Re: [patch] notifiers: fix blocking_notifier_call_chain() scalability

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2007-01-23 at 10:45 +0100, Ingo Molnar wrote:
> Subject: [patch] notifiers: fix blocking_notifier_call_chain() scalability
> From: Ingo Molnar <mingo@elte.hu>
> 
> while lock-profiling the -rt kernel i noticed weird contention during 
> mmap-intense workloads, and the tracer showed the following gem, in one 
> of our MM hotpaths:
> 
>  threaded-2771  1....   65us : sys_munmap (sysenter_do_call)
>  threaded-2771  1....   66us : profile_munmap (sys_munmap)
>  threaded-2771  1....   66us : blocking_notifier_call_chain (profile_munmap)
>  threaded-2771  1....   66us : rt_down_read (blocking_notifier_call_chain)
> 
> ouch! a global rw-semaphore taken in one of the most 
> performance-sensitive codepaths of the kernel. And i dont even have 
> oprofile enabled! All distro kernels have CONFIG_PROFILING enabled, so 
> this scalability problem affects the majority of Linux users.
> 
> The fix is to enhance blocking_notifier_call_chain() to only take the 
> lock if there appears to be work on the call-chain.
> 
> With this patch applied i get nicely saturated system, and much higher 
> munmap performance, on SMP systems.
> 
> And as a bonus this also fixes a similar scalability bottleneck in the 
> thread-exit codepath: profile_task_exit() ...
> 
> Signed-off-by: Ingo Molnar <mingo@elte.hu>

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

> ---
>  kernel/sys.c |   15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> Index: linux/kernel/sys.c
> ===================================================================
> --- linux.orig/kernel/sys.c
> +++ linux/kernel/sys.c
> @@ -325,11 +325,18 @@ EXPORT_SYMBOL_GPL(blocking_notifier_chai
>  int blocking_notifier_call_chain(struct blocking_notifier_head *nh,
>  		unsigned long val, void *v)
>  {
> -	int ret;
> +	int ret = NOTIFY_DONE;
>  
> -	down_read(&nh->rwsem);
> -	ret = notifier_call_chain(&nh->head, val, v);
> -	up_read(&nh->rwsem);
> +	/*
> +	 * We check the head outside the lock, but if this access is
> +	 * racy then it does not matter what the result of the test
> +	 * is, we re-check the list after having taken the lock anyway:
> +	 */
> +	if (rcu_dereference(nh->head)) {
> +		down_read(&nh->rwsem);
> +		ret = notifier_call_chain(&nh->head, val, v);
> +		up_read(&nh->rwsem);
> +	}
>  	return ret;
>  }