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Message-ID: <20180830141032.76efd12c@gandalf.local.home>
Date: Thu, 30 Aug 2018 14:10:32 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, mingo@...nel.org,
jiangshanlai@...il.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
oleg@...hat.com, joel@...lfernandes.org,
Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH tip/core/rcu 01/19] rcu: Refactor
rcu_{nmi,irq}_{enter,exit}()
On Wed, 29 Aug 2018 15:20:29 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> This commit also changes order of execution from this:
>
> rcu_dynticks_task_exit();
> rcu_dynticks_eqs_exit();
> trace_rcu_dyntick();
> rcu_cleanup_after_idle();
>
> To this:
>
> rcu_dynticks_task_exit();
> rcu_dynticks_eqs_exit();
> rcu_cleanup_after_idle();
> trace_rcu_dyntick();
>
> In other words, the calls to trace_rcu_dyntick() and trace_rcu_dyntick()
How is trace_rcu_dyntick() and trace_rcu_dyntick reversed ? ;-)
> are reversed. This has no functional effect because the real
> concern is whether a given call is before or after the call to
> rcu_dynticks_eqs_exit(), and this patch does not change that. Before the
> call to rcu_dynticks_eqs_exit(), RCU is not yet watching the current
> CPU and after that call RCU is watching.
>
> A similar switch in calling order happens on the idle-entry path, with
> similar lack of effect for the same reasons.
>
> Suggested-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Byungchul Park <byungchul.park@....com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
> kernel/rcu/tree.c | 61 +++++++++++++++++++++++++++++++----------------
> 1 file changed, 41 insertions(+), 20 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 0b760c1369f7..0adf77923e8b 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -771,17 +771,18 @@ void rcu_user_enter(void)
> #endif /* CONFIG_NO_HZ_FULL */
>
> /**
> - * rcu_nmi_exit - inform RCU of exit from NMI context
> + * rcu_nmi_exit_common - inform RCU of exit from NMI context
> + * @irq: Is this call from rcu_irq_exit?
> *
> * If we are returning from the outermost NMI handler that interrupted an
> * RCU-idle period, update rdtp->dynticks and rdtp->dynticks_nmi_nesting
> * to let the RCU grace-period handling know that the CPU is back to
> * being RCU-idle.
> *
> - * If you add or remove a call to rcu_nmi_exit(), be sure to test
> + * If you add or remove a call to rcu_nmi_exit_common(), be sure to test
> * with CONFIG_RCU_EQS_DEBUG=y.
As this is a static function, this description doesn't make sense. You
need to move the description down to the new rcu_nmi_exit() below.
Other than that...
Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
-- Steve
> */
> -void rcu_nmi_exit(void)
> +static __always_inline void rcu_nmi_exit_common(bool irq)
> {
> struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
>
> @@ -807,7 +808,22 @@ void rcu_nmi_exit(void)
> /* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
> trace_rcu_dyntick(TPS("Startirq"), rdtp->dynticks_nmi_nesting, 0, rdtp->dynticks);
> WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
> +
> + if (irq)
> + rcu_prepare_for_idle();
> +
> rcu_dynticks_eqs_enter();
> +
> + if (irq)
> + rcu_dynticks_task_enter();
> +}
> +
> +/**
> + * rcu_nmi_exit - inform RCU of exit from NMI context
> + */
> +void rcu_nmi_exit(void)
> +{
> + rcu_nmi_exit_common(false);
> }
>
> /**
> @@ -831,14 +847,8 @@ void rcu_nmi_exit(void)
> */
> void rcu_irq_exit(void)
> {
> - struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> -
> lockdep_assert_irqs_disabled();
> - if (rdtp->dynticks_nmi_nesting == 1)
> - rcu_prepare_for_idle();
> - rcu_nmi_exit();
> - if (rdtp->dynticks_nmi_nesting == 0)
> - rcu_dynticks_task_enter();
> + rcu_nmi_exit_common(true);
> }
>
> /*
> @@ -921,7 +931,8 @@ void rcu_user_exit(void)
> #endif /* CONFIG_NO_HZ_FULL */
>
> /**
> - * rcu_nmi_enter - inform RCU of entry to NMI context
> + * rcu_nmi_enter_common - inform RCU of entry to NMI context
> + * @irq: Is this call from rcu_irq_enter?
> *
> * If the CPU was idle from RCU's viewpoint, update rdtp->dynticks and
> * rdtp->dynticks_nmi_nesting to let the RCU grace-period handling know
> @@ -929,10 +940,10 @@ void rcu_user_exit(void)
> * long as the nesting level does not overflow an int. (You will probably
> * run out of stack space first.)
> *
> - * If you add or remove a call to rcu_nmi_enter(), be sure to test
> + * If you add or remove a call to rcu_nmi_enter_common(), be sure to test
> * with CONFIG_RCU_EQS_DEBUG=y.
> */
> -void rcu_nmi_enter(void)
> +static __always_inline void rcu_nmi_enter_common(bool irq)
> {
> struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> long incby = 2;
> @@ -949,7 +960,15 @@ void rcu_nmi_enter(void)
> * period (observation due to Andy Lutomirski).
> */
> if (rcu_dynticks_curr_cpu_in_eqs()) {
> +
> + if (irq)
> + rcu_dynticks_task_exit();
> +
> rcu_dynticks_eqs_exit();
> +
> + if (irq)
> + rcu_cleanup_after_idle();
> +
> incby = 1;
> }
> trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
> @@ -960,6 +979,14 @@ void rcu_nmi_enter(void)
> barrier();
> }
>
> +/**
> + * rcu_nmi_enter - inform RCU of entry to NMI context
> + */
> +void rcu_nmi_enter(void)
> +{
> + rcu_nmi_enter_common(false);
> +}
> +
> /**
> * rcu_irq_enter - inform RCU that current CPU is entering irq away from idle
> *
> @@ -984,14 +1011,8 @@ void rcu_nmi_enter(void)
> */
> void rcu_irq_enter(void)
> {
> - struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> -
> lockdep_assert_irqs_disabled();
> - if (rdtp->dynticks_nmi_nesting == 0)
> - rcu_dynticks_task_exit();
> - rcu_nmi_enter();
> - if (rdtp->dynticks_nmi_nesting == 1)
> - rcu_cleanup_after_idle();
> + rcu_nmi_enter_common(true);
> }
>
> /*
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