Re: [RFC][PATCH 4/7] smp: Optimize send_call_function_single_ipi()
From: Paul E. McKenney
Date: Wed May 27 2020 - 11:57:47 EST
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
> 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)
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
rcu_pending(): Only called from rcu_sched_clock_irq(), so still no problem.
rcu_flavor_sched_clock_irq(): Ditto for both definitions.