On Wed, Jul 24, 2019 at 04:30:13PM +0200, Frederic Weisbecker wrote: > 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); Does this get cleared automatically if preempted, or should I clear this is one of RCU's scheduler hooks? (My guess is the latter, but I figured I should ask.) Thanx, Paul