Hi, I created an epoll wakeup test project, listed some possible cases, and any other corner cases needs to be added? https://github.com/heiher/epoll-wakeup/blob/master/README.md On Wed, Sep 4, 2019 at 10:02 PM Heiher <r@xxxxxx> wrote: > > Hi, > > On Wed, Sep 4, 2019 at 8:02 PM Jason Baron <jbaron@xxxxxxxxxx> wrote: > > > > > > > > On 9/4/19 5:57 AM, Roman Penyaev wrote: > > > On 2019-09-03 23:08, Jason Baron wrote: > > >> On 9/2/19 11:36 AM, Roman Penyaev wrote: > > >>> Hi, > > >>> > > >>> This is indeed a bug. (quick side note: could you please remove efd[1] > > >>> from your test, because it is not related to the reproduction of a > > >>> current bug). > > >>> > > >>> Your patch lacks a good description, what exactly you've fixed. Let > > >>> me speak out loud and please correct me if I'm wrong, my understanding > > >>> of epoll internals has become a bit rusty: when epoll fds are nested > > >>> an attempt to harvest events (ep_scan_ready_list() call) produces a > > >>> second (repeated) event from an internal fd up to an external fd: > > >>> > > >>> epoll_wait(efd[0], ...): > > >>> ep_send_events(): > > >>> ep_scan_ready_list(depth=0): > > >>> ep_send_events_proc(): > > >>> ep_item_poll(): > > >>> ep_scan_ready_list(depth=1): > > >>> ep_poll_safewake(): > > >>> ep_poll_callback() > > >>> list_add_tail(&epi, &epi->rdllist); > > >>> ^^^^^^ > > >>> repeated event > > >>> > > >>> > > >>> In your patch you forbid wakeup for the cases, where depth != 0, i.e. > > >>> for all nested cases. That seems clear. But what if we can go further > > >>> and remove the whole chunk, which seems excessive: > > >>> > > >>> @@ -885,26 +886,11 @@ static __poll_t ep_scan_ready_list(struct > > >>> eventpoll *ep, > > >>> > > >>> - > > >>> - if (!list_empty(&ep->rdllist)) { > > >>> - /* > > >>> - * Wake up (if active) both the eventpoll wait list and > > >>> - * the ->poll() wait list (delayed after we release the > > >>> lock). > > >>> - */ > > >>> - if (waitqueue_active(&ep->wq)) > > >>> - wake_up(&ep->wq); > > >>> - if (waitqueue_active(&ep->poll_wait)) > > >>> - pwake++; > > >>> - } > > >>> write_unlock_irq(&ep->lock); > > >>> > > >>> if (!ep_locked) > > >>> mutex_unlock(&ep->mtx); > > >>> > > >>> - /* We have to call this outside the lock */ > > >>> - if (pwake) > > >>> - ep_poll_safewake(&ep->poll_wait); > > >>> > > >>> > > >>> I reason like that: by the time we've reached the point of scanning events > > >>> for readiness all wakeups from ep_poll_callback have been already fired and > > >>> new events have been already accounted in ready list (ep_poll_callback() > > >>> calls > > >>> the same ep_poll_safewake()). Here, frankly, I'm not 100% sure and probably > > >>> missing some corner cases. > > >>> > > >>> Thoughts? > > >> > > >> So the: 'wake_up(&ep->wq);' part, I think is about waking up other > > >> threads that may be in waiting in epoll_wait(). For example, there may > > >> be multiple threads doing epoll_wait() on the same epoll fd, and the > > >> logic above seems to say thread 1 may have processed say N events and > > >> now its going to to go off to work those, so let's wake up thread 2 now > > >> to handle the next chunk. > > > > > > Not quite. Thread which calls ep_scan_ready_list() processes all the > > > events, and while processing those, removes them one by one from the > > > ready list. But if event mask is !0 and event belongs to > > > Level Triggered Mode descriptor (let's say default mode) it tails event > > > again back to the list (because we are in level mode, so event should > > > be there). So at the end of this traversing loop ready list is likely > > > not empty, and if so, wake up again is called for nested epoll fds. > > > But, those nested epoll fds should get already all the notifications > > > from the main event callback ep_poll_callback(), regardless any thread > > > which traverses events. > > > > > > I suppose this logic exists for decades, when Davide (the author) was > > > reshuffling the code here and there. > > > > > > But I do not feel confidence to state that this extra wakeup is bogus, > > > I just have a gut feeling that it looks excessive. > > > > Note that I was talking about the wakeup done on ep->wq not ep->poll_wait. > > The path that I'm concerned about is let's say that there are N events > > queued on the ready list. A thread that was woken up in epoll_wait may > > decide to only process say N/2 of then. Then it will call wakeup on ep->wq > > and this will wakeup another thread to process the remaining N/2. Without > > the wakeup, the original thread isn't going to process the events until > > it finishes with the original N/2 and gets back to epoll_wait(). So I'm not > > sure how important that path is but I wanted to at least note the change > > here would impact that behavior. > > > > Thanks, > > > > -Jason > > > > > > > > > >> So I think removing all that even for the > > >> depth 0 case is going to change some behavior here. So perhaps, it > > >> should be removed for all depths except for 0? And if so, it may be > > >> better to make 2 patches here to separate these changes. > > >> > > >> For the nested wakeups, I agree that the extra wakeups seem unnecessary > > >> and it may make sense to remove them for all depths. I don't think the > > >> nested epoll semantics are particularly well spelled out, and afaict, > > >> nested epoll() has behaved this way for quite some time. And the current > > >> behavior is not bad in the way that a missing wakeup or false negative > > >> would be. > > > > > > That's 100% true! For edge mode extra wake up is not a bug, not optimal > > > for userspace - yes, but that can't lead to any lost wakeups. > > > > > > -- > > > Roman > > > > > I tried to remove the whole chunk of code that Roman said, and it > seems that there > are no obvious problems with the two test programs below: > > Test case 1: > t0 > | > e0 > | > e1 (et) > | > s0 (lt) > > When s0 is readable, the thread 0 can only read once event from e0. > > #include <stdio.h> > #include <unistd.h> > #include <sys/epoll.h> > #include <sys/socket.h> > > int main(int argc, char *argv[]) > { > int sfd[2]; > int efd[2]; > int nfds; > struct epoll_event e; > > if (socketpair(AF_UNIX, SOCK_STREAM, 0, sfd) < 0) > goto out; > > efd[0] = epoll_create(1); > if (efd[0] < 0) > goto out; > > efd[1] = epoll_create(1); > if (efd[1] < 0) > goto out; > > e.events = EPOLLIN; > if (epoll_ctl(efd[1], EPOLL_CTL_ADD, sfd[0], &e) < 0) > goto out; > > e.events = EPOLLIN | EPOLLET; > if (epoll_ctl(efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0) > goto out; > > if (write(sfd[1], "w", 1) != 1) > goto out; > > nfds = epoll_wait(efd[0], &e, 1, 0); > if (nfds != 1) > goto out; > > nfds = epoll_wait(efd[0], &e, 1, 0); > if (nfds != 0) > goto out; > > nfds = epoll_wait(efd[1], &e, 1, 0); > if (nfds != 1) > goto out; > > nfds = epoll_wait(efd[1], &e, 1, 0); > if (nfds != 1) > goto out; > > close(efd[1]); > close(efd[0]); > close(sfd[0]); > close(sfd[1]); > > printf("PASS\n"); > return 0; > > out: > printf("FAIL\n"); > return -1; > } > > Test case 2: > t0 t1 > \ / > e0 > / \ > (et) e1 e2 (et) > | | > (lt) s0 s2 (lt) > > When s0 and s2 are readable, both thread 0 and thread 1 can read an > event from e0. > > #include <stdio.h> > #include <unistd.h> > #include <pthread.h> > #include <sys/epoll.h> > #include <sys/socket.h> > > static int efd[3]; > static int sfd[4]; > static int count; > > static void * > thread_handler(void *data) > { > struct epoll_event e; > > if (epoll_wait(efd[0], &e, 1, -1) == 1) > count++; > > return NULL; > } > > static void * > emit_handler(void *data) > { > usleep (100000); > > write(sfd[1], "w", 1); > write(sfd[3], "w", 1); > > return NULL; > } > > int main(int argc, char *argv[]) > { > struct epoll_event e; > pthread_t tw, te; > > if (socketpair(AF_UNIX, SOCK_STREAM, 0, &sfd[0]) < 0) > goto out; > > if (socketpair(AF_UNIX, SOCK_STREAM, 0, &sfd[2]) < 0) > goto out; > > efd[0] = epoll_create(1); > if (efd[0] < 0) > goto out; > > efd[1] = epoll_create(1); > if (efd[1] < 0) > goto out; > > efd[2] = epoll_create(1); > if (efd[2] < 0) > goto out; > > e.events = EPOLLIN; > if (epoll_ctl(efd[1], EPOLL_CTL_ADD, sfd[0], &e) < 0) > goto out; > > e.events = EPOLLIN; > if (epoll_ctl(efd[2], EPOLL_CTL_ADD, sfd[2], &e) < 0) > goto out; > > e.events = EPOLLIN | EPOLLET; > if (epoll_ctl(efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0) > goto out; > > e.events = EPOLLIN | EPOLLET; > if (epoll_ctl(efd[0], EPOLL_CTL_ADD, efd[2], &e) < 0) > goto out; > > if (pthread_create(&tw, NULL, thread_handler, NULL) < 0) > goto out; > > if (pthread_create(&te, NULL, emit_handler, NULL) < 0) > goto out; > > if (epoll_wait(efd[0], &e, 1, -1) == 1) > count++; > > if (pthread_join(tw, NULL) < 0) > goto out; > > if (count != 2) > goto out; > > close(efd[0]); > close(efd[1]); > close(efd[2]); > close(sfd[0]); > close(sfd[1]); > close(sfd[2]); > close(sfd[3]); > > printf ("PASS\n"); > return 0; > > out: > printf("FAIL\n"); > return -1; > } > > t0: thread0 > t1: thread1 > e0: epoll0 (efd[0]) > e1: epoll1 (efd[1]) > e2: epoll2 (efd[2]) > s0: socket0 (sfd[0]) > s2: socket2 (sfd[2]) > > Is it possible to prove that this modification is correct, or any > other corner cases are missing? > > -- > Best regards! > Hev > https://hev.cc -- Best regards! Hev https://hev.cc
