> On Feb 28, 2024, at 4:52 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Wed, Feb 28, 2024 at 04:27:47PM -0500, Joel Fernandes wrote: >> >> >>>> On Feb 28, 2024, at 4:13 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: >>> >>> On Wed, Feb 28, 2024 at 03:14:34PM -0500, Joel Fernandes wrote: >>>>> On Wed, Feb 28, 2024 at 12:18 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: >>>>> >>>>> On Wed, Feb 28, 2024 at 10:37:51AM -0600, Yan Zhai wrote: >>>>>> On Wed, Feb 28, 2024 at 9:37 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: >>>>>>> Also optionally, I wonder if calling rcu_tasks_qs() directly is better >>>>>>> (for documentation if anything) since the issue is Tasks RCU specific. Also >>>>>>> code comment above the rcu_softirq_qs() call about cond_resched() not taking >>>>>>> care of Tasks RCU would be great! >>>>>>> >>>>>> Yes it's quite surprising to me that cond_resched does not help here, >>>>> >>>>> In theory, it would be possible to make cond_resched() take care of >>>>> Tasks RCU. In practice, the lazy-preemption work is looking to get rid >>>>> of cond_resched(). But if for some reason cond_resched() needs to stay >>>>> around, doing that work might make sense. >>>> >>>> In my opinion, cond_resched() doing Tasks-RCU QS does not make sense >>>> (to me), because cond_resched() is to inform the scheduler to run >>>> something else possibly of higher priority while the current task is >>>> still runnable. On the other hand, what's not permitted in a Tasks RCU >>>> reader is a voluntary sleep. So IMO even though cond_resched() is a >>>> voluntary call, it is still not a sleep but rather a preemption point. >>> >>> From the viewpoint of Task RCU's users, the point is to figure out >>> when it is OK to free an already-removed tracing trampoline. The >>> current Task RCU implementation relies on the fact that tracing >>> trampolines do not do voluntary context switches. >> >> Yes. >> >>> >>>> So a Tasks RCU reader should perfectly be able to be scheduled out in >>>> the middle of a read-side critical section (in current code) by >>>> calling cond_resched(). It is just like involuntary preemption in the >>>> middle of a RCU reader, in disguise, Right? >>> >>> You lost me on this one. This for example is not permitted: >>> >>> rcu_read_lock(); >>> cond_resched(); >>> rcu_read_unlock(); >>> >>> But in a CONFIG_PREEMPT=y kernel, that RCU reader could be preempted. >>> >>> So cond_resched() looks like a voluntary context switch to me. Recall >>> that vanilla non-preemptible RCU will treat them as quiescent states if >>> the grace period extends long enough. >>> >>> What am I missing here? >> >> That we are discussing Tasks-RCU read side section? Sorry I should have been more clear. I thought sleeping was not permitted in Tasks RCU reader, but non-sleep context switches (example involuntarily getting preempted were). > > Well, to your initial point, cond_resched() does eventually invoke > preempt_schedule_common(), so you are quite correct that as far as > Tasks RCU is concerned, cond_resched() is not a quiescent state. Thanks for confirming. :-) - Joel > > Thanx, Paul
