On Wed, Feb 28, 2024 at 05:10:43PM -0500, Joel Fernandes wrote:

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On Feb 28, 2024, at 4:52 PM, Paul E. McKenney [off-list ref] wrote:

On Wed, Feb 28, 2024 at 04:27:47PM -0500, Joel Fernandes wrote:

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On Feb 28, 2024, at 4:13 PM, Paul E. McKenney [off-list ref] wrote:

On Wed, Feb 28, 2024 at 03:14:34PM -0500, Joel Fernandes wrote:

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On Wed, Feb 28, 2024 at 12:18 PM Paul E. McKenney [off-list ref] wrote:

On Wed, Feb 28, 2024 at 10:37:51AM -0600, Yan Zhai wrote:

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On Wed, Feb 28, 2024 at 9:37 AM Joel Fernandes [off-list ref] wrote:

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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.

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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