From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@kernel.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()
Date: Wed, 27 May 2020 18:35:43 +0200 [thread overview]
Message-ID: <20200527163543.GA706478@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200527155656.GU2869@paulmck-ThinkPad-P72>
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.
next prev parent reply other threads:[~2020-05-27 16:36 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-26 16:10 [RFC][PATCH 0/7] Fix the scheduler-IPI mess Peter Zijlstra
2020-05-26 16:10 ` [RFC][PATCH 1/7] sched: Fix smp_call_function_single_async() usage for ILB Peter Zijlstra
2020-05-26 23:56 ` Frederic Weisbecker
2020-05-27 10:23 ` Vincent Guittot
2020-05-27 11:28 ` Frederic Weisbecker
2020-05-27 12:07 ` Vincent Guittot
2020-05-29 15:26 ` Valentin Schneider
2020-06-01 9:52 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2020-06-01 11:40 ` Frederic Weisbecker
2020-05-26 16:10 ` [RFC][PATCH 2/7] smp: Optimize flush_smp_call_function_queue() Peter Zijlstra
2020-05-28 12:28 ` Frederic Weisbecker
2020-06-01 9:52 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2020-05-26 16:11 ` [RFC][PATCH 3/7] smp: Move irq_work_run() out of flush_smp_call_function_queue() Peter Zijlstra
2020-05-29 13:04 ` Frederic Weisbecker
2020-06-01 9:52 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2020-05-26 16:11 ` [RFC][PATCH 4/7] smp: Optimize send_call_function_single_ipi() Peter Zijlstra
2020-05-27 9:56 ` Peter Zijlstra
2020-05-27 10:15 ` Peter Zijlstra
2020-05-27 15:56 ` Paul E. McKenney
2020-05-27 16:35 ` Peter Zijlstra [this message]
2020-05-27 17:12 ` Peter Zijlstra
2020-05-27 19:39 ` Paul E. McKenney
2020-05-28 1:35 ` Joel Fernandes
2020-05-28 8:59 ` [tip: core/rcu] rcu: Allow for smp_call_function() running callbacks from idle tip-bot2 for Peter Zijlstra
2021-01-21 16:56 ` [RFC][PATCH 4/7] smp: Optimize send_call_function_single_ipi() Peter Zijlstra
2021-01-22 0:20 ` Paul E. McKenney
2021-01-22 8:31 ` Peter Zijlstra
2021-01-22 15:35 ` Paul E. McKenney
2020-05-29 13:01 ` Frederic Weisbecker
2020-06-01 9:52 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2020-05-26 16:11 ` [RFC][PATCH 5/7] irq_work, smp: Allow irq_work on call_single_queue Peter Zijlstra
2020-05-28 23:40 ` Frederic Weisbecker
2020-05-29 13:36 ` Peter Zijlstra
2020-06-05 9:37 ` Peter Zijlstra
2020-06-05 15:02 ` Frederic Weisbecker
2020-06-05 16:17 ` Peter Zijlstra
2020-06-05 15:24 ` Kees Cook
2020-06-10 13:24 ` Frederic Weisbecker
2020-06-01 9:52 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2020-05-26 16:11 ` [RFC][PATCH 6/7] sched: Add rq::ttwu_pending Peter Zijlstra
2020-06-01 9:52 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2020-05-26 16:11 ` [RFC][PATCH 7/7] sched: Replace rq::wake_list Peter Zijlstra
2020-05-29 15:10 ` Valdis Klētnieks
2020-06-01 9:52 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2020-06-02 15:16 ` Frederic Weisbecker
2020-06-04 14:18 ` [RFC][PATCH 7/7] " Guenter Roeck
2020-06-05 0:24 ` Eric Biggers
2020-06-05 7:41 ` Peter Zijlstra
2020-06-05 16:15 ` Eric Biggers
2020-06-06 23:13 ` Guenter Roeck
2020-06-09 20:21 ` Eric Biggers
2020-06-09 21:25 ` Guenter Roeck
2020-06-09 21:38 ` Eric Biggers
2020-06-09 22:06 ` Peter Zijlstra
2020-06-09 23:03 ` Guenter Roeck
2020-06-10 9:09 ` Peter Zijlstra
2020-06-18 17:57 ` Steven Rostedt
2020-06-18 19:06 ` Guenter Roeck
2020-06-09 22:07 ` Peter Zijlstra
2020-06-05 8:10 ` Peter Zijlstra
2020-06-05 13:33 ` Guenter Roeck
2020-06-05 14:09 ` Peter Zijlstra
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