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Date: Thu, 21 May 2015 23:53:21 +0200
From: Denys Vlasenko <dvlasenk@...hat.com>
To: paulmck@...ux.vnet.ibm.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 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@...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?
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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