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SubjectRe: [RFC][PATCH 4/7] smp: Optimize send_call_function_single_ipi()
On Wed, May 27, 2020 at 12:39:14PM -0700, Paul E. McKenney wrote:
> On Wed, May 27, 2020 at 07:12:36PM +0200, Peter Zijlstra wrote:
> > On Wed, May 27, 2020 at 06:35:43PM +0200, Peter Zijlstra wrote:
> > > Right, I went though them, didn't find anything obvious amiss. OK, let
> > > me do a nicer patch.
> >
> > something like so then?
> >
> > ---
> > Subject: rcu: Allow for smp_call_function() running callbacks from idle
> >
> > Current RCU hard relies on smp_call_function() callbacks running from
> > interrupt context. A pending optimization is going to break that, it
> > will allow idle CPUs to run the callbacks from the idle loop. This
> > avoids raising the IPI on the requesting CPU and avoids handling an
> > exception on the receiving CPU.
> >
> > Change rcu_is_cpu_rrupt_from_idle() to also accept task context,
> > provided it is the idle task.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>
> Looks good to me!
>
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

- Joel


>
> > ---
> > kernel/rcu/tree.c | 25 +++++++++++++++++++------
> > kernel/sched/idle.c | 4 ++++
> > 2 files changed, 23 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index d8e9dbbefcfa..c716eadc7617 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -418,16 +418,23 @@ void rcu_momentary_dyntick_idle(void)
> > EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle);
> >
> > /**
> > - * rcu_is_cpu_rrupt_from_idle - see if interrupted from idle
> > + * rcu_is_cpu_rrupt_from_idle - see if 'interrupted' from idle
> > *
> > * If the current CPU is idle and running at a first-level (not nested)
> > - * interrupt from idle, return true. The caller must have at least
> > - * disabled preemption.
> > + * interrupt, or directly, from idle, return true.
> > + *
> > + * The caller must have at least disabled IRQs.
> > */
> > static int rcu_is_cpu_rrupt_from_idle(void)
> > {
> > - /* Called only from within the scheduling-clock interrupt */
> > - lockdep_assert_in_irq();
> > + long nesting;
> > +
> > + /*
> > + * Usually called from the tick; but also used from smp_function_call()
> > + * for expedited grace periods. This latter can result in running from
> > + * the idle task, instead of an actual IPI.
> > + */
> > + lockdep_assert_irqs_disabled();
> >
> > /* Check for counter underflows */
> > RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) < 0,
> > @@ -436,9 +443,15 @@ static int rcu_is_cpu_rrupt_from_idle(void)
> > "RCU dynticks_nmi_nesting counter underflow/zero!");
> >
> > /* Are we at first interrupt nesting level? */
> > - if (__this_cpu_read(rcu_data.dynticks_nmi_nesting) != 1)
> > + nesting = __this_cpu_read(rcu_data.dynticks_nmi_nesting);
> > + if (nesting > 1)
> > return false;
> >
> > + /*
> > + * If we're not in an interrupt, we must be in the idle task!
> > + */
> > + WARN_ON_ONCE(!nesting && !is_idle_task(current));
> > +
> > /* Does CPU appear to be idle from an RCU standpoint? */
> > return __this_cpu_read(rcu_data.dynticks_nesting) == 0;
> > }
> > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > index e9cef84c2b70..05deb81bb3e3 100644
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -289,6 +289,10 @@ static void do_idle(void)
> > */
> > smp_mb__after_atomic();
> >
> > + /*
> > + * RCU relies on this call to be done outside of an RCU read-side
> > + * critical section.
> > + */
> > flush_smp_call_function_from_idle();
> > schedule_idle();
> >

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