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

---

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);
*/
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)
return false;

/* Does CPU appear to be idle from an RCU standpoint? */
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