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) <j...@joelfernandes.org>
>
> 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
> >
>
