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! ;-)
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);
}
