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. > 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? Thanx, Paul
