On Thu, Mar 21, 2013 at 8:24 PM, Eric Wong <normalperson@xxxxxxxx> wrote: > Arve Hjønnevåg <arve@xxxxxxxxxxx> wrote: >> On Thu, Mar 21, 2013 at 4:52 AM, Eric Wong <normalperson@xxxxxxxx> wrote: >> > Changes since v2: >> > * epi->state is no longer atomic, we only cmpxchg in ep_poll_callback >> > now and rely on implicit barriers in other places for reading. >> > * intermediate EP_STATE_DEQUEUE removed, this (with xchg) caused too >> > much overhead in the ep_send_events loop and could not eliminate >> > starvation dangers from improper EPOLLET usage (the original code >> > had this problem, too, the window is just a few cycles larger, now). >> > * minor code cleanups > >> > /* >> > * Activate ep->ws before deactivating epi->ws to prevent >> >> Does anything deactivate ep->ws now? > > Oops, I left that out when I killed ep_scan_ready_list. > But I think we need a different approach to wakeup sources in > this series... > >> > + /* >> > + * reset item state for EPOLLONESHOT and EPOLLET >> > + * no barrier here, rely on ep->mtx release for write barrier >> > + */ >> >> What happens if ep_poll_callback runs before you set epi->state below? >> It used to queue on ep->ovflist and call __pm_stay_awake on ep->ws, >> but now it does not appear to do anything. >> >> > + epi->state = EP_STATE_IDLE; >> > } >> > >> > return eventcnt; >> > } >> > > > With EPOLLET and improper usage (not hitting EAGAIN), the event now > has a larger window to be lost (as mentioned in my changelog). > What about the case where EPOLLET is not set? The old code did not drop events in that case. > As far as correct __pm_stay_awake/__pm_relax handling, perhaps adding > an atomic counter to struct eventpoll (or each epitem) will work? > The wakeup_source should stay in sync with the epoll state. I don't think any additional state is needed. > If we go with atomic counter in struct eventpoll, is per-epitem > wakeup_source still necessary? We have space in epitem now, but > maybe one day we will might need it. > The wakeup_source per epitem is useful for accounting reasons. If suspend fails, it is useful to know which device caused it. > Thanks for looking at this patch. > > Btw, I'm curious; which applications use EPOLLWAKEUP? > > My epoll work is focused on network servers with thousands of clients, > and I don't think any of them use (or have use for) EPOLLWAKEUP. > But I will keep EPOLLWAKEUP users in mind when working on epoll :) EPOLLWAKEUP is only needed on systems that use suspend. I don't know if it is currently in use, but it is intended to at least replace the evdev wakelock in the android kernel, but user-space needs to be updated before we can drop that patch. -- Arve Hjønnevåg -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
