Re: [PATCH] rcu: mark debug_lockdep_rcu_enabled() as pure

[Paul E. McKenney]

On Fri, May 19, 2017 at 10:03:59AM +0300, Konstantin Khlebnikov wrote:
> This allows to get rid of unneeded invocations.
> 
> Function debug_lockdep_rcu_enabled() becomes really hot if several
> debug options are enabled together with CONFIG_PROVE_RCU.
> 
> Hottest path ends with:
>   debug_lockdep_rcu_enabled
>   is_ftrace_trampoline
>   __kernel_text_address
> 
> Here debug_lockdep_rcu_enabled() is called from condition
> (debug_lockdep_rcu_enabled() && !__warned && (c)) inside macro
> do_for_each_ftrace_op(), where "c" is false.
> 
> With this patch "netperf -H localhost" shows boost from 2400 to 2500.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>

Nice performance increase!

The gcc documentation says that __attribute__((pure)) functions are
supposed to have return values that depend only at the function's
arguments and the values of global variables.  However, it also says:

	Interesting non-pure functions are functions with infinite loops
	or those depending on volatile memory or other system resource,
	that may change between two consecutive calls (such as feof in
	a multithreading environment).

This is OK for current->lockdep_recursion because this variable is changed
only by the current task (I think so, anyway).

It is sort of OK for debug_locks.  This could be set to zero at any time
by any other task, but if we have a race condition that very rarely causes
two lockdep splats instead of just one, so what?  (But I am sure that
some of the people on CC will correct me if I am wrong here.)

It should be OK for rcu_scheduler_active because the transition from
RCU_SCHEDULER_INACTIVE to RCU_SCHEDULER_INIT happens before the first
context switch, and the various barrier() calls, implied as well as
explicit, should keep things straight.

But I don't totally trust my analysis.  Could you please get someone
more gcc-savvy to review this and give their ack/review?  Given that,
I will queue it.

							Thanx, Paul

> ---
>  include/linux/rcupdate.h |    2 +-
>  kernel/rcu/update.c      |    2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index e1e5d002fdb9..9ecb3cb715bd 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -472,7 +472,7 @@ extern struct lockdep_map rcu_lock_map;
>  extern struct lockdep_map rcu_bh_lock_map;
>  extern struct lockdep_map rcu_sched_lock_map;
>  extern struct lockdep_map rcu_callback_map;
> -int debug_lockdep_rcu_enabled(void);
> +int __pure debug_lockdep_rcu_enabled(void);
> 
>  int rcu_read_lock_held(void);
>  int rcu_read_lock_bh_held(void);
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 273e869ca21d..a0c30abefdcd 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -292,7 +292,7 @@ struct lockdep_map rcu_callback_map =
>  	STATIC_LOCKDEP_MAP_INIT("rcu_callback", &rcu_callback_key);
>  EXPORT_SYMBOL_GPL(rcu_callback_map);
> 
> -int notrace debug_lockdep_rcu_enabled(void)
> +int __pure notrace debug_lockdep_rcu_enabled(void)
>  {
>  	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&
>  	       current->lockdep_recursion == 0;
>