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rcu: Refactor rcu_{nmi,irq}_{enter,exit}()

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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.

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 rcu_cleanup_after_idle() and
trace_rcu_dyntick() 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@linux.vnet.ibm.com>
Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Applied Steven Rostedt feedback. ]
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Byungchul Park 2018-06-22 15:12:06 +09:00 committed by Paul E. McKenney
parent 5b394b2ddf
commit cf7614e13c
1 changed files with 44 additions and 22 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

66
kernel/rcu/tree.c
View file

@ -770,18 +770,16 @@ void rcu_user_enter(void)
}
#endif /* CONFIG_NO_HZ_FULL */
/**
* rcu_nmi_exit - 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);
@ -807,7 +805,26 @@ 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
* @irq: Is this call from rcu_irq_exit?
*
* If you add or remove a call to rcu_nmi_exit(), be sure to test
* with CONFIG_RCU_EQS_DEBUG=y.
*/
void rcu_nmi_exit(void)
{
rcu_nmi_exit_common(false);
}
/**
@ -831,14 +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);
}
/*
@ -921,7 +932,8 @@ 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 +941,10 @@ 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 +961,15 @@ 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 +980,14 @@ 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 +1012,8 @@ 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);
}
/*