linux-kernel - Re: [PATCH v2 5/6] osnoise: handle quiescent states for PREEMPT

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Message-ID: <87seraeimb.fsf@oracle.com>
Date: Thu, 28 Nov 2024 21:03:56 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        peterz@...radead.org, tglx@...utronix.de, paulmck@...nel.org,
        mingo@...nel.org, bigeasy@...utronix.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com, efault@....de, sshegde@...ux.ibm.com,
        boris.ostrovsky@...cle.com,
        Daniel Bristot de
 Oliveira <bristot@...nel.org>
Subject: Re: [PATCH v2 5/6] osnoise: handle quiescent states for
 PREEMPT_RCU=n, PREEMPTION=y


Frederic Weisbecker <frederic@...nel.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@...nel.org>
>> Cc: Daniel Bristot de Oliveira <bristot@...nel.org>
>> Cc: Steven Rostedt <rostedt@...dmis.org>
>> Suggested-by: Paul E. McKenney <paulmck@...nel.org>
>> Acked-by: Daniel Bristot de Oliveira <bristot@...nel.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.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.

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