %d\n", dev->state))

Similarily this output will not be useful, and should say something
like: "Can only enter ONESHOT_STOPPED from ONESHOT.  Current state: %d\n".

> +			return -EINVAL;
> +
> +		if (dev->set_state_oneshot_stopped)
> +			return dev->set_state_oneshot_stopped(dev);
> +		else
> +			return -ENOSYS;
> +
>  	default:
>  		return -ENOSYS;
>  	}

Kevin
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