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Date: Thu, 21 May 2015 08:26:12 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Denys Vlasenko <dvlasenk@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
Josh Triplett <josh@...htriplett.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <laijs@...fujitsu.com>,
linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH] rcu: Deinline rcu_read_lock_sched_held() if
DEBUG_LOCK_ALLOC
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@...hat.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@...hat.com>
> >> CC: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> >> CC: Josh Triplett <josh@...htriplett.org>
> >> CC: Steven Rostedt <rostedt@...dmis.org>
> >> CC: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> >> CC: Lai Jiangshan <laijs@...fujitsu.com>
> >> CC: linux-kernel@...r.kernel.org
> >> CC: Tejun Heo <tj@...nel.org>
> >> CC: Oleg Nesterov <oleg@...hat.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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