On Mon, Jul 09, 2018 at 09:34:32AM -0700, Paul E. McKenney wrote: > On Mon, Jul 09, 2018 at 05:26:32PM +0200, Peter Zijlstra wrote: > > On Mon, Jul 09, 2018 at 07:29:32AM -0700, Paul E. McKenney wrote: > > > OK, so here are our options: > > > > > > 1. Add the RCU conditional to need_resched(), as David suggests. > > > Peter has concerns about overhead. > > > > > > 2. Create a new need_resched_rcu_qs() that is to be used when > > > deciding whether or not to do cond_resched(). This would > > > exact the overhead only where it is needed, but is one more > > > thing for people to get wrong. > > > > Also, with the crypto guys checking need_resched() in asm that won't > > really work either. > > Fair point! Ease of use is a good thing, even within the Linux kernel. > Or maybe especially within the Linux kernel... > > > > 3. Revert my changes to de-emphasize cond_resched_rcu_qs(), > > > and go back to sprinkling cond_resched_rcu_qs() throughout > > > the code. This also is one more thing for people to get wrong, > > > and might well eventually convert all cond_resched() calls to > > > cond_resched_rcu_qs(), which sure seems like a failure mode to me. > > > > 4a. use resched_cpu() more agressive > > 4b. use the tick to set TIF_NEED_RESCHED when it finds rcu_urgent_qs > > (avoids the IPI at the 'cost' of a slight delay in processing) > > 4b sounds eminently reasonable to me! Something like the (untested, > probably doesn't even build) patch below? > > David, any reason why this wouldn't work? Seems to me that this would > make need_resched() respond to RCU's need for quiescent states in a > timely manner without need_resched() having to become heavier weight, > but figured I should ask. > > > 5. make guest mode a quiescent state (like supposedly already done > > for NOHZ_FULL) (but this would not help the crypto guys). > > > > 6. .... > > > > ok I ran out of ideas here I think. > > > > > > So for PREEMPT the tick can check preempt_count() == 0 and if so, know > > it _could_ have rescheduled and advance the qs, right? But since we > > don't have a preempt count for !PREEMPT_COUNT kernels this doesn't work. > > > > And thus we need to invoke actual scheduling events and then through the > > schedule() callback RCU knows things happened. > > > > 4b seems like something worth trying for !PREEMPT kernels I suppose > > David is running a !PREEMPT kernel. > > For PREEMPT kernels, the patch below results in a quiescent state for > the CPU, and the forced schedule queues the task. This queuing enables > later RCU priority boosting (if enabled) once all other CPUs sharing > the same leaf rcu_node structure have passed through quiescent states. > > And yes, for PREEMPT kernels the scheduling-clock interrupt handler > already checks for a quiescent state using a combination of > preempt_count() (as you say, but ignoring the hardirq bits because > we are in an interrupt handler) and current->rcu_read_lock_nesting. > > So I believe that this will cover it. > > Thoughts?
Updated per Peter's feedback on IRC. Thanx, Paul ------------------------------------------------------------------------ diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 51919985f6cf..ccde5f8aff61 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2496,6 +2496,12 @@ void rcu_check_callbacks(int user) { trace_rcu_utilization(TPS("Start scheduler-tick")); raw_cpu_inc(rcu_data.ticks_this_gp); + if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs)) && + !is_idle_task(current)) { + set_tsk_need_resched(current); + set_preempt_need_resched(); + } + __this_cpu_write(rcu_dynticks.rcu_urgent_qs, false); rcu_flavor_check_callbacks(user); if (rcu_pending()) invoke_rcu_core();