Messages in this thread | | | Date | Wed, 27 May 2020 08:56:56 -0700 | From | "Paul E. McKenney" <> | 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
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