On Wed, Feb 28, 2024 at 05:10:43PM -0500, Joel Fernandes wrote: > > > > 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. :-) However, given that the current Tasks RCU use cases wait for trampolines to be evacuated, Tasks RCU could make the choice that cond_resched() be a quiescent state, for example, by adjusting rcu_all_qs() and .rcu_urgent_qs accordingly. But this seems less pressing given the chance that cond_resched() might go away in favor of lazy preemption. Thanx, Paul
