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

[Paul E. McKenney]

On Sun, May 21, 2017 at 11:51:39AM +0300, Konstantin Khlebnikov wrote:
> On 20.05.2017 19:42, Paul E. McKenney wrote:
> >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
> 
> Thank you for analisys.
> 
> __attribute__((pure)) allows compiler to deduplicate / eliminate calls.
> This indeed might expand race windows when global switches like debug_locks
> changes their state. But strict synchronization here isn't required.

Any objections to this patch?

						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;
> >>
> >
>