Date: Thu, 21 May 2015 09:45:41 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <laijs@cn.fujitsu.com>, linux-kernel@vger.kernel.org,
        Tejun Heo <tj@kernel.org>, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH] rcu: Deinline rcu_read_lock_sched_held() if
 DEBUG_LOCK_ALLOC
Message-ID: <20150521094541.0eb0ce34@gandalf.local.home>
In-Reply-To: <1432202647-20201-1-git-send-email-dvlasenk@redhat.com>
References: <1432202647-20201-1-git-send-email-dvlasenk@redhat.com>
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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.


> +#endif

Hmm, you added two #ifdef, and one #endif. How does this even compile??

-- Steve

> +
>  #ifdef CONFIG_PREEMPT_RCU
>  
>  /*

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