On Tue, Aug 27, 2019 at 08:58:13AM -0700, Paul E. McKenney wrote: > On Tue, Aug 27, 2019 at 09:08:53AM -0400, Joel Fernandes wrote: > > On Tue, Aug 27, 2019 at 11:23:33AM +0200, Sebastian Andrzej Siewior wrote: > > [snip] > > > > However, if this was instead an rcu_read_lock() critical section within > > > > a PREEMPT=y kernel, then if a schedule() occured within stop_one_task(), > > > > RCU would consider that critical section to be preempted. This means > > > > that any RCU grace period that is blocked by this RCU read-side critical > > > > section would remain blocked until stop_one_cpu() resumed, returned, > > > > and so on until the matching rcu_read_unlock() was reached. In other > > > > words, RCU would consider that RCU read-side critical section to span > > > > the call to stop_one_cpu() even if stop_one_cpu() invoked schedule(). > > > > > > Isn't that my example from above and what we do in RT? My understanding > > > is that this is the reason why we need BOOST on RT otherwise the RCU > > > critical section could remain blocked for some time. > > > > Not just for boost, it is needed to block the grace period itself on > > PREEMPT=y. On PREEMPT=y, if rcu_note_context_switch() happens in middle of a > > rcu_read_lock() reader section, then the task is added to a blocked list > > (rcu_preempt_ctxt_queue). Then just after that, the CPU reports a QS state > > (rcu_qs()) as you can see in the PREEMPT=y implementation of > > rcu_note_context_switch(). Even though the CPU has reported a QS, the grace > > period will not end because the preempted (or block as could be in -rt) task > > is still blocking the grace period. This is fundamental to the function of > > Preemptible-RCU where there is the concept of tasks blocking a grace period, > > not just CPUs. > > > > I think what Paul is trying to explain AIUI (Paul please let me know if I > > missed something): > > > > (1) Anyone calling rcu_note_context_switch() and expecting it to respect > > RCU-readers that are readers as a result of interrupt disabled regions, have > > incorrect expectations. So calling rcu_note_context_switch() has to be done > > carefully. > > > > (2) Disabling interrupts is "generally" implied as an RCU-sched flavor > > reader. However, invoking rcu_note_context_switch() from a disabled interrupt > > region is *required* for rcu_note_context_switch() to work correctly. > > > > (3) On PREEMPT=y kernels, invoking rcu_note_context_switch() from an > > interrupt disabled region does not mean that that the task will be added to a > > blocked list (unless it is also in an RCU-preempt reader) so > > rcu_note_context_switch() may immediately report a quiescent state and > > nothing blockings the grace period. > > So callers of rcu_note_context_switch() must be aware of this behavior. > > > > (4) On PREEMPT=n, unlike PREEMPT=y, there is no blocked list handling and so > > nothing will block the grace period once rcu_note_context_switch() is called. > > So any path calling rcu_note_context_switch() on a PREEMPT=n kernel, in the > > middle of something that is expected to be an RCU reader would be really bad > > from an RCU view point. > > > > Probably, we should add this all to documentation somewhere. > > I think that Sebastian understands this and was using the example of RCU > priority boosting to confirm his understanding. But documentation would > be good. Extremely difficult to keep current, but good. I believe that > the requirements document does cover this. Oh ok, got it. Sorry about the noise then! (In a way, I was just thinking out loud since this is a slightly confusing topic :-P and an archive link to this discussion serves a great purpose in my notes :-D :-)). thanks! - Joel
