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Date:   Wed, 27 May 2020 08:56:56 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, frederic@kernel.org,
        linux-kernel@vger.kernel.org, x86@kernel.org, cai@lca.pw,
        mgorman@techsingularity.net, joel@joelfernandes.org
Subject: Re: [RFC][PATCH 4/7] smp: Optimize send_call_function_single_ipi()
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On Wed, May 27, 2020 at 12:15:13PM +0200, Peter Zijlstra wrote:
> On Wed, May 27, 2020 at 11:56:45AM +0200, Peter Zijlstra wrote:
> 
> > This is rcu_is_cpu_rrupt_from_idle()'s lockdep_assert_in_irq() tripping
> > up (it's comment is obviously a bit antiquated).
> > 
> > Now, if I read that code correctly, it actually relies on
> > rcu_irq_enter() and thus really wants to be in an interrupt. Is there
> > any way this code can be made to work from the new context too?
> > 
> > After all, all that really is different is not having gone throught he
> > bother of setting up the IRQ context, nothing else changed -- it just so
> > happens you actually relied on that ;/
> 
> At first glance, something like the below could work. But obviously I
> might have overlooked something more subtle than a brick :-)

This can work, but only if the call from the idle loop is a place where
either RCU isn't watching on the one hand or that cannot be in an RCU
read-side critical section on the other.  Because rcu_exp_handler()
assumes that if this function returns true, we are not in an RCU read-side
critical section.  (I would expect this to be the case, but I figured
that I should make it explicit.)

> ---
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 90c8be22d57a..0792c032a972 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -426,8 +426,11 @@ EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle);
>   */

Could we please have a comment noting the change in semantics and
the reason?

>  static int rcu_is_cpu_rrupt_from_idle(void)
>  {
> -	/* Called only from within the scheduling-clock interrupt */
> -	lockdep_assert_in_irq();
> +	/*
> +	 * Usually called from the tick; but also used from smp_call_function()
> +	 * for expedited grace periods.
> +	 */
> +	lockdep_assert_irqs_disabled();
>  
>  	/* Check for counter underflows */
>  	RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nesting) < 0,
> @@ -435,8 +438,11 @@ static int rcu_is_cpu_rrupt_from_idle(void)
>  	RCU_LOCKDEP_WARN(__this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 0,
>  			 "RCU dynticks_nmi_nesting counter underflow/zero!");
>  
> -	/* Are we at first interrupt nesting level? */
> -	if (__this_cpu_read(rcu_data.dynticks_nmi_nesting) != 1)
> +	/*
> +	 * Are we at first interrupt nesting level? -- or below, when running
> +	 * directly from the idle loop itself.
> +	 */
> +	if (__this_cpu_read(rcu_data.dynticks_nmi_nesting) > 1)

Wouldn't it also be a good idea to check that we are in the context of
an idle thread?  Just in case some idiot like me drops a call to this
function in the wrong place, for example, if I were to mistakenly remember
the old semantics where it would return false from process context?

Maybe something like this?

	nesting = __this_cpu_read(rcu_data.dynticks_nmi_nesting;
	if (nesting > 1)
		return false;
	WARN_ON_ONCE(!nesting && !is_idle_task(current));

>  		return false;
>  
>  	/* Does CPU appear to be idle from an RCU standpoint? */

And let's check the other callers:

rcu_sched_clock_irq():  This will always be called from IRQ (right?), so
	no problem.

rcu_pending():  Only called from rcu_sched_clock_irq(), so still no problem.

rcu_flavor_sched_clock_irq(): Ditto for both definitions.

							Thanx, Paul