Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()

Joel Fernandes
2020-02-13T20:44:44Z

On Thu, Feb 13, 2020 at 10:56:12AM -0800, Paul E. McKenney wrote:
[...]
> > > It might well be that I could make these functions be NMI-safe, but
> > > rcu_prepare_for_idle() in particular would be a bit ugly at best.
> > > So, before looking into that, I have a question. Given these proposed
> > > changes, will rcu_nmi_exit_common() and rcu_nmi_enter_common() be able
> > > to just use in_nmi()?
> >
> > That _should_ already be the case today. That is, if we end up in a
> > tracer and in_nmi() is unreliable we're already screwed anyway.
>
> So something like this, then? This is untested, probably doesn't even
> build, and could use some careful review from both Peter and Steve,
> at least. As in the below is the second version of the patch, the first
> having been missing a couple of important "!" characters.

I removed the static from rcu_nmi_enter()/exit() as it is called from
outside, that makes it build now. Updated below is Paul's diff. I also added
NOKPROBE_SYMBOL() to rcu_nmi_exit() to match rcu_nmi_enter() since it seemed
asymmetric.

---8<-----------------------

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d91c9156fab2e..bbcc7767f18ee 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -614,16 +614,18 @@ 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 rdp->dynticks and rdp->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_common(), be sure to test
+ * If you add or remove a call to rcu_nmi_exit(), be sure to test
* with CONFIG_RCU_EQS_DEBUG=y.
*/
-static __always_inline void rcu_nmi_exit_common(bool irq)
+__always_inline void rcu_nmi_exit(void)
{
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);

@@ -651,25 +653,15 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks));
WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */

- if (irq)
+ if (!in_nmi())
rcu_prepare_for_idle();

rcu_dynticks_eqs_enter();

- if (irq)
+ if (!in_nmi())
rcu_dynticks_task_enter();
}
-
-/**
- * rcu_nmi_exit - inform RCU of exit from NMI context
- *
- * 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);
-}
+NOKPROBE_SYMBOL(rcu_nmi_exit);

/**
* rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle
@@ -693,7 +685,7 @@ void rcu_nmi_exit(void)
void rcu_irq_exit(void)
{
lockdep_assert_irqs_disabled();
- rcu_nmi_exit_common(true);
+ rcu_nmi_exit();
}

/*
@@ -777,7 +769,7 @@ void rcu_user_exit(void)
#endif /* CONFIG_NO_HZ_FULL */

/**
- * rcu_nmi_enter_common - inform RCU of entry to NMI context
+ * rcu_nmi_enter - 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 rdp->dynticks and
@@ -786,10 +778,10 @@ 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_common(), be sure to test
+ * If you add or remove a call to rcu_nmi_enter(), be sure to test
* with CONFIG_RCU_EQS_DEBUG=y.
*/
-static __always_inline void rcu_nmi_enter_common(bool irq)
+__always_inline void rcu_nmi_enter(void)
{
long incby = 2;
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
@@ -807,12 +799,12 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
*/
if (rcu_dynticks_curr_cpu_in_eqs()) {

- if (irq)
+ if (!in_nmi())
rcu_dynticks_task_exit();

rcu_dynticks_eqs_exit();

- if (irq)
+ if (!in_nmi())
rcu_cleanup_after_idle();

incby = 1;
@@ -834,14 +826,6 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
rdp->dynticks_nmi_nesting + incby);
barrier();
}
-
-/**
- * rcu_nmi_enter - inform RCU of entry to NMI context
- */
-void rcu_nmi_enter(void)
-{
- rcu_nmi_enter_common(false);
-}
NOKPROBE_SYMBOL(rcu_nmi_enter);

/**
@@ -869,7 +853,7 @@ NOKPROBE_SYMBOL(rcu_nmi_enter);
void rcu_irq_enter(void)
{
lockdep_assert_irqs_disabled();
- rcu_nmi_enter_common(true);
+ rcu_nmi_enter();
}

/*