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

[Ankur Arora <ankur.a.arora@oracle.com>]

Frederic Weisbecker <frederic@kernel.org> writes:

> 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.

Ah, yes. That makes sense.

Ankur

>>
>> 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


--
ankur