Messages in this thread | | | Date | Thu, 21 May 2015 08:26:12 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu: Deinline rcu_read_lock_sched_held() if DEBUG_LOCK_ALLOC |
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On Thu, May 21, 2015 at 05:09:27PM +0200, Denys Vlasenko wrote: > 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.
Boot/run as well as build?
Thanx, Paul
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