Re: [RFC v2] rcu/tree: Try to invoke_rcu_core() if in_irq() during unlock

[Joel Fernandes]

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
> > 
>