On Wed, Feb 28, 2024 at 05:33:07PM -0500, Steven Rostedt wrote:

On Wed, 28 Feb 2024 14:19:11 -0800
"Paul E. McKenney" [off-list ref] wrote:

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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.

Although cond_resched() is technically a "preemption point" and not truly a
voluntary schedule, I would be happy to state that it's not allowed to be
called from trampolines, or their callbacks. Now the question is, does BPF
programs ever call cond_resched()? I don't think they do.

Nor do I, but I too must defer to Alexei.  ;-)

[ Added Alexei ]

The other issue with making cond_resched() be a Tasks RCU quiescent
state is that the CONFIG_PREEMPTION=y version of cond_resched() would
need to stop being a complete no-op.  Which actually might be OK.

							Thanx, Paul