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 <[email protected]>
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.
---
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;
