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Fri, 29 Nov 2024 05:03:59 +0000 Received: from CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e]) by CO6PR10MB5409.namprd10.prod.outlook.com ([fe80::25a9:32c2:a7b0:de9e%3]) with mapi id 15.20.8207.014; Fri, 29 Nov 2024 05:03:59 +0000 References: <20241106201758.428310-1-ankur.a.arora@oracle.com> <20241106201758.428310-6-ankur.a.arora@oracle.com> User-agent: mu4e 1.4.10; emacs 27.2 From: Ankur Arora To: Frederic Weisbecker Cc: Ankur Arora , linux-kernel@vger.kernel.org, peterz@infradead.org, tglx@linutronix.de, paulmck@kernel.org, mingo@kernel.org, bigeasy@linutronix.de, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, efault@gmx.de, sshegde@linux.ibm.com, boris.ostrovsky@oracle.com, Daniel Bristot de Oliveira Subject: Re: [PATCH v2 5/6] osnoise: handle quiescent states for PREEMPT_RCU=n, PREEMPTION=y In-reply-to: Date: Thu, 28 Nov 2024 21:03:56 -0800 Message-ID: <87seraeimb.fsf@oracle.com> Content-Type: text/plain; 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And, for PREEMPT_RCU=3Dy configurations does this >> by directly calling rcu_momentary_eqs(). >> >> With PREEMPT_LAZY=3Dy, however, we can have configurations with >> (PREEMPTION=3Dy, PREEMPT_RCU=3Dn), 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 >> Cc: Daniel Bristot de Oliveira >> Cc: Steven Rostedt >> Suggested-by: Paul E. McKenney >> Acked-by: Daniel Bristot de Oliveira >> Signed-off-by: Ankur Arora >> --- >> 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 thr= ough * cond_resched() */ - if (IS_ENABLED(CONFIG_PREEMPT_RCU)) { + if (IS_ENABLED(CONFIG_PREEMPTION)) { if (!disable_irq) local_irq_disable(); -- ankur