On Sun, Aug 18, 2019 at 03:12:10PM -0700, Paul E. McKenney wrote: > On Sun, Aug 18, 2019 at 05:49:48PM -0400, Joel Fernandes (Google) wrote: > > When we're in hard interrupt context in rcu_read_unlock_special(), we > > can still benefit from invoke_rcu_core() doing wake ups of rcuc > > threads when the !use_softirq parameter is passed. This is safe > > to do so because: > > > > 1. We avoid the scheduler deadlock issues thanks to the deferred_qs bit > > introduced in commit 23634ebc1d94 ("rcu: Check for wakeup-safe > > conditions in rcu_read_unlock_special()") by checking for the same in > > this patch. > > > > 2. in_irq() implies in_interrupt() which implies raising softirq will > > not do any wake ups. > > > > The rcuc thread which is awakened will run when the interrupt returns. > > > > We also honor 25102de ("rcu: Only do rcu_read_unlock_special() wakeups > > if expedited") thus doing the rcuc awakening only when none of the > > following are true: > > 1. Critical section is blocking an expedited GP. > > 2. A nohz_full CPU. > > If neither of these cases are true (exp == false), then the "else" block > > will run to do the irq_work stuff. > > > > This commit is based on a partial revert of d143b3d1cd89 ("rcu: Simplify > > rcu_read_unlock_special() deferred wakeups") with an additional in_irq() > > check added. > > > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > OK, I will bite... If it is safe to wake up an rcuc kthread, why > is it not safe to do raise_softirq()? Because raise_softirq should not be done and/or doesn't do anything if use_softirq == false. In fact, RCU_SOFTIRQ doesn't even existing if use_softirq == false. The "else if" condition of this patch uses for use_softirq. Or, did I miss your point? > And from the nit department, looks like some whitespace damage on the > comments. I will fix all of these in the change log, it was just a quick RFC I sent with the idea, tagged as RFC and not yet for merging. I should also remove the comment about " in_irq() implies in_interrupt() which implies raising softirq" from the changelog since this patch is only concerned with the rcuc kthread. thanks! - Joel > Thanx, Paul > > > --- > > v1->v2: Some minor character encoding issues in changelog corrected. > > > > Note that I am still testing this patch, but I sent an early RFC for your > > feedback. Thanks! > > > > kernel/rcu/tree_plugin.h | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 2defc7fe74c3..f4b3055026dc 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -621,6 +621,11 @@ static void rcu_read_unlock_special(struct task_struct *t) > > // Using softirq, safe to awaken, and we get > > // no help from enabling irqs, unlike bh/preempt. > > raise_softirq_irqoff(RCU_SOFTIRQ); > > + } else if (exp && in_irq() && !use_softirq && > > + !t->rcu_read_unlock_special.b.deferred_qs) { > > + // Safe to awaken rcuc kthread which will be > > + // scheduled in from the interrupt return path. > > + invoke_rcu_core(); > > } else { > > // Enabling BH or preempt does reschedule, so... > > // Also if no expediting or NO_HZ_FULL, slow is OK. > > -- > > 2.23.0.rc1.153.gdeed80330f-goog > > >