linux-kernel - Re: [PATCHv7 01/11] clockevents: Prefer CPU local devices over global devices

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Date:	Thu, 06 Jun 2013 17:12:40 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Stephen Boyd <sboyd@...eaurora.org>
CC:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCHv7 01/11] clockevents: Prefer CPU local devices over global
 devices

On 06/03/2013 10:33 PM, Stephen Boyd wrote:
> On an SMP system with only one global clockevent and a dummy
> clockevent per CPU we run into problems. We want the dummy
> clockevents to be registered as the per CPU tick devices, but
> we can only achieve that if we register the dummy clockevents
> before the global clockevent or if we artificially inflate the
> rating of the dummy clockevents to be higher than the rating
> of the global clockevent. Failure to do so leads to boot
> hangs when the dummy timers are registered on all other CPUs
> besides the CPU that accepted the global clockevent as its tick
> device and there is no broadcast timer to poke the dummy
> devices.
> 
> If we're registering multiple clockevents and one clockevent is
> global and the other is local to a particular CPU we should
> choose to use the local clockevent regardless of the rating of
> the device. This way, if the clockevent is a dummy it will take
> the tick device duty as long as there isn't a higher rated tick
> device and any global clockevent will be bumped out into
> broadcast mode, fixing the problem described above.

It is not clear the connection between the changelog, the patch and the
comment. Could you clarify a bit ?

Thanks
  -- Daniel



> Reported-by: Mark Rutland <mark.rutland@....com>
> Tested-by: Mark Rutland <mark.rutland@....com>
> Tested-by: Sören Brinkmann <soren.brinkmann@...inx.com>
> Acked-by: Marc Zyngier <marc.zyngier@....com>,
> Cc: John Stultz <john.stultz@...aro.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> ---
>  kernel/time/tick-common.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
> index 5d3fb10..3da62de 100644
> --- a/kernel/time/tick-common.c
> +++ b/kernel/time/tick-common.c
> @@ -254,9 +254,10 @@ static int tick_check_new_device(struct clock_event_device *newdev)
>  		    !(newdev->features & CLOCK_EVT_FEAT_ONESHOT))
>  			goto out_bc;
>  		/*
> -		 * Check the rating
> +		 * Check the rating, but prefer CPU local devices
>  		 */
> -		if (curdev->rating >= newdev->rating)
> +		if (curdev->rating >= newdev->rating &&
> +		    cpumask_equal(curdev->cpumask, newdev->cpumask))
>  			goto out_bc;
>  	}
>  
> 


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