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

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Feb 28, 2024 at 12:18 PM Paul E. McKenney <paulmck@kernel.org> 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@joelfernandes.org> 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.

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?

thanks,

 - Joel