Re: [RFC][PATCH 4/7] smp: Optimize send_call_function_single_ipi()

[Joel Fernandes]

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@xxxxxxxxxxxxx>
> 
> Looks good to me!
> 
> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

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();
> >