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SubjectRe: [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@linaro.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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