On Wed, Jul 24, 2019 at 06:52:19AM -0700, Paul E. McKenney wrote:
> On Wed, Jul 24, 2019 at 03:22:59PM +0200, Frederic Weisbecker wrote:
> > On Wed, Jul 24, 2019 at 04:53:31AM -0700, Paul E. McKenney wrote:
> > > Hello!
> > >
> > > One of the callback-invocation forward-progress issues turns out to
> > > be nohz_full CPUs not turning their scheduling-clock interrupt back on
> > > when running in kernel mode. Given that callback floods can cause RCU's
> > > callback-invocation loop to run for some time, it would be good for this
> > > loop to re-enable this interrupt. Of course, this problem applies to
> > > pretty much any kernel code that might loop for an extended time period,
> > > not just RCU.
> > >
> > > I took a quick look at kernel/time/tick-sched.c and the closest thing
> > > I found was tick_nohz_full_kick_cpu(), except that (1) it isn't clear
> > > that this does much when invoked on the current CPU and (2) it doesn't
> > > help in rcutorture TREE04. In contrast, disabling NO_HZ_FULL and using
> > > RCU_NOCB_CPU instead works quite well.
> > >
> > > So what should I be calling instead of tick_nohz_full_kick_cpu() to
> > > re-enable the current CPU's scheduling-clock interrupt?
> >
> > Indeed, kernel code is assumed to be quick enough (between two extended grace
> > periods) to avoid running the tick for RCU. But some long lasting kernel code
> > may require to tick temporarily.
> >
> > You can use tick_nohz_dep_set_cpu(cpu, TICK_DEP_MASK_RCU) with the
> > following:
> >
> > diff --git a/include/linux/tick.h b/include/linux/tick.h
> > index f92a10b5e112..3f476e2a4bf7 100644
> > --- a/include/linux/tick.h
> > +++ b/include/linux/tick.h
> > @@ -108,7 +108,8 @@ enum tick_dep_bits {
> > TICK_DEP_BIT_POSIX_TIMER = 0,
> > TICK_DEP_BIT_PERF_EVENTS = 1,
> > TICK_DEP_BIT_SCHED = 2,
> > - TICK_DEP_BIT_CLOCK_UNSTABLE = 3
> > + TICK_DEP_BIT_CLOCK_UNSTABLE = 3,
> > + TICK_DEP_BIT_RCU = 4
> > };
> >
> > #define TICK_DEP_MASK_NONE 0
> > @@ -116,6 +117,7 @@ enum tick_dep_bits {
> > #define TICK_DEP_MASK_PERF_EVENTS (1 << TICK_DEP_BIT_PERF_EVENTS)
> > #define TICK_DEP_MASK_SCHED (1 << TICK_DEP_BIT_SCHED)
> > #define TICK_DEP_MASK_CLOCK_UNSTABLE (1 << TICK_DEP_BIT_CLOCK_UNSTABLE)
> > +#define TICK_DEP_MASK_RCU (1 << TICK_DEP_BIT_RCU)
> >
> > #ifdef CONFIG_NO_HZ_COMMON
> > extern bool tick_nohz_enabled;
>
> I will give this a try, thank you! (Testing will take a few days.)
Sure!
For the background: expect a self-IPI to fire which then restart the tick on IRQ exit.
Once you're later done with the work, don't forget to remove the tick dependency:
tick_nohz_dep_clear_cpu(cpu, TICK_DEP_MASK_RCU);
Thanks.
