Messages in this thread | | | Date | Wed, 27 May 2020 18:35:43 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH 4/7] smp: Optimize send_call_function_single_ipi() |
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On Wed, May 27, 2020 at 08:56:56AM -0700, Paul E. McKenney wrote: > On Wed, May 27, 2020 at 12:15:13PM +0200, Peter Zijlstra wrote:
> > 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.
Guaranteed no RCU read side, although the call is in a place where RCU is active again, is that a problem? I think with a bit of work I can move it to where RCU is still idle.
> 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.)
Indeed, I shall put a comment in the idle look to make sure it stays that way.
> > --- > > > > 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?
A Changelog you mean? Sure, I can do, but I wasn't nowhere confident enough in the change to even bother trying to write one.
> > 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));
Yep, that should do.
> > 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.
Right, I went though them, didn't find anything obvious amiss. OK, let me do a nicer patch.
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