On Mon, Jul 01, 2019 at 01:03:10PM -0700, Paul E. McKenney wrote:
> On Mon, Jul 01, 2019 at 12:04:14AM -0400, Joel Fernandes (Google) wrote:
> > The rcu_preempt_note_context_switch() tries to handle cases where
> > __rcu_read_unlock() got preempted and then the context switch path does
> > the reporting of the quiscent state along with clearing any bits in the
> > rcu_read_unlock_special union.
> >
> > This can be handled by just calling rcu_deferred_qs() which was added
> > during the RCU consolidation work and already does these checks.
> >
> > Tested RCU config TREE03 for an hour which succeeds.
> >
> > Cc: rcu@xxxxxxxxxxxxxxx
> > Cc: kernel-team@xxxxxxxxxxx
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>
> My first reaction was "that cannot possibly work", but after a bit of
> digging, it really does appear to work just fine. I therefore expanded
> the commit log a bit, so please check it to catch any messups on my part.
>
> Very cool, thank you very much! ;-)
Awesome! Thanks. I am glad you agree with the change and I agree with your
changes to the commit log.
thanks,
- Joel
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit ce547cb41ed7662f70d0b07d4c7f7555ba130c61
> Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Date: Mon Jul 1 00:04:14 2019 -0400
>
> rcu: Simplify rcu_note_context_switch exit from critical section
>
> Because __rcu_read_unlock() can be preempted just before the call to
> rcu_read_unlock_special(), it is possible for a task to be preempted just
> before it would have fully exited its RCU read-side critical section.
> This would result in a needless extension of that critical section until
> that task was resumed, which might in turn result in a needlessly
> long grace period, needless RCU priority boosting, and needless
> force-quiescent-state actions. Therefore, rcu_note_context_switch()
> invokes __rcu_read_unlock() followed by rcu_preempt_deferred_qs() when
> it detects this situation. This action by rcu_note_context_switch()
> ends the RCU read-side critical section immediately.
>
> Of course, once the task resumes, it will invoke rcu_read_unlock_special()
> redundantly. This is harmless because the fact that a preemption
> happened means that interrupts, preemption, and softirqs cannot
> have been disabled, so there would be no deferred quiescent state.
> While ->rcu_read_lock_nesting remains less than zero, none of the
> ->rcu_read_unlock_special.b bits can be set, and they were all zeroed by
> the call to rcu_note_context_switch() at task-preemption time. Therefore,
> setting ->rcu_read_unlock_special.b.exp_hint to false has no effect.
>
> Therefore, the extra call to rcu_preempt_deferred_qs_irqrestore()
> would return immediately. With one possible exception, which is
> if an expedited grace period started just as the task was being
> resumed, which could leave ->exp_deferred_qs set. This will cause
> rcu_preempt_deferred_qs_irqrestore() to invoke rcu_report_exp_rdp(),
> reporting the quiescent state, just as it should. (Such an expedited
> grace period won't affect the preemption code path due to interrupts
> having already been disabled.)
>
> But when rcu_note_context_switch() invokes __rcu_read_unlock(), it
> is doing so with preemption disabled, hence __rcu_read_unlock() will
> unconditionally defer the quiescent state, only to immediately invoke
> rcu_preempt_deferred_qs(), thus immediately reporting the deferred
> quiescent state. It turns out to be safe (and faster) to instead
> just invoke rcu_preempt_deferred_qs() without the __rcu_read_unlock()
> middleman.
>
> Because this is the invocation during the preemption (as opposed to
> the invocation just after the resume), at least one of the bits in
> ->rcu_read_unlock_special.b must be set and ->rcu_read_lock_nesting
> must be negative. This means that rcu_preempt_need_deferred_qs() must
> return true, avoiding the early exit from rcu_preempt_deferred_qs().
> Thus, rcu_preempt_deferred_qs_irqrestore() will be invoked immediately,
> as required.
>
> This commit therefore simplifies the CONFIG_PREEMPT=y version of
> rcu_note_context_switch() by removing the "else if" branch of its
> "if" statement. This change means that all callers that would have
> invoked rcu_read_unlock_special() followed by rcu_preempt_deferred_qs()
> will now simply invoke rcu_preempt_deferred_qs(), thus avoiding the
> rcu_read_unlock_special() middleman when __rcu_read_unlock() is preempted.
>
> Cc: rcu@xxxxxxxxxxxxxxx
> Cc: kernel-team@xxxxxxxxxxx
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 187dc076c497..214e4689c29d 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -313,15 +313,6 @@ void rcu_note_context_switch(bool preempt)
> ? rnp->gp_seq
> : rcu_seq_snap(&rnp->gp_seq));
> rcu_preempt_ctxt_queue(rnp, rdp);
> - } else if (t->rcu_read_lock_nesting < 0 &&
> - t->rcu_read_unlock_special.s) {
> -
> - /*
> - * Complete exit from RCU read-side critical section on
> - * behalf of preempted instance of __rcu_read_unlock().
> - */
> - rcu_read_unlock_special(t);
> - rcu_preempt_deferred_qs(t);
> } else {
> rcu_preempt_deferred_qs(t);
> }
>
