Messages in this thread | | | Date | Thu, 21 May 2015 23:53:21 +0200 | From | Denys Vlasenko <> | Subject | Re: [PATCH] rcu: Deinline rcu_read_lock_sched_held() if DEBUG_LOCK_ALLOC |
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On 05/21/2015 05:26 PM, Paul E. McKenney wrote: > 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?
I did not even dare to try booting 128 megabyte allyesconfig monstrosity, before you asked.
It took me 4 hours of rebuilds to figure out that CONFIG_CMDLINE_BOOL needs to be disabled for my qemu boot to succeed :) :)
So, yes, patched allyesconfig kernel seems to boot... it takes 250 seconds in qemu.
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