Re: [PATCH 2/2 v2] epoll: Do not take global 'epmutex' for simpletopologies

[Paul E. McKenney]

On Tue, Oct 01, 2013 at 05:08:14PM +0000, Jason Baron wrote:
> When calling EPOLL_CTL_ADD for an epoll file descriptor that is attached
> directly to a wakeup source, we do not need to take the global 'epmutex',
> unless the epoll file descriptor is nested. The purpose of taking
> the 'epmutex' on add is to prevent complex topologies such as loops and
> deep wakeup paths from forming in parallel through multiple EPOLL_CTL_ADD
> operations. However, for the simple case of an epoll file descriptor
> attached directly to a wakeup source (with no nesting), we do not need
> to hold the 'epmutex'.
> 
> This patch along with 'epoll: optimize EPOLL_CTL_DEL using rcu' improves
> scalability on larger systems. Quoting Nathan Zimmer's mail on SPECjbb
> performance:
> 
> "
> On the 16 socket run the performance went from 35k jOPS to 125k jOPS.
> In addition the benchmark when from scaling well on 10 sockets to scaling well
> on just over 40 sockets.
> 
> ...
> 
> Currently the benchmark stops scaling at around 40-44 sockets but it seems like
> I found a second unrelated bottleneck.
> "

Some questions and comments below.

> Tested-by: Nathan Zimmer <nzimmer@xxxxxxx>
> Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>
> ---
>  fs/eventpoll.c | 94 ++++++++++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 68 insertions(+), 26 deletions(-)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index dd9fae1..0f25162 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -595,8 +595,7 @@ static inline void ep_pm_stay_awake_rcu(struct epitem *epi)
>  static int ep_scan_ready_list(struct eventpoll *ep,
>  			      int (*sproc)(struct eventpoll *,
>  					   struct list_head *, void *),
> -			      void *priv,
> -			      int depth)
> +			      void *priv, int depth, int ep_locked)
>  {
>  	int error, pwake = 0;
>  	unsigned long flags;
> @@ -607,7 +606,9 @@ static int ep_scan_ready_list(struct eventpoll *ep,
>  	 * We need to lock this because we could be hit by
>  	 * eventpoll_release_file() and epoll_ctl().
>  	 */
> -	mutex_lock_nested(&ep->mtx, depth);
> +
> +	if (!ep_locked)
> +		mutex_lock_nested(&ep->mtx, depth);
> 
>  	/*
>  	 * Steal the ready list, and re-init the original one to the
> @@ -671,7 +672,8 @@ static int ep_scan_ready_list(struct eventpoll *ep,
>  	}
>  	spin_unlock_irqrestore(&ep->lock, flags);
> 
> -	mutex_unlock(&ep->mtx);
> +	if (!ep_locked)
> +		mutex_unlock(&ep->mtx);
> 
>  	/* We have to call this outside the lock */
>  	if (pwake)

OK, allowing calls to ep_scan_ready_list() to be made under the lock.

Usually you would use a __ep_scan_ready_list() instead of adding the
flag, but you do have the wakeup that needs to be outside of the lock.

But doesn't that mean that you need something like?

	if (pwake && !ep_locked)

And then wouldn't you also need some way to inform the caller to
do the wakeup after releasing ep->mtx?

Or is it now safe to do the wakeup under ep->mtx?  If so, why?

> @@ -829,15 +831,34 @@ static int ep_read_events_proc(struct eventpoll *ep, struct list_head *head,
>  	return 0;
>  }
> 
> +static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
> +				 poll_table *pt);
> +
> +struct readyevents_arg {
> +	struct eventpoll *ep;
> +	int locked;
> +};
> +
>  static int ep_poll_readyevents_proc(void *priv, void *cookie, int call_nests)
>  {

OK, I will bite...  What is constraining the arguments of the
ep_poll_readyevents_proc()?  I don't see any use of it except in this
file.  Nor do I see any use of ep_call_nested() except in this file.
Might it be a bit cleaner to add a flag to the argument list?

I don't feel strongly about this, just asking the question.

> -	return ep_scan_ready_list(priv, ep_read_events_proc, NULL, call_nests + 1);
> +	struct readyevents_arg *arg = (struct readyevents_arg *)priv;
> +
> +	return ep_scan_ready_list(arg->ep, ep_read_events_proc, NULL,
> +				  call_nests + 1, arg->locked);
>  }
> 
>  static unsigned int ep_eventpoll_poll(struct file *file, poll_table *wait)
>  {
>  	int pollflags;
>  	struct eventpoll *ep = file->private_data;
> +	struct readyevents_arg arg;
> +
> +	/*
> +	 * During ep_insert() we already hold the ep->mtx for the tfile.
> +	 * Prevent re-aquisition.
> +	 */
> +	arg.locked = ((wait && (wait->_qproc == ep_ptable_queue_proc)) ? 1 : 0);
> +	arg.ep = ep;
> 
>  	/* Insert inside our poll wait queue */
>  	poll_wait(file, &ep->poll_wait, wait);
> @@ -849,7 +870,7 @@ static unsigned int ep_eventpoll_poll(struct file *file, poll_table *wait)
>  	 * could re-enter here.
>  	 */
>  	pollflags = ep_call_nested(&poll_readywalk_ncalls, EP_MAX_NESTS,
> -				   ep_poll_readyevents_proc, ep, ep, current);
> +				   ep_poll_readyevents_proc, &arg, ep, current);
> 
>  	return pollflags != -1 ? pollflags : 0;
>  }
> @@ -1250,7 +1271,7 @@ static noinline void ep_destroy_wakeup_source(struct epitem *epi)
>   * Must be called with "mtx" held.
>   */
>  static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
> -		     struct file *tfile, int fd)
> +		     struct file *tfile, int fd, int full_check)
>  {
>  	int error, revents, pwake = 0;
>  	unsigned long flags;
> @@ -1318,7 +1339,7 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
> 
>  	/* now check if we've created too many backpaths */
>  	error = -EINVAL;
> -	if (reverse_path_check())
> +	if (full_check && reverse_path_check())
>  		goto error_remove_epi;
> 
>  	/* We have to drop the new item inside our item list to keep track of it */
> @@ -1542,7 +1563,7 @@ static int ep_send_events(struct eventpoll *ep,
>  	esed.maxevents = maxevents;
>  	esed.events = events;
> 
> -	return ep_scan_ready_list(ep, ep_send_events_proc, &esed, 0);
> +	return ep_scan_ready_list(ep, ep_send_events_proc, &esed, 0, 0);
>  }
> 
>  static inline struct timespec ep_set_mstimeout(long ms)
> @@ -1813,11 +1834,12 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
>  		struct epoll_event __user *, event)
>  {
>  	int error;
> -	int did_lock_epmutex = 0;
> +	int full_check = 0;
>  	struct fd f, tf;
>  	struct eventpoll *ep;
>  	struct epitem *epi;
>  	struct epoll_event epds;
> +	struct eventpoll *tep = NULL;
> 
>  	error = -EFAULT;
>  	if (ep_op_has_event(op) &&
> @@ -1866,23 +1888,40 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
>  	 * and hang them on the tfile_check_list, so we can check that we
>  	 * haven't created too many possible wakeup paths.
>  	 *
> -	 * We need to hold the epmutex across ep_insert to prevent
> -	 * multple adds from creating loops in parallel.
> +	 * We do not need to take the global 'epumutex' on EPOLL_CTL_ADD when
> +	 * the epoll file descriptor is attaching directly to a wakeup source,
> +	 * unless the epoll file descriptor is nested. The purpose of taking the
> +	 * 'epmutex' on add is to prevent complex toplogies such as loops and
> +	 * deep wakeup paths from forming in parallel through multiple
> +	 * EPOLL_CTL_ADD operations.
>  	 */
> +	mutex_lock_nested(&ep->mtx, 0);
>  	if (op == EPOLL_CTL_ADD) {
> -		mutex_lock(&epmutex);
> -		did_lock_epmutex = 1;
> -		if (is_file_epoll(tf.file)) {
> -			error = -ELOOP;
> -			if (ep_loop_check(ep, tf.file) != 0) {
> -				clear_tfile_check_list();
> -				goto error_tgt_fput;
> +		if (!list_empty(&f.file->f_ep_links) ||
> +						is_file_epoll(tf.file)) {
> +			full_check = 1;
> +			mutex_unlock(&ep->mtx);
> +			mutex_lock(&epmutex);
> +			if (is_file_epoll(tf.file)) {
> +				error = -ELOOP;
> +				if (ep_loop_check(ep, tf.file) != 0) {
> +					clear_tfile_check_list();
> +					goto error_tgt_fput;
> +				}
> +			} else
> +				list_add(&tf.file->f_tfile_llink,
> +							&tfile_check_list);
> +			mutex_lock_nested(&ep->mtx, 0);
> +			if (is_file_epoll(tf.file)) {
> +				tep = tf.file->private_data;
> +				mutex_lock_nested(&tep->mtx, 1);
>  			}
> -		} else
> -			list_add(&tf.file->f_tfile_llink, &tfile_check_list);
> +		}
> +	}
> +	if (op == EPOLL_CTL_DEL && is_file_epoll(tf.file)) {
> +		tep = tf.file->private_data;
> +		mutex_lock_nested(&tep->mtx, 1);
>  	}
> -
> -	mutex_lock_nested(&ep->mtx, 0);
> 
>  	/*
>  	 * Try to lookup the file inside our RB tree, Since we grabbed "mtx"
> @@ -1896,10 +1935,11 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
>  	case EPOLL_CTL_ADD:
>  		if (!epi) {
>  			epds.events |= POLLERR | POLLHUP;
> -			error = ep_insert(ep, &epds, tf.file, fd);
> +			error = ep_insert(ep, &epds, tf.file, fd, full_check);
>  		} else
>  			error = -EEXIST;
> -		clear_tfile_check_list();
> +		if (full_check)
> +			clear_tfile_check_list();
>  		break;
>  	case EPOLL_CTL_DEL:
>  		if (epi)
> @@ -1915,10 +1955,12 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
>  			error = -ENOENT;
>  		break;
>  	}
> +	if (tep != NULL)
> +		mutex_unlock(&tep->mtx);
>  	mutex_unlock(&ep->mtx);
> 
>  error_tgt_fput:
> -	if (did_lock_epmutex)
> +	if (full_check)
>  		mutex_unlock(&epmutex);
> 
>  	fdput(tf);
> -- 
> 1.8.2
> 
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