Re: [PATCH v2 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN

[Jason Baron]

On 02/18/2015 03:07 AM, Ingo Molnar wrote:
> * Jason Baron <jbaron@xxxxxxxxxx> wrote:
>
>> Epoll file descriptors that are added to a shared wakeup 
>> source are always added in a non-exclusive manner. That 
>> means that when we have multiple epoll fds attached to a 
>> shared wakeup source they are all woken up. This can lead 
>> to excessive cpu usage and uneven load distribution.
>>
>> This patch introduces two new 'events' flags that are 
>> intended to be used with EPOLL_CTL_ADD operations. 
>> EPOLLEXCLUSIVE, adds the epoll fd to the event source in 
>> an exclusive manner such that the minimum number of 
>> threads are woken. EPOLLROUNDROBIN, which depends on 
>> EPOLLEXCLUSIVE also being set, can also be added to the 
>> 'events' flag, such that we round robin through the set 
>> of waiting threads.
>>
>> An implementation note is that in the epoll wakeup 
>> routine, 'ep_poll_callback()', if EPOLLROUNDROBIN is set, 
>> we return 1, for a successful wakeup, only when there are 
>> current waiters. The idea is to use this additional 
>> heuristic in order minimize wakeup latencies.
>>
>> Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>
>> ---
>>  fs/eventpoll.c                 | 25 ++++++++++++++++++++-----
>>  include/uapi/linux/eventpoll.h |  6 ++++++
>>  2 files changed, 26 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
>> index d77f944..382c832 100644
>> --- a/fs/eventpoll.c
>> +++ b/fs/eventpoll.c
>> @@ -92,7 +92,8 @@
>>   */
>>  
>>  /* Epoll private bits inside the event mask */
>> -#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET)
>> +#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET | \
>> +			 EPOLLEXCLUSIVE | EPOLLROUNDROBIN)
>>  
>>  /* Maximum number of nesting allowed inside epoll sets */
>>  #define EP_MAX_NESTS 4
>> @@ -1002,6 +1003,7 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
>>  	unsigned long flags;
>>  	struct epitem *epi = ep_item_from_wait(wait);
>>  	struct eventpoll *ep = epi->ep;
>> +	int ewake = 0;
>>  
>>  	if ((unsigned long)key & POLLFREE) {
>>  		ep_pwq_from_wait(wait)->whead = NULL;
>> @@ -1066,8 +1068,10 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
>>  	 * Wake up ( if active ) both the eventpoll wait list and the ->poll()
>>  	 * wait list.
>>  	 */
>> -	if (waitqueue_active(&ep->wq))
>> +	if (waitqueue_active(&ep->wq)) {
>> +		ewake = 1;
>>  		wake_up_locked(&ep->wq);
>> +	}
>>  	if (waitqueue_active(&ep->poll_wait))
>>  		pwake++;
>>  
>> @@ -1078,6 +1082,8 @@ out_unlock:
>>  	if (pwake)
>>  		ep_poll_safewake(&ep->poll_wait);
>>  
>> +	if (epi->event.events & EPOLLROUNDROBIN)
>> +		return ewake;
>>  	return 1;
>>  }
>>  
>> @@ -1095,7 +1101,12 @@ static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
>>  		init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
>>  		pwq->whead = whead;
>>  		pwq->base = epi;
>> -		add_wait_queue(whead, &pwq->wait);
>> +		if (epi->event.events & EPOLLROUNDROBIN)
>> +			add_wait_queue_rr(whead, &pwq->wait);
>> +		else if (epi->event.events & EPOLLEXCLUSIVE)
>> +			add_wait_queue_exclusive(whead, &pwq->wait);
>> +		else
>> +			add_wait_queue(whead, &pwq->wait);
>>  		list_add_tail(&pwq->llink, &epi->pwqlist);
>>  		epi->nwait++;
>>  	} else {
>> @@ -1820,8 +1831,7 @@ SYSCALL_DEFINE1(epoll_create, int, size)
>>  SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
>>  		struct epoll_event __user *, event)
>>  {
>> -	int error;
>> -	int full_check = 0;
>> +	int error, full_check = 0, wait_flags = 0;
>>  	struct fd f, tf;
>>  	struct eventpoll *ep;
>>  	struct epitem *epi;
>> @@ -1861,6 +1871,11 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
>>  	if (f.file == tf.file || !is_file_epoll(f.file))
>>  		goto error_tgt_fput;
>>  
>> +	wait_flags = epds.events & (EPOLLEXCLUSIVE | EPOLLROUNDROBIN);
>> +	if (wait_flags && ((op == EPOLL_CTL_MOD) || ((op == EPOLL_CTL_ADD) &&
>> +	    ((wait_flags == EPOLLROUNDROBIN) || (is_file_epoll(tf.file))))))
>> +		goto error_tgt_fput;
>> +
>>  	/*
>>  	 * At this point it is safe to assume that the "private_data" contains
>>  	 * our own data structure.
>> diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
>> index bc81fb2..10260a1 100644
>> --- a/include/uapi/linux/eventpoll.h
>> +++ b/include/uapi/linux/eventpoll.h
>> @@ -26,6 +26,12 @@
>>  #define EPOLL_CTL_DEL 2
>>  #define EPOLL_CTL_MOD 3
>>  
>> +/* Balance wakeups for a shared event source */
>> +#define EPOLLROUNDROBIN (1 << 27)
>> +
>> +/* Add exclusively */
>> +#define EPOLLEXCLUSIVE (1 << 28)
>> +
>>  /*
>>   * Request the handling of system wakeup events so as to prevent system suspends
>>   * from happening while those events are being processed.
> So let me rephrase the justification of your two patches:
>
> Unlike regular waitqueue usage (where threads remove 
> themselves from the waitqueue once they receive a wakeup), 
> epoll waitqueues are static and 'persistent': epoll target 
> threads are on the poll waitqueue indefinitely, only 
> register/unregister removes threads from them.
>
> So they are not really 'wait queues', but static 'task 
> lists', and are thus exposed to classic thundering herd 
> scheduling problems and scheduling assymetries: a single 
> event on a shared event source will wake all epoll 
> 'task-lists', and not only will it wake them, but due to 
> the static nature of the lists, even an exclusive wakeup 
> will iterate along the list with O(N) overhead, until it 
> finds a wakeable thread.
>
> As the number of lists and the number of threads in the 
> lists increases this scales suboptimally, and it also looks 
> slightly odd that a random set of epoll worker threads is 
> 'more equal' than the others, in receiving a comparably 
> higher proportion of events.

yes,  in fact we are currently working around these imbalances
by doing register/unregister (EPOLL_CTL_ADD/EPOLL_CTL_DEL),
periodically to re-set the order of the queues. This resolves the
balancing to an extent, but not the spurious wakeups.

>
> The solution is to add this new ABI to allow epoll events 
> to be actively load-balanced both between the persistent 
> 'task lists', and to also allow the individual task lists 
> to act as dynamic runqueues: the head of the list is likely 
> to be sleeping, newly woken tasks get moved to the tail of 
> the list.
>
> This has two main advantages: firstly it solves the O(N) 
> (micro-)problem, but it also more evenly distributes events 
> both between task-lists and within epoll groups as tasks as 
> well.

Its solving 2 issues - spurious wakeups, and more
even loading of threads. The event distribution is more
even between 'epoll groups' with this patch, however,
if multiple threads are blocking on a single 'epoll group',
this patch does not affect the the event distribution there.
Currently, threads are added to 'epoll group' as exclusive
already, so when you have multiple threads blocking on
an epoll group, only one wakes up. In our use case, we
have a 1-to-1 mapping b/w threads and epoll groups, so
we don't have spurious or un-balanced wakeups there.

That suggests the alternative user-space model for
addressing this problem. That is, to have a single epoll
group added with EPOLLONESHOT. In this way threads
can pull work or events off of a single queue, work on
the event and then re-arm (such that other threads don't
see events from that source in the meantime).
This, however, means all threads work on all events, and
they all have to synchronize to an extent on the single queue.
That is all register/unregister and re-arm event to that queue
have to be visible or synchronized for all the waiters. This
model also doesn't allow userspace to partition events that
are naturally local to thread, since there a single epoll group.

The second userspace model is to have worker threads with
their own separate epoll groups, and then have separate
thread(s) to address the shared wakeup sources. Then the
threads that are waiting on the shared wakeup sources can
distribute the events fairly to the worker threads. This involves
extra context switching for shared events, and I think ends up
degenerating back into the original problem.

> The disadvantages: slightly higher management micro-costs, 
> plus a global waitqueue list, which used to be read-mostly, 
> is now actively dirtied by every event, adding more global 
> serialization. The latter is somewhat muted by the fact 
> that the waitqueue lock itself is already a global 
> serialization point today and got dirtied by every event, 
> and the list head is next to it, usually in the same 
> cacheline.

Yes, I'm a bit concerned about the changes to the core
wakeup function, however in the non-rotate case, the
only additional write is to initialize the 'rotate_list' on entry.
I measured the latency of the __wake_up_common() for
the case where this code was added and we were not
doing 'rotate', and I didn't measure any additional latency
with ftrace, but it perhaps warrants more careful testing.

The outstanding issues I have are:

1) Does epoll need 2 new flags here - EPOLLEXCLUSIVE and
EPOLLROUNDROBIN. IE should they just be combined since
EPOLLROUNDROBIN depends on EPOLLEXCLUSIVE, or is there
a valid use for just EPOLLEXCLUSIVE (wake up the first waiter
but don't do the balancing)?

2) The concern Andy raised regarding potential starvation.
That is could a adversarial thread cause us to miss wakeups
if it can add itself exclusively to the shared wakeup source.
Currently, the adversarial thread would need to simply be
able to open the file in question. For things like pipe, this is
not an issue, but perhaps it is for files in the global
namespace...

Thanks,

-Jason
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