linux-kernel - Re: [PATCHv4 4/4] cpuidle: coupled: add parallel barrier function

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Message-Id: <201205112032.21117.rjw@sisk.pl>
Date:	Fri, 11 May 2012 20:32:20 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Colin Cross <ccross@...roid.com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-pm@...ts.linux-foundation.org, Kevin Hilman <khilman@...com>,
	Len Brown <len.brown@...el.com>,
	Trinabh Gupta <g.trinabh@...il.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	Kay Sievers <kay.sievers@...y.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Arnd Bergmann <arnd.bergmann@...aro.org>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCHv4 4/4] cpuidle: coupled: add parallel barrier function

On Friday, May 11, 2012, Colin Cross wrote:
> On Wed, May 9, 2012 at 2:31 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > On Tuesday, May 08, 2012, Colin Cross wrote:
> >> Adds cpuidle_coupled_parallel_barrier, which can be used by coupled
> >> cpuidle state enter functions to handle resynchronization after
> >> determining if any cpu needs to abort.  The normal use case will
> >> be:
> >>
> >> static bool abort_flag;
> >> static atomic_t abort_barrier;
> >>
> >> int arch_cpuidle_enter(struct cpuidle_device *dev, ...)
> >> {
> >>       if (arch_turn_off_irq_controller()) {
> >>               /* returns an error if an irq is pending and would be lost
> >>                  if idle continued and turned off power */
> >>               abort_flag = true;
> >>       }
> >>
> >>       cpuidle_coupled_parallel_barrier(dev, &abort_barrier);
> >>
> >>       if (abort_flag) {
> >>               /* One of the cpus didn't turn off it's irq controller */
> >>               arch_turn_on_irq_controller();
> >>               return -EINTR;
> >>       }
> >>
> >>       /* continue with idle */
> >>       ...
> >> }
> >>
> >> This will cause all cpus to abort idle together if one of them needs
> >> to abort.
> >>
> >> Reviewed-by: Santosh Shilimkar <santosh.shilimkar@...com>
> >> Tested-by: Santosh Shilimkar <santosh.shilimkar@...com>
> >> Reviewed-by: Kevin Hilman <khilman@...com>
> >> Tested-by: Kevin Hilman <khilman@...com>
> >> Signed-off-by: Colin Cross <ccross@...roid.com>
> >> ---
> >>  drivers/cpuidle/coupled.c |   37 +++++++++++++++++++++++++++++++++++++
> >>  include/linux/cpuidle.h   |    4 ++++
> >>  2 files changed, 41 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/drivers/cpuidle/coupled.c b/drivers/cpuidle/coupled.c
> >> index 93101fb..3e65de1 100644
> >> --- a/drivers/cpuidle/coupled.c
> >> +++ b/drivers/cpuidle/coupled.c
> >> @@ -130,6 +130,43 @@ struct cpuidle_coupled {
> >>  static cpumask_t cpuidle_coupled_poked_mask;
> >>
> >>  /**
> >> + * cpuidle_coupled_parallel_barrier - synchronize all online coupled cpus
> >> + * @dev: cpuidle_device of the calling cpu
> >> + * @a:   atomic variable to hold the barrier
> >> + *
> >> + * No caller to this function will return from this function until all online
> >> + * cpus in the same coupled group have called this function.  Once any caller
> >> + * has returned from this function, the barrier is immediately available for
> >> + * reuse.
> >> + *
> >> + * The atomic variable a must be initialized to 0 before any cpu calls
> >> + * this function, will be reset to 0 before any cpu returns from this function.
> >> + *
> >> + * Must only be called from within a coupled idle state handler
> >> + * (state.enter when state.flags has CPUIDLE_FLAG_COUPLED set).
> >> + *
> >> + * Provides full smp barrier semantics before and after calling.
> >> + */
> >> +void cpuidle_coupled_parallel_barrier(struct cpuidle_device *dev, atomic_t *a)
> >> +{
> >> +     int n = dev->coupled->online_count;
> >> +
> >> +     smp_mb__before_atomic_inc();
> >> +     atomic_inc(a);
> >> +
> >> +     while (atomic_read(a) < n)
> >> +             cpu_relax();
> >> +
> >> +     if (atomic_inc_return(a) == n * 2) {
> >> +             atomic_set(a, 0);
> >> +             return;
> >> +     }
> >> +
> >> +     while (atomic_read(a) > n)
> >> +             cpu_relax();
> >> +}
> >
> > Well, this looks like "wait until all CPUs execute this code".  Don't we have
> > anything like this already somewhere?
> >
> >> +
> >> +/**
> >>   * cpuidle_state_is_coupled - check if a state is part of a coupled set
> >>   * @dev: struct cpuidle_device for the current cpu
> >>   * @drv: struct cpuidle_driver for the platform
> >> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
> >> index 6038448..5ab7183 100644
> >> --- a/include/linux/cpuidle.h
> >> +++ b/include/linux/cpuidle.h
> >> @@ -183,6 +183,10 @@ static inline int cpuidle_wrap_enter(struct cpuidle_device *dev,
> >>
> >>  #endif
> >>
> >> +#ifdef CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED
> >> +void cpuidle_coupled_parallel_barrier(struct cpuidle_device *dev, atomic_t *a);
> >> +#endif
> >
> > Why exactly is the extra Kconfig option necessary?
> 
> It prevents compiling in coupled.o (2k text section) on the majority
> of kernels that will never use it.

OK, sorry, for some unknown reason it seemed to me that the option was added by
this patch.

Thanks,
Rafael
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