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. Thanx, Paul
