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author | Jason Baron <jbaron@akamai.com> | 2017-11-17 15:29:02 -0800 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2017-11-17 16:10:02 -0800 |
commit | 57a173bdf5baab48e8e78825c7366c634acd087c (patch) | |
tree | f55b20df760de9869b11ee620f1e87f5f96f9df6 | |
parent | 2ae928a9441a3b5f13952e1e8a97d03cb23ea603 (diff) | |
download | linux-57a173bdf5baab48e8e78825c7366c634acd087c.tar.bz2 |
epoll: avoid calling ep_call_nested() from ep_poll_safewake()
ep_poll_safewake() is used to wakeup potentially nested epoll file
descriptors. The function uses ep_call_nested() to prevent entering the
same wake up queue more than once, and to prevent excessively deep
wakeup paths (deeper than EP_MAX_NESTS). However, this is not necessary
since we are already preventing these conditions during EPOLL_CTL_ADD.
This saves extra function calls, and avoids taking a global lock during
the ep_call_nested() calls.
I have, however, left ep_call_nested() for the CONFIG_DEBUG_LOCK_ALLOC
case, since ep_call_nested() keeps track of the nesting level, and this
is required by the call to spin_lock_irqsave_nested(). It would be nice
to remove the ep_call_nested() calls for the CONFIG_DEBUG_LOCK_ALLOC
case as well, however its not clear how to simply pass the nesting level
through multiple wake_up() levels without more surgery. In any case, I
don't think CONFIG_DEBUG_LOCK_ALLOC is generally used for production.
This patch, also apparently fixes a workload at Google that Salman Qazi
reported by completely removing the poll_safewake_ncalls->lock from
wakeup paths.
Link: http://lkml.kernel.org/r/1507920533-8812-1-git-send-email-jbaron@akamai.com
Signed-off-by: Jason Baron <jbaron@akamai.com>
Acked-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Salman Qazi <sqazi@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r-- | fs/eventpoll.c | 47 |
1 files changed, 18 insertions, 29 deletions
diff --git a/fs/eventpoll.c b/fs/eventpoll.c index a45360444895..dc15bb02ee2a 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -276,9 +276,6 @@ static DEFINE_MUTEX(epmutex); /* Used to check for epoll file descriptor inclusion loops */ static struct nested_calls poll_loop_ncalls; -/* Used for safe wake up implementation */ -static struct nested_calls poll_safewake_ncalls; - /* Used to call file's f_op->poll() under the nested calls boundaries */ static struct nested_calls poll_readywalk_ncalls; @@ -551,40 +548,21 @@ out_unlock: * this special case of epoll. */ #ifdef CONFIG_DEBUG_LOCK_ALLOC -static inline void ep_wake_up_nested(wait_queue_head_t *wqueue, - unsigned long events, int subclass) + +static struct nested_calls poll_safewake_ncalls; + +static int ep_poll_wakeup_proc(void *priv, void *cookie, int call_nests) { unsigned long flags; + wait_queue_head_t *wqueue = (wait_queue_head_t *)cookie; - spin_lock_irqsave_nested(&wqueue->lock, flags, subclass); - wake_up_locked_poll(wqueue, events); + spin_lock_irqsave_nested(&wqueue->lock, flags, call_nests + 1); + wake_up_locked_poll(wqueue, POLLIN); spin_unlock_irqrestore(&wqueue->lock, flags); -} -#else -static inline void ep_wake_up_nested(wait_queue_head_t *wqueue, - unsigned long events, int subclass) -{ - wake_up_poll(wqueue, events); -} -#endif -static int ep_poll_wakeup_proc(void *priv, void *cookie, int call_nests) -{ - ep_wake_up_nested((wait_queue_head_t *) cookie, POLLIN, - 1 + call_nests); return 0; } -/* - * Perform a safe wake up of the poll wait list. The problem is that - * with the new callback'd wake up system, it is possible that the - * poll callback is reentered from inside the call to wake_up() done - * on the poll wait queue head. The rule is that we cannot reenter the - * wake up code from the same task more than EP_MAX_NESTS times, - * and we cannot reenter the same wait queue head at all. This will - * enable to have a hierarchy of epoll file descriptor of no more than - * EP_MAX_NESTS deep. - */ static void ep_poll_safewake(wait_queue_head_t *wq) { int this_cpu = get_cpu(); @@ -595,6 +573,15 @@ static void ep_poll_safewake(wait_queue_head_t *wq) put_cpu(); } +#else + +static void ep_poll_safewake(wait_queue_head_t *wq) +{ + wake_up_poll(wq, POLLIN); +} + +#endif + static void ep_remove_wait_queue(struct eppoll_entry *pwq) { wait_queue_head_t *whead; @@ -2315,8 +2302,10 @@ static int __init eventpoll_init(void) */ ep_nested_calls_init(&poll_loop_ncalls); +#ifdef CONFIG_DEBUG_LOCK_ALLOC /* Initialize the structure used to perform safe poll wait head wake ups */ ep_nested_calls_init(&poll_safewake_ncalls); +#endif /* Initialize the structure used to perform file's f_op->poll() calls */ ep_nested_calls_init(&poll_readywalk_ncalls); |