linux-kernel - Re: [PATCH v2 2/9] rcu: Mark rcu_dynticks_curr_cpu_in

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Date:   Wed, 12 Feb 2020 17:38:18 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        rostedt@...dmis.org, mingo@...nel.org, gregkh@...uxfoundation.org,
        gustavo@...eddedor.com, tglx@...utronix.de, paulmck@...nel.org,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com
Subject: Re: [PATCH v2 2/9] rcu: Mark rcu_dynticks_curr_cpu_in_eqs() inline

On Wed, Feb 12, 2020 at 10:01:41PM +0100, Peter Zijlstra wrote:
> Since rcu_is_watching() is notrace (and needs to be, as it can be
> called from the tracers), make sure everything it in turn calls is
> notrace too.
> 
> To that effect, mark rcu_dynticks_curr_cpu_in_eqs() inline, which
> implies notrace, as the function is tiny.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/rcu/tree.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -294,7 +294,7 @@ static void rcu_dynticks_eqs_online(void
>   *
>   * No ordering, as we are sampling CPU-local information.
>   */
> -static bool rcu_dynticks_curr_cpu_in_eqs(void)
> +static inline bool rcu_dynticks_curr_cpu_in_eqs(void)

I think there are ways to turn off function inlining, such as gcc's:
-fkeep-inline-functions

And just to be sure weird compilers (clang *cough*) don't screw this up,
could we make it static inline notrace?

Build tested it on the tip tree on top of your patch:

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

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f3cb824fe5bbf..078d56951c8e7 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -294,7 +294,7 @@ static void rcu_dynticks_eqs_online(void)
  *
  * No ordering, as we are sampling CPU-local information.
  */
-static inline bool rcu_dynticks_curr_cpu_in_eqs(void)
+static inline notrace bool rcu_dynticks_curr_cpu_in_eqs(void)
 {
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);