linux-kernel - Re: [PATCH 2/4][V2] time : set broadcast irq affinity

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Date:	Tue, 5 Mar 2013 21:40:09 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
cc:	john.stultz@...aro.org, viresh.kumar@...aro.org,
	jacob.jun.pan@...ux.intel.com,
	linux-arm-kernel@...ts.infradead.org, santosh.shilimkar@...com,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	rickard.andersson@...ricsson.com, vincent.guittot@...aro.org,
	linus.walleij@...ricsson.com
Subject: Re: [PATCH 2/4][V2] time : set broadcast irq affinity

On Sat, 2 Mar 2013, Daniel Lezcano wrote:
> When a cpu goes to a deep idle state where its local timer is shutdown,
> it notifies the time frame work to use the broadcast timer instead.
> 
> Unfortunately, the broadcast device could wake up any CPU, including an
> idle one which is not concerned by the wake up at all.
> 
> This implies, in the worst case, an idle CPU will wake up to send an IPI
> to another idle cpu.
> 
> This patch solves this by setting the irq affinity to the cpu concerned
> by the nearest timer event, by this way, the CPU which is wake up is
> guarantee to be the one concerned by the next event and we are safe with
> unnecessary wakeup for another idle CPU.
> 
> As the irq affinity is not supported by all the archs, a flag is needed
> to specify which clocksource can handle it : CLOCK_EVT_FEAT_DYNIRQ
> 
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> ---
>  include/linux/clockchips.h   |    5 +++++
>  kernel/time/tick-broadcast.c |   40 +++++++++++++++++++++++++++++++++-------
>  2 files changed, 38 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
> index 6634652..c93e2a6 100644
> --- a/include/linux/clockchips.h
> +++ b/include/linux/clockchips.h
> @@ -55,6 +55,11 @@ enum clock_event_nofitiers {
>  #define CLOCK_EVT_FEAT_C3STOP		0x000008
>  #define CLOCK_EVT_FEAT_DUMMY		0x000010
>  
> +/*
> + * Clock event device can set its irq affinity dynamically
> + */
> +#define CLOCK_EVT_FEAT_DYNIRQ		0x000020
> +
>  /**
>   * struct clock_event_device - clock event device descriptor
>   * @event_handler:	Assigned by the framework to be called by the low
> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
> index 6197ac0..9ca8ff5 100644
> --- a/kernel/time/tick-broadcast.c
> +++ b/kernel/time/tick-broadcast.c
> @@ -406,13 +406,37 @@ struct cpumask *tick_get_broadcast_oneshot_mask(void)
>  	return to_cpumask(tick_broadcast_oneshot_mask);
>  }
>  
> -static int tick_broadcast_set_event(struct clock_event_device *bc,
> +/*
> + * Set broadcast interrupt affinity
> + */
> +static void tick_broadcast_set_affinity(struct clock_event_device *bc,
> +					const struct cpumask *cpumask)
> +{
> +	if (!(bc->features & CLOCK_EVT_FEAT_DYNIRQ))
> +		return;
> +
> +	if (cpumask_equal(bc->cpumask, cpumask))
> +		return;
> +
> +	bc->cpumask = cpumask;

This breaks with CONFIG_CPUMASK_OFFSTACK=y. cpumask_copy() is your friend!

Thanks,

	tglx
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