On Mon, Apr 01, 2019 at 10:22:57AM -0700, Paul E. McKenney wrote: > On Mon, Apr 01, 2019 at 10:32:11AM +0200, Peter Zijlstra wrote: > > On Fri, Mar 29, 2019 at 11:26:34AM -0700, Paul E. McKenney wrote: > > > When RCU core processing is offloaded from RCU_SOFTIRQ to the rcuc > > > kthreads, a full and unconditional wakeup is required to initiate RCU > > > core processing. In contrast, when RCU core processing is carried > > > out by RCU_SOFTIRQ, a raise_softirq() suffices. Of course, there are > > > situations where raise_softirq() does a full wakeup, but these do not > > > occur with normal usage of rcu_read_unlock(). > > > > Do we have a comment somewhere explaining why? > > First, thank you for reviewing this! > > The "why" is because people normally don't do things like the code > sequence shown below, but where the scheduler holds locks across the > second RCU read-side critical section. (If they did, lockdep would > complain. Nevertheless, it is good to avoid this potential problem.) > > > > The initial solution to this problem was to use set_tsk_need_resched() and > > > set_preempt_need_resched() to force a future context switch, which allows > > > rcu_preempt_note_context_switch() to report the deferred quiescent state > > > to RCU's core processing. Unfortunately for expedited grace periods, > > > there can be a significant delay between the call for a context switch > > > and the actual context switch. > > > > This is all PREEMPT=y kernels, right? Where is the latency coming from? > > Because PREEMPT=y _should_ react quite quickly. > > Yes, PREEMPT=y. It happens like this: > > 1. rcu_read_lock() with everything enabled. > > 2. Preemption then resumption. > > 3. local_irq_disable(). > > 4. rcu_read_unlock(). > > 5. local_irq_enable(). > > From what I know, the scheduler doesn't see anything until the next > interrupt, local_bh_enable(), return to userspace, etc. Because this > is PREEMPT=y, preempt_enable() and cond_resched() do nothing. So > it could be some time (milliseconds, depending on HZ, NO_HZ_FULL, and > so on) until the scheduler responds. With NO_HZ_FULL, last I knew, > the delay can be extremely long. > > Or am I missing something that gets the scheduler on the job faster? > > Hmmm... If your point is that this amount of delay matters only for > expedited grace periods, you are quite right. So perhaps I shouldn't be > doing any of the expensive stuff unless there is an expedited grace period > in flight. Or if NO_HZ_FULL. See below for an updated (and untested) > patch to this effect. > > > > This commit therefore introduces a ->deferred_qs flag to the task_struct > > > structure's rcu_special structure. This flag is initially false, and > > > is set to true by the first call to rcu_read_unlock() requiring special > > > attention, then finally reset back to false when the quiescent state is > > > finally reported. Then rcu_read_unlock() attempts full wakeups only when > > > ->deferred_qs is false, that is, on the first rcu_read_unlock() requiring > > > special attention. Note that a chain of RCU readers linked by some other > > > sort of reader may find that a later rcu_read_unlock() is once again able > > > to do a full wakeup, courtesy of an intervening preemption: > > > > > > rcu_read_lock(); > > > /* preempted */ > > > local_irq_disable(); > > > rcu_read_unlock(); /* Can do full wakeup, sets ->deferred_qs. */ > > > rcu_read_lock(); > > > local_irq_enable(); > > > preempt_disable() > > > rcu_read_unlock(); /* Cannot do full wakeup, ->deferred_qs set. */ > > > rcu_read_lock(); > > > preempt_enable(); > > > /* preempted, >deferred_qs reset. */ > > > > As it would have without ->deferred_sq and just having done the above > > which was deemed insufficient. > > > > So I'm really puzzled by the need for all this. > > On the first round, without the ->deferred_qs, we know the scheduler > cannot be holding any of its pi or rq locks because if it did, it would > have disabled interrupts across the entire RCU read-side critical section. > Wakeups are therefore safe in this case, whether via softirq or wakeup. > Afterwards, we don't have that guarantee because an earlier critical > section might have been preempted and the scheduler might have held one > of its locks across the entire just-ended critical section. > > And I believe you are right that we should avoid the wakeups unless > there is an expedited grace period in flight or in a NO_HZ_FULL kernel. > Hence the patch shown below. Which is a stupid patch. It assumes ancient times when expedited grace periods were guaranteed to preempted pre-existing RCU read-side critical sections. Back to the drawing board... Thanx, Paul > ----------------------------------------------------------------------- > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index 2e52a77af6be..582c6d88aaa0 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -606,20 +606,26 @@ static void rcu_read_unlock_special(struct task_struct *t) > local_irq_save(flags); > irqs_were_disabled = irqs_disabled_flags(flags); > if (preempt_bh_were_disabled || irqs_were_disabled) { > + bool exp; > + > t->rcu_read_unlock_special.b.exp_hint = false; > + exp = !!READ_ONCE(this_cpu_ptr(&rcu_data)->mynode->exp_tasks); > // Need to defer quiescent state until everything is enabled. > - if (irqs_were_disabled && use_softirq && > + if ((exp || IS_ENABLED(CONFIG_NO_HZ_FULL)) && > + irqs_were_disabled && use_softirq && > (in_irq() || !t->rcu_read_unlock_special.b.deferred_qs)) { > // Using softirq, safe to awaken, and we get > // no help from enabling irqs, unlike bh/preempt. > raise_softirq_irqoff(RCU_SOFTIRQ); > - } else if (irqs_were_disabled && !use_softirq && > + } else if ((exp || IS_ENABLED(CONFIG_NO_HZ_FULL)) && > + irqs_were_disabled && !use_softirq && > !t->rcu_read_unlock_special.b.deferred_qs) { > // Safe to awaken and we get no help from enabling > // irqs, unlike bh/preempt. > invoke_rcu_core(); > } else { > // Enabling BH or preempt does reschedule, so... > + // Also if no expediting or NO_HZ_FULL, slow is OK. > set_tsk_need_resched(current); > set_preempt_need_resched(); > }