Re: [PATCH v2 5/6] osnoise: handle quiescent states for PREEMPT_RCU=n, PREEMPTION=y

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, Nov 28, 2024 at 09:03:56PM -0800, Ankur Arora a écrit :
> 
> Frederic Weisbecker <frederic@kernel.org> writes:
> 
> > Le Wed, Nov 06, 2024 at 12:17:57PM -0800, Ankur Arora a écrit :
> >> To reduce RCU noise for nohz_full configurations, osnoise depends
> >> on cond_resched() providing quiescent states for PREEMPT_RCU=n
> >> configurations. And, for PREEMPT_RCU=y configurations does this
> >> by directly calling rcu_momentary_eqs().
> >>
> >> With PREEMPT_LAZY=y, however, we can have configurations with
> >> (PREEMPTION=y, PREEMPT_RCU=n), which means neither of the above
> >> can help.
> >>
> >> Handle that by fallback to the explicit quiescent states via
> >> rcu_momentary_eqs().
> >>
> >> Cc: Paul E. McKenney <paulmck@kernel.org>
> >> Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
> >> Cc: Steven Rostedt <rostedt@goodmis.org>
> >> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> >> Acked-by: Daniel Bristot de Oliveira <bristot@kernel.org>
> >> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> >> ---
> >>  kernel/trace/trace_osnoise.c | 22 ++++++++++++----------
> >>  1 file changed, 12 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
> >> index a50ed23bee77..15e9600d231d 100644
> >> --- a/kernel/trace/trace_osnoise.c
> >> +++ b/kernel/trace/trace_osnoise.c
> >> @@ -1538,18 +1538,20 @@ static int run_osnoise(void)
> >>  		/*
> >>  		 * In some cases, notably when running on a nohz_full CPU with
> >>  		 * a stopped tick PREEMPT_RCU has no way to account for QSs.
> >> -		 * This will eventually cause unwarranted noise as PREEMPT_RCU
> >> -		 * will force preemption as the means of ending the current
> >> -		 * grace period. We avoid this problem by calling
> >> -		 * rcu_momentary_eqs(), which performs a zero duration
> >> -		 * EQS allowing PREEMPT_RCU to end the current grace period.
> >> -		 * This call shouldn't be wrapped inside an RCU critical
> >> -		 * section.
> >> +		 * This will eventually cause unwarranted noise as RCU forces
> >> +		 * preemption as the means of ending the current grace period.
> >> +		 * We avoid this by calling rcu_momentary_eqs(), which performs
> >> +		 * a zero duration EQS allowing RCU to end the current grace
> >> +		 * period. This call shouldn't be wrapped inside an RCU
> >> +		 * critical section.
> >>  		 *
> >> -		 * Note that in non PREEMPT_RCU kernels QSs are handled through
> >> -		 * cond_resched()
> >> +		 * For non-PREEMPT_RCU kernels with cond_resched() (non-
> >> +		 * PREEMPT_LAZY configurations), QSs are handled through
> >> +		 * cond_resched(). For PREEMPT_LAZY kernels, we fallback to
> >> +		 * the zero duration QS via rcu_momentary_eqs().
> >>  		 */
> >> -		if (IS_ENABLED(CONFIG_PREEMPT_RCU)) {
> >> +		if (IS_ENABLED(CONFIG_PREEMPT_RCU) ||
> >> +		    (!IS_ENABLED(CONFIG_PREEMPT_RCU) && IS_ENABLED(CONFIG_PREEMPTION))) {
> >>  			if (!disable_irq)
> >>  				local_irq_disable();
> >
> > How about making this unconditional so it works everywhere and doesn't
> > rely on cond_resched() Kconfig/preempt-dynamic mood?
> 
> I think it's a minor matter given that this isn't a hot path, but
> we don't really need it for the !PREEMPT_RCU configuration.

Well if you make it unconditional, cond_resched() / rcu_all_qs() won't do its
own rcu_momentary_qs(), because rcu_data.rcu_urgent_qs should
be false. So that essentially unify the behaviours for all configurations.

Thanks.

> 
> Still, given that both of those clauses imply CONFIG_PREEMPTION, we
> can just simplify this to (with an appropriately adjusted comment):
> 
> --- a/kernel/trace/trace_osnoise.c
> +++ b/kernel/trace/trace_osnoise.c
> @@ -1543,7 +1543,7 @@ static int run_osnoise(void)
>                  * Note that in non PREEMPT_RCU kernels QSs are handled through
>                  * cond_resched()
>                  */
> -               if (IS_ENABLED(CONFIG_PREEMPT_RCU)) {
> +               if (IS_ENABLED(CONFIG_PREEMPTION)) {
>                         if (!disable_irq)
>                                 local_irq_disable();
> 
> --
> ankur