Re: [PATCH] rcu: Refactor rcu_{nmi,irq}_{enter,exit}()

Byungchul Park
2018-06-25T08:22:10Z

On Sat, Jun 23, 2018 at 10:49:54AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 22, 2018 at 03:23:51PM +0900, Byungchul Park wrote:
> > On Fri, Jun 22, 2018 at 03:12:06PM +0900, Byungchul Park wrote:
> > > When passing through irq or NMI contexts, the current code uses
> > > ->dynticks_nmi_nesting to detect if it's in the ourmost at the moment.
> > >
> > > Here, the thing is that all the related functions, rcu_irq_enter(),
> > > rcu_nmi_enter(), rcu_irq_exit() and rcu_nmi_exit() are carrying out
> > > the check within each under the following call relationship so there
> > > must be redundant conditional branches:
> > >
> > > rcu_irq_enter()
> > > /* A conditional branch with ->dynticks_nmi_nesting */
> > > rcu_nmi_enter()
> > > /* A conditional branch with ->dynticks_nmi_nesting */
> > ^
> > Precisely, ->dynticks
> > > /* A conditional branch with ->dynticks_nmi_nesting */
> > >
> > > rcu_irq_exit()
> > > /* A conditional branch with ->dynticks_nmi_nesting */
> > > rcu_nmi_exit()
> > > /* A conditional branch with ->dynticks_nmi_nesting */
> > > /* A conditional branch with ->dynticks_nmi_nesting */
> > >
> > > rcu_nmi_enter()
> > > /* A conditional branch with ->dynticks_nmi_nesting */
> > ^
> > Precisely, ->dynticks
> > >
> > > rcu_nmi_exit()
> > > /* A conditional branch with ->dynticks_nmi_nesting */
> > >
> > > In order to make the code more readable and remove redundant
> > > conditional branches, we need to refactor rcu_{nmi,irq}_{enter,exit}()
> > > so they use a common function for each like:
> > >
> > > rcu_irq_enter() inlining rcu_nmi_enter_common(irq)
> > > /* A conditional branch with ->dynticks_nmi_nesting */
> > ^
> > Precisely, ->dynticks
> > >
> > > rcu_irq_exit() inlining rcu_nmi_exit_common(irq)
> > > /* A conditional branch with ->dynticks_nmi_nesting */
> > >
> > > rcu_nmi_enter() inlining rcu_nmi_enter_common(nmi)
> > > /* A conditional branch with ->dynticks_nmi_nesting */
> > ^
> > Precisely, ->dynticks
> > >
> > > rcu_nmi_exit() inlining rcu_nmi_exit_common(nmi)
> > > /* A conditional branch with ->dynticks_nmi_nesting */
>
> I queued this for testing and further review, updating the commit log
> as follows. Please let me know if I messed up something.

Much better. Thanks a lot.

> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 5e5ea52645b197fb7ae2f59f7927079b91e91aa0
> Author: Byungchul Park <byungchul.park@lge.com>
> Date: Fri Jun 22 15:12:06 2018 +0900
>
> rcu: Refactor rcu_{nmi,irq}_{enter,exit}()
>
> When entering or exiting irq or NMI handlers, the current code uses
> ->dynticks_nmi_nesting to detect if it is in the outermost handler,
> that is, the one interrupting or returning to an RCU-idle context (the
> idle loop or nohz_full usermode execution). When entering the outermost
> handler via an interrupt (as opposed to NMI), it is necessary to invoke
> rcu_dynticks_task_exit() just before the CPU is marked non-idle from an
> RCU perspective and to invoke rcu_cleanup_after_idle() just after the
> CPU is marked non-idle. Similarly, when exiting the outermost handler
> via an interrupt, it is necessary to invoke rcu_prepare_for_idle() just
> before marking the CPU idle and to invoke rcu_dynticks_task_enter()
> just after marking the CPU idle.
>
> The decision to execute these four functions is currently taken in
> rcu_irq_enter() and rcu_irq_exit() as follows:
>
> rcu_irq_enter()
> /* A conditional branch with ->dynticks_nmi_nesting */
> rcu_nmi_enter()
> /* A conditional branch with ->dynticks */
> /* A conditional branch with ->dynticks_nmi_nesting */
>
> rcu_irq_exit()
> /* A conditional branch with ->dynticks_nmi_nesting */
> rcu_nmi_exit()
> /* A conditional branch with ->dynticks_nmi_nesting */
> /* A conditional branch with ->dynticks_nmi_nesting */
>
> rcu_nmi_enter()
> /* A conditional branch with ->dynticks */
>
> rcu_nmi_exit()
> /* A conditional branch with ->dynticks_nmi_nesting */
>
> This works, but the conditional branches in rcu_irq_enter() and
> rcu_irq_exit() are redundant with those in rcu_nmi_enter() and
> rcu_nmi_exit(), respectively. Redundant branches are not something
> we want in the to/from-idle fastpaths, so this commit refactors
> rcu_{nmi,irq}_{enter,exit}() so they use a common inlined function passed
> a constant argument as follows:
>
> rcu_irq_enter() inlining rcu_nmi_enter_common(irq=true)
> /* A conditional branch with ->dynticks */
>
> rcu_irq_exit() inlining rcu_nmi_exit_common(irq=true)
> /* A conditional branch with ->dynticks_nmi_nesting */
>
> rcu_nmi_enter() inlining rcu_nmi_enter_common(irq=false)
> /* A conditional branch with ->dynticks */
>
> rcu_nmi_exit() inlining rcu_nmi_exit_common(irq=false)
> /* A conditional branch with ->dynticks_nmi_nesting */
>
> The combination of the constant function argument and the inlining allows
> the compiler to discard the conditionals that previously controlled
> execution of rcu_dynticks_task_exit(), rcu_cleanup_after_idle(),
> rcu_prepare_for_idle(), and rcu_dynticks_task_enter(). This reduces both
> the to-idle and from-idle path lengths by two conditional branches each,
> and improves readability as well.
>
> Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8788ddbc0d13..4ed74f10c852 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -773,17 +773,17 @@ 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
> *
> * 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.
> */
> -void rcu_nmi_exit(void)
> +static __always_inline void rcu_nmi_exit_common(bool irq)
> {
> struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
>
> @@ -809,7 +809,22 @@ 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);
> }
>
> /**
> @@ -833,14 +848,8 @@ 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);
> }
>
> /*
> @@ -923,7 +932,7 @@ 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
> *
> * 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
> @@ -931,10 +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;
> @@ -951,7 +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("++="),
> @@ -962,6 +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
> *
> @@ -986,14 +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);
> }
>
> /*