Re: [PATCH] sched/core: fix formatting issues in sched_can_stop_tick()

Frederic Weisbecker
2014-04-14T22:48:37Z

On Mon, Apr 14, 2014 at 08:38:38PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 14, 2014 at 09:47:41PM +0530, Viresh Kumar wrote:
> > sched_can_stop_tick() was using 7 spaces instead of 8 spaces or a 'tab' at the
> > beginning of each line. Which doesn't align with the Coding Guidelines.
> >
> > Also it removes the *rq variable as it was used at only one place and hence we
> > can directly use this_rq() instead.
> >
> > Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> > ---
> > I don't think rq = tihs_rq() has to be done before smp_mb(), in case yes sorry
> > for this patch :(
> >
> > kernel/sched/core.c | 16 ++++++----------
> > 1 file changed, 6 insertions(+), 10 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 268a45e..13299c5 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -666,18 +666,14 @@ static inline bool got_nohz_idle_kick(void)
> > #ifdef CONFIG_NO_HZ_FULL
> > bool sched_can_stop_tick(void)
> > {
> > + /* Make sure rq->nr_running update is visible after the IPI */
> > + smp_rmb();
> >
> > + /* More than one running task need preemption */
> > + if (this_rq()->nr_running > 1)
> > + return false;
> >
> > + return true;
> > }
> > #endif /* CONFIG_NO_HZ_FULL */
>
> AFAICT the smp_rmb() is entirely spurious, arch interrupts should ensure
> consistency on their own. That is:
>
> CPU 0 CPU 1
>
> [w] X = 1
> IPI 1 <int>
> [r] r = X
>
> Should act as if there was a full memory barrier, making it so that the
> read on CPU1 observes the write on CPU0.

Right, I have a pending patch for that: