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

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Date:	Tue, 18 Jun 2013 11:22:57 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	"soren.brinkmann@...inx.com" <soren.brinkmann@...inx.com>
Subject: Re: [PATCHv7 01/11] clockevents: Prefer CPU local devices over
 global devices

On Thu, Jun 13, 2013 at 07:39:50PM +0100, Stephen Boyd wrote:
> On 06/13, Thomas Gleixner wrote:
> > On Thu, 13 Jun 2013, Daniel Lezcano wrote:
> > > I prefer Thomas to have a look at it and ack it. I changed Cc to To for
> > > Thomas.
> > 
> > The patch does not apply on tip timers/core. The code has been
> > reworked a month ago. Please work against tip timers/core. That's
> > where this stuff ends up.
> > 
> 
> Ah, I thought your patch series had stalled. Here is a refreshed
> patch. Every other patch in this series applies cleanly to tip
> timers/core so I don't want to resend them again unless
> absolutely necessary.
> 
> -----8<-----
> Subject: [PATCH v8] clockevents: Prefer CPU local devices over global devices
> 
> 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.
> 
> Reported-by: Mark Rutland <mark.rutland@....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>

I've just tested this atop of tip/timers/core on a tc2, using only the
sp804. As previously, without the patch boot hangs, and with the patch
I'm able to reach userspace and do useful things.

Tested-by: Mark Rutland <mark.rutland@....com>

Thanks for working on this, Stephen.

Mark.

> ---
>  kernel/time/tick-common.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
> index 5edfb48..edd45f6 100644
> --- a/kernel/time/tick-common.c
> +++ b/kernel/time/tick-common.c
> @@ -243,8 +243,13 @@ static bool tick_check_preferred(struct clock_event_device *curdev,
>  			return false;
>  	}
>  
> -	/* Use the higher rated one */
> -	return !curdev || newdev->rating > curdev->rating;
> +	/*
> +	 * Use the higher rated one, but prefer a CPU local device with a lower
> +	 * rating than a non-CPU local device
> +	 */
> +	return !curdev ||
> +		newdev->rating > curdev->rating ||
> +	       !cpumask_equal(curdev->cpumask, newdev->cpumask);
>  }
>  
>  /*
> 
> -- 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> hosted by The Linux Foundation
> 
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