Messages in this thread | | | Date | Thu, 21 May 2015 17:09:27 +0200 | From | Denys Vlasenko <> | Subject | Re: [PATCH] rcu: Deinline rcu_read_lock_sched_held() if DEBUG_LOCK_ALLOC |
| |
On 05/21/2015 03:45 PM, Steven Rostedt wrote: > On Thu, 21 May 2015 12:04:07 +0200 > Denys Vlasenko <dvlasenk@redhat.com> wrote: > >> DEBUG_LOCK_ALLOC=y is not a production setting, but it is >> not very unusual either. Many developers routinely >> use kernels built with it enabled. >> >> Apart from being selected by hand, it is also auto-selected by >> PROVE_LOCKING "Lock debugging: prove locking correctness" and >> LOCK_STAT "Lock usage statistics" config options. >> LOCK STAT is necessary for "perf lock" to work. >> >> I wouldn't spend too much time optimizing it, but this particular >> function has a very large cost in code size: when it is deinlined, >> code size decreases by 830,000 bytes: >> >> text data bss dec hex filename >> 85674192 22294776 20627456 128596424 7aa39c8 vmlinux.before >> 84837612 22294424 20627456 127759492 79d7484 vmlinux >> >> (with this config: http://busybox.net/~vda/kernel_config) >> >> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com> >> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> >> CC: Josh Triplett <josh@joshtriplett.org> >> CC: Steven Rostedt <rostedt@goodmis.org> >> CC: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> >> CC: Lai Jiangshan <laijs@cn.fujitsu.com> >> CC: linux-kernel@vger.kernel.org >> CC: Tejun Heo <tj@kernel.org> >> CC: Oleg Nesterov <oleg@redhat.com> >> --- >> include/linux/rcupdate.h | 40 ++----------------------------------- >> kernel/rcu/update.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++ >> 2 files changed, 54 insertions(+), 38 deletions(-) >> >> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h >> index 7809749..6024a65 100644 >> --- a/include/linux/rcupdate.h >> +++ b/include/linux/rcupdate.h >> @@ -439,46 +439,10 @@ int rcu_read_lock_bh_held(void); >> * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an >> * RCU-sched read-side critical section. In absence of >> * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side >> - * critical section unless it can prove otherwise. Note that disabling >> - * of preemption (including disabling irqs) counts as an RCU-sched >> - * read-side critical section. This is useful for debug checks in functions >> - * that required that they be called within an RCU-sched read-side >> - * critical section. >> - * >> - * Check debug_lockdep_rcu_enabled() to prevent false positives during boot >> - * and while lockdep is disabled. >> - * >> - * Note that if the CPU is in the idle loop from an RCU point of >> - * view (ie: that we are in the section between rcu_idle_enter() and >> - * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU >> - * did an rcu_read_lock(). The reason for this is that RCU ignores CPUs >> - * that are in such a section, considering these as in extended quiescent >> - * state, so such a CPU is effectively never in an RCU read-side critical >> - * section regardless of what RCU primitives it invokes. This state of >> - * affairs is required --- we need to keep an RCU-free window in idle >> - * where the CPU may possibly enter into low power mode. This way we can >> - * notice an extended quiescent state to other CPUs that started a grace >> - * period. Otherwise we would delay any grace period as long as we run in >> - * the idle task. >> - * >> - * Similarly, we avoid claiming an SRCU read lock held if the current >> - * CPU is offline. >> + * critical section unless it can prove otherwise. >> */ >> #ifdef CONFIG_PREEMPT_COUNT >> -static inline int rcu_read_lock_sched_held(void) >> -{ >> - int lockdep_opinion = 0; >> - >> - if (!debug_lockdep_rcu_enabled()) >> - return 1; >> - if (!rcu_is_watching()) >> - return 0; >> - if (!rcu_lockdep_current_cpu_online()) >> - return 0; >> - if (debug_locks) >> - lockdep_opinion = lock_is_held(&rcu_sched_lock_map); >> - return lockdep_opinion || preempt_count() != 0 || irqs_disabled(); >> -} >> +int rcu_read_lock_sched_held(void); >> #else /* #ifdef CONFIG_PREEMPT_COUNT */ >> static inline int rcu_read_lock_sched_held(void) >> { >> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c >> index e0d31a3..e02218f 100644 >> --- a/kernel/rcu/update.c >> +++ b/kernel/rcu/update.c >> @@ -62,6 +62,58 @@ MODULE_ALIAS("rcupdate"); >> >> module_param(rcu_expedited, int, 0); >> >> +#ifdef CONFIG_DEBUG_LOCK_ALLOC >> +/** >> + * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section? >> + * >> + * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an >> + * RCU-sched read-side critical section. In absence of >> + * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side >> + * critical section unless it can prove otherwise. Note that disabling >> + * of preemption (including disabling irqs) counts as an RCU-sched >> + * read-side critical section. This is useful for debug checks in functions >> + * that required that they be called within an RCU-sched read-side >> + * critical section. >> + * >> + * Check debug_lockdep_rcu_enabled() to prevent false positives during boot >> + * and while lockdep is disabled. >> + * >> + * Note that if the CPU is in the idle loop from an RCU point of >> + * view (ie: that we are in the section between rcu_idle_enter() and >> + * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU >> + * did an rcu_read_lock(). The reason for this is that RCU ignores CPUs >> + * that are in such a section, considering these as in extended quiescent >> + * state, so such a CPU is effectively never in an RCU read-side critical >> + * section regardless of what RCU primitives it invokes. This state of >> + * affairs is required --- we need to keep an RCU-free window in idle >> + * where the CPU may possibly enter into low power mode. This way we can >> + * notice an extended quiescent state to other CPUs that started a grace >> + * period. Otherwise we would delay any grace period as long as we run in >> + * the idle task. >> + * >> + * Similarly, we avoid claiming an SRCU read lock held if the current >> + * CPU is offline. >> + */ >> +#ifdef CONFIG_PREEMPT_COUNT >> +int rcu_read_lock_sched_held(void) >> +{ >> + int lockdep_opinion = 0; >> + >> + if (!debug_lockdep_rcu_enabled()) >> + return 1; >> + if (!rcu_is_watching()) >> + return 0; >> + if (!rcu_lockdep_current_cpu_online()) >> + return 0; >> + if (debug_locks) >> + lockdep_opinion = lock_is_held(&rcu_sched_lock_map); >> + return lockdep_opinion || preempt_count() != 0 || irqs_disabled(); >> +} >> +EXPORT_SYMBOL(rcu_read_lock_sched_held); >> +#else >> +/* !CONFIG_PREEMPT_COUNT - the function is inlined to always return 1 */ > > Nuke the #else. It's not needed and this is a common enough practice to > have the static inline foo() { } when disabled that we do not need to > comment about it here.
Sending patch v2 in a few minutes.
>> +#endif > > Hmm, you added two #ifdef, and one #endif. How does this even compile??
Er... it... doesn't.
There was "#if defined CONFIG_DEBUG_LOCK_ALLOC && defined CONFIG_PREEMPT_COUNT" but then I split it into two #ifs to have a nice explanatory empty #else clause. And I did not test it after that edit ("what could possibly go wrong?"). Sorry.
Patch v2 I'm sending _is_ tested.
| |