[PATCH] microoptimize by trying to avoid locking a locked mutex

[Konstantin Belousov]

On Thu, Nov 05, 2015 at 12:32:18AM +0100, Mateusz Guzik wrote:
> mtx_lock will unconditionally try to grab the lock and if that fails,
> will call __mtx_lock_sleep which will immediately try to do the same
> atomic op again.
> 
> So, the obvious microoptimization is to check the state in
> __mtx_lock_sleep and avoid the operation if the lock is not free.
> 
> This gives me ~40% speedup in a microbenchmark of 40 find processes
> traversing tmpfs and contending on mount mtx (only used as an easy
> benchmark, I have WIP patches to get rid of it).
> 
> Second part of the patch is optional and just checks the state of the
> lock prior to doing any atomic operations, but it gives a very modest
> speed up when applied on top of the __mtx_lock_sleep change. As such,
> I'm not going to defend this part.
Shouldn't the same consideration applied to all spinning loops, i.e.
also to the spin/thread mutexes, and to the spinning parts of sx and
lockmgr ?

> 
> x vanilla
> + patched
> +--------------------------------------------------------------------------------------------------------------------------------------+
> |               +                                                                                                                      |
> |+       ++  ++ ++   +                                                                         x        x x           xx xx        x  x|
> |      |_____AM____|                                                                                    |____________A_M__________|    |
> +--------------------------------------------------------------------------------------------------------------------------------------+
>     N           Min           Max        Median           Avg        Stddev
> x   9        13.845        16.148        15.271     15.133889    0.75997096
> +   9         8.363          9.56         9.126     9.0643333    0.34198501
> Difference at 95.0% confidence
> 	-6.06956 +/- 0.588917
> 	-40.1057% +/- 3.89138%
> 	(Student's t, pooled s = 0.589283)
> 
> x patched
> + patched2
> +--------------------------------------------------------------------------------------------------------------------------------------+
> |                                                                         +                                                            |
> |+                                                    * +    +         +  +           x x        + + x   x     x x  x                 x|
> |                                   |____________________________A_____M______|_______________|______A___M__________________|          |
> +--------------------------------------------------------------------------------------------------------------------------------------+
>     N           Min           Max        Median           Avg        Stddev
> x   9         8.363          9.56         9.126     9.0643333    0.34198501
> +   9         7.563         9.038         8.611     8.5256667    0.43365885
> Difference at 95.0% confidence
> 	-0.538667 +/- 0.390278
> 	-5.94271% +/- 4.30565%
> 	(Student's t, pooled s = 0.390521)
> 
> diff --git a/sys/kern/kern_mutex.c b/sys/kern/kern_mutex.c
> index bec8f6b..092aaae 100644
> --- a/sys/kern/kern_mutex.c
> +++ b/sys/kern/kern_mutex.c
> @@ -419,7 +419,10 @@ __mtx_lock_sleep(volatile uintptr_t *c, uintptr_t tid, int opts,
>  	all_time -= lockstat_nsecs(&m->lock_object);
>  #endif
>  
> -	while (!_mtx_obtain_lock(m, tid)) {
> +	for (;;) {
> +		v = m->mtx_lock;
> +		if (v == MTX_UNOWNED && _mtx_obtain_lock(m, tid))
> +			break;
>  #ifdef KDTRACE_HOOKS
>  		spin_cnt++;
>  #endif
> @@ -428,7 +431,6 @@ __mtx_lock_sleep(volatile uintptr_t *c, uintptr_t tid, int opts,
>  		 * If the owner is running on another CPU, spin until the
>  		 * owner stops running or the state of the lock changes.
>  		 */
> -		v = m->mtx_lock;
>  		if (v != MTX_UNOWNED) {
You could restructure the code to only do one comparision with MTX_UNOWNED,
effectively using 'else' instead of the if() on the previous line.

>  			owner = (struct thread *)(v & ~MTX_FLAGMASK);
>  			if (TD_IS_RUNNING(owner)) {
> diff --git a/sys/sys/mutex.h b/sys/sys/mutex.h
> index a9ec072..4208d5f 100644
> --- a/sys/sys/mutex.h
> +++ b/sys/sys/mutex.h
> @@ -184,12 +184,11 @@ void	thread_lock_flags_(struct thread *, int, const char *, int);
>  /* Lock a normal mutex. */
>  #define __mtx_lock(mp, tid, opts, file, line) do {			\
>  	uintptr_t _tid = (uintptr_t)(tid);				\
> -									\
> -	if (!_mtx_obtain_lock((mp), _tid))				\
> -		_mtx_lock_sleep((mp), _tid, (opts), (file), (line));	\
> -	else								\
> +	if (((mp)->mtx_lock == MTX_UNOWNED && _mtx_obtain_lock((mp), _tid))) \
>  		LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(adaptive__acquire,	\
>  		    mp, 0, 0, file, line);				\
> +	else								\
> +		_mtx_lock_sleep((mp), _tid, (opts), (file), (line));	\
>  } while (0)
>  
>  /*
> -- 
> Mateusz Guzik <mjguzik gmail.com>
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