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

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Date:	Wed, 6 Mar 2013 10:48:08 +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 Wed, 6 Mar 2013, Daniel Lezcano wrote:
> On 03/05/2013 09:40 PM, Thomas Gleixner wrote:
> > 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!
> 
> This instruction copies the pointer, not the cpumask content.
> 
> bc->cpumask is defined as a const struct cpumask * and is used to copy a
> cpumask pointer not the content.
> 
> The cpumask parameter is a pointer to a global cpumask provided by the
> cpumask_of macro.
> 
> But to be in the safe side, I compiled tested with
> CONFIG_CPUMASK_OFFSTACK=y without problem.
> 
> Did I missed something ?

No, I misinterpreted the patch. Assigning a pointer is safe. 

Thanks,

	tglx
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