Re: [RFC PATCH 1/1] epoll: use rwlock in order to reduce ep_poll_callback() contention

From: Roman Penyaev
Date: Wed Dec 05 2018 - 06:22:30 EST

On 2018-12-04 20:02, Paul E. McKenney wrote:
On Tue, Dec 04, 2018 at 12:23:08PM -0500, Jason Baron wrote:


On 12/3/18 6:02 AM, Roman Penyaev wrote:
> Hi all,
>
> The goal of this patch is to reduce contention of ep_poll_callback() which
> can be called concurrently from different CPUs in case of high events
> rates and many fds per epoll. Problem can be very well reproduced by
> generating events (write to pipe or eventfd) from many threads, while
> consumer thread does polling. In other words this patch increases the
> bandwidth of events which can be delivered from sources to the poller by
> adding poll items in a lockless way to the list.
>
> The main change is in replacement of the spinlock with a rwlock, which is
> taken on read in ep_poll_callback(), and then by adding poll items to the
> tail of the list using xchg atomic instruction. Write lock is taken
> everywhere else in order to stop list modifications and guarantee that list
> updates are fully completed (I assume that write side of a rwlock does not
> starve, it seems qrwlock implementation has these guarantees).
>
> The following are some microbenchmark results based on the test [1] which
> starts threads which generate N events each. The test ends when all
> events are successfully fetched by the poller thread:
>
> spinlock
> ========
>
> threads run time events per ms
> ------- --------- -------------
> 8 13191ms 6064/ms
> 16 30758ms 5201/ms
> 32 44315ms 7220/ms
>
> rwlock + xchg
> =============
>
> threads run time events per ms
> ------- --------- -------------
> 8 8581ms 9323/ms
> 16 13800ms 11594/ms
> 32 24167ms 13240/ms
>
> According to the results bandwidth of delivered events is significantly
> increased, thus execution time is reduced.
>
> This is RFC because I did not run any benchmarks comparing current
> qrwlock and spinlock implementations (4.19 kernel), although I did
> not notice any epoll performance degradations in other benchmarks.
>
> Also I'm not quite sure where to put very special lockless variant
> of adding element to the list (list_add_tail_lockless() in this
> patch). Seems keeping it locally is safer.
>
> [1] https://github.com/rouming/test-tools/blob/master/stress-epoll.c
>
> Signed-off-by: Roman Penyaev <rpenyaev@xxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
> fs/eventpoll.c | 107 +++++++++++++++++++++++++++++++------------------
> 1 file changed, 69 insertions(+), 38 deletions(-)
>
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index 42bbe6824b4b..89debda47aca 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -50,10 +50,10 @@
> *
> * 1) epmutex (mutex)
> * 2) ep->mtx (mutex)
> - * 3) ep->wq.lock (spinlock)
> + * 3) ep->lock (rwlock)
> *
> * The acquire order is the one listed above, from 1 to 3.
> - * We need a spinlock (ep->wq.lock) because we manipulate objects
> + * We need a rwlock (ep->lock) because we manipulate objects
> * from inside the poll callback, that might be triggered from
> * a wake_up() that in turn might be called from IRQ context.
> * So we can't sleep inside the poll callback and hence we need
> @@ -85,7 +85,7 @@
> * of epoll file descriptors, we use the current recursion depth as
> * the lockdep subkey.
> * It is possible to drop the "ep->mtx" and to use the global
> - * mutex "epmutex" (together with "ep->wq.lock") to have it working,
> + * mutex "epmutex" (together with "ep->lock") to have it working,
> * but having "ep->mtx" will make the interface more scalable.
> * Events that require holding "epmutex" are very rare, while for
> * normal operations the epoll private "ep->mtx" will guarantee
> @@ -182,8 +182,6 @@ struct epitem {
> * This structure is stored inside the "private_data" member of the file
> * structure and represents the main data structure for the eventpoll
> * interface.
> - *
> - * Access to it is protected by the lock inside wq.
> */
> struct eventpoll {
> /*
> @@ -203,13 +201,16 @@ struct eventpoll {
> /* List of ready file descriptors */
> struct list_head rdllist;
>
> + /* Lock which protects rdllist and ovflist */
> + rwlock_t lock;
> +
> /* RB tree root used to store monitored fd structs */
> struct rb_root_cached rbr;
>
> /*
> * This is a single linked list that chains all the "struct epitem" that
> * happened while transferring ready events to userspace w/out
> - * holding ->wq.lock.
> + * holding ->lock.
> */
> struct epitem *ovflist;
>
> @@ -697,17 +698,17 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
> * because we want the "sproc" callback to be able to do it
> * in a lockless way.
> */
> - spin_lock_irq(&ep->wq.lock);
> + write_lock_irq(&ep->lock);
> list_splice_init(&ep->rdllist, &txlist);
> ep->ovflist = NULL;
> - spin_unlock_irq(&ep->wq.lock);
> + write_unlock_irq(&ep->lock);
>
> /*
> * Now call the callback function.
> */
> res = (*sproc)(ep, &txlist, priv);
>
> - spin_lock_irq(&ep->wq.lock);
> + write_lock_irq(&ep->lock);
> /*
> * During the time we spent inside the "sproc" callback, some
> * other events might have been queued by the poll callback.
> @@ -722,7 +723,8 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
> * contain them, and the list_splice() below takes care of them.
> */
> if (!ep_is_linked(epi)) {
> - list_add_tail(&epi->rdllink, &ep->rdllist);
> + /* Reverse ->ovflist, events should be in FIFO */
> + list_add(&epi->rdllink, &ep->rdllist);
> ep_pm_stay_awake(epi);
> }
> }
> @@ -745,11 +747,11 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
> * the ->poll() wait list (delayed after we release the lock).
> */
> if (waitqueue_active(&ep->wq))
> - wake_up_locked(&ep->wq);
> + wake_up(&ep->wq);
> if (waitqueue_active(&ep->poll_wait))
> pwake++;
> }
> - spin_unlock_irq(&ep->wq.lock);
> + write_unlock_irq(&ep->lock);
>
> if (!ep_locked)
> mutex_unlock(&ep->mtx);
> @@ -789,10 +791,10 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
>
> rb_erase_cached(&epi->rbn, &ep->rbr);
>
> - spin_lock_irq(&ep->wq.lock);
> + write_lock_irq(&ep->lock);
> if (ep_is_linked(epi))
> list_del_init(&epi->rdllink);
> - spin_unlock_irq(&ep->wq.lock);
> + write_unlock_irq(&ep->lock);
>
> wakeup_source_unregister(ep_wakeup_source(epi));
> /*
> @@ -842,7 +844,7 @@ static void ep_free(struct eventpoll *ep)
> * Walks through the whole tree by freeing each "struct epitem". At this
> * point we are sure no poll callbacks will be lingering around, and also by
> * holding "epmutex" we can be sure that no file cleanup code will hit
> - * us during this operation. So we can avoid the lock on "ep->wq.lock".
> + * us during this operation. So we can avoid the lock on "ep->lock".
> * We do not need to lock ep->mtx, either, we only do it to prevent
> * a lockdep warning.
> */
> @@ -1023,6 +1025,7 @@ static int ep_alloc(struct eventpoll **pep)
> goto free_uid;
>
> mutex_init(&ep->mtx);
> + rwlock_init(&ep->lock);
> init_waitqueue_head(&ep->wq);
> init_waitqueue_head(&ep->poll_wait);
> INIT_LIST_HEAD(&ep->rdllist);
> @@ -1112,10 +1115,38 @@ struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd,
> }
> #endif /* CONFIG_CHECKPOINT_RESTORE */
>
> +/*
> + * Adds a new entry to the tail of the list in a lockless way, i.e.
> + * multiple CPUs are allowed to call this function concurrently.
> + *
> + * Beware: it is necessary to prevent any other modifications of the
> + * existing list until all changes are completed, in other words
> + * concurrent list_add_tail_lockless() calls should be protected
> + * with a read lock, where write lock acts as a barrier which
> + * makes sure all list_add_tail_lockless() calls are fully
> + * completed.
> + */
> +static inline void list_add_tail_lockless(struct list_head *new,
> + struct list_head *head)
> +{
> + struct list_head *prev;
> +
> + new->next = head;
> + prev = xchg(&head->prev, new);
> + prev->next = new;
> + new->prev = prev;
> +}
> +
> /*
> * This is the callback that is passed to the wait queue wakeup
> * mechanism. It is called by the stored file descriptors when they
> * have events to report.
> + *
> + * This callback takes a read lock in order not to content with concurrent
> + * events from another file descriptors, thus all modifications to ->rdllist
> + * or ->ovflist are lockless. Read lock is paired with the write lock from
> + * ep_scan_ready_list(), which stops all list modifications and guarantees
> + * that lists won't be corrupted.
> */
> static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)
> {
> @@ -1126,7 +1157,7 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
> __poll_t pollflags = key_to_poll(key);
> int ewake = 0;
>
> - spin_lock_irqsave(&ep->wq.lock, flags);
> + read_lock_irqsave(&ep->lock, flags);
>
> ep_set_busy_poll_napi_id(epi);
>
> @@ -1156,8 +1187,8 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
> */
> if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
> if (epi->next == EP_UNACTIVE_PTR) {
> - epi->next = ep->ovflist;
> - ep->ovflist = epi;
> + /* Atomically exchange tail */
> + epi->next = xchg(&ep->ovflist, epi);

This also relies on the fact that the same epi can't be added to the
list in parallel, because the wait queue doing the wakeup will have the
wait_queue_head lock. That was a little confusing for me b/c we only had
the read_lock() above.

I missed this as well.

I was also concerned about "fly-by" wakeups where the to-be-awoken task
never really goes to sleep, but it looks like tasks are unconditionally
queued, which seems like it should avoid that problem.

Please do some testing with artificial delays in the lockless queuing
code, for example, via udelay() or similar. If there are races, this
would help force them to happen.

Yep, that what I am doing right now, experimenting with different
polling scenarios and stressing with random delays. Thanks.

--
Roman