Re: [PATCH] net: raise RCU qs after each threaded NAPI poll

[Paul E. McKenney]

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" <paulmck@xxxxxxxxxx> wrote:
> 
> > > > 
> > > > 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