Hi Michael, On 01/29/2016 03:14 AM, Michael Kerrisk (man-pages) wrote: > Hello Jason, > On 01/28/2016 06:57 PM, Jason Baron wrote: >> Hi, >> >> On 01/28/2016 02:16 AM, Michael Kerrisk (man-pages) wrote: >>> Hi Jason, >>> >>> On 12/08/2015 04:23 AM, Jason Baron wrote: >>>> Hi, >>>> >>>> Re-post of an old series addressing thundering herd issues when sharing >>>> an event source fd amongst multiple epoll fds. Last posting was here >>>> for reference: https://lkml.org/lkml/2015/2/25/56 >>>> >>>> The patch herein drops the core scheduler 'rotate' changes I had previously >>>> proposed as this patch seems performant without those. >>>> >>>> I was prompted to re-post this because Madars Vitolins reported some good >>>> speedups with this patch using Enduro/X application. His writeup is here: >>>> https://mvitolin.wordpress.com/2015/12/05/endurox-testing-epollexclusive-flag/ >>>> >>>> Thanks, >>>> >>>> -Jason >>>> >>>> Sample epoll_clt text: >>> >>> Thanks for the proposed text. I have some questions about points >>> that are not quite clear to me. >>> >>>> EPOLLEXCLUSIVE >>>> Sets an exclusive wakeup mode for the epfd file descriptor that is >>>> being attached to the target file descriptor, fd. Thus, when an >>>> event occurs and multiple epfd file descriptors are attached to the >>>> same target file using EPOLLEXCLUSIVE, one or more epfds will receive >>>> an event with epoll_wait(2). The default in this scenario (when >>>> EPOLLEXCLUSIVE is not set) is for all epfds to receive an event. >>>> EPOLLEXLUSVIE may only be specified with the op EPOLL_CTL_ADD. >>> >>> So, assuming an FD is present in the interest list of multiple (say 6) >>> epoll FDs, and some (say 3) of those attachments were done using >>> EPOLLEXCLUSVE. Which of the following statements are correct: >>> >>> (a) It's guaranteed that *none* of the epoll FDs that did NOT specify >>> EPOLLEXCLUSIVE will receive an event. >>> >>> (b) It's guaranteed that *all* of the epoll FDs that did NOT specify >>> EPOLLEXCLUSIVE will receive an event. >>> >>> (c) From 1 to 3 of the epoll FDs that did specify EPOLLEXCLUSIVE >>> will receive an event. >>> >>> (d) Exactly one epoll FD that did specify EPOLLEXCLUSIVE will get >>> an event, and it is indeterminate which one. >>> >> >> So b and c. All the non-exclusive adds will get it and at least 1 of the >> exclusive adds will as well. > > So is it fair to say that the expected use case is that all epoll sets > would use EPOLLEXCLUSIVE? > >>> I suppose one point I'm trying to uncover in the above is: what is >>> the scope of EPOLLEXCLUSIVE? Is it just applicable for one process's >>> FD, or is it setting an attribute in the epoll "interest list" record >>> for that FD that affects notification behavior across all processes? >>> >> >> Right - so 'EPOLLEXCLUSIVE' will affect other epoll sets that are also >> using 'EPOLLEXCLUSIVE' against the the same fd, but will have no affect >> on epoll sets connected to fd that do not specify it. >> >> >>> And then: >>> >>> (1) What are the semantics of EPOLLEXCLUSIVE if the added FD becomes >>> disabled via EPOLLONESHOT (or explicitly via EPOLL_CTL_MOD with >>> the 'events' field set to 0)? >>> >> >> In the case of EPOLLEXCLUSIVE and EPOLLONESHOT, one would have to re-arm >> at least 1 of threads that was woken up by doing EPOLL_CTL_MOD to >> guarantee further wakeups. >> >> And like-wise with an EPOLL_CTL_MOD with 'events' all set to 0, one >> would need to either re-arm the thread that set the 'events' field to 0 >> (by setting back to non-zero), or re-arm in at least one other thread >> via EPOLL_CTL_MOD (or delete and add). > > Okay -- so when an EPOLLEXCLUSIVE FD becomes disarmed it is possible > to re-enable rith EPOLL_CTL_MOD; one doesn't need to delete and re-add > the FD. > >>> (2) The source code contains a comment "we do not currently supported >>> nested exclusive wakeups". Could you elaborate on this point? It >>> sounds like something that should be documented. >> >> So I was just trying to say that we return -EINVAL if you try to do and >> EPOLL_CTL_ADD with EPOLLEXCLUSIVE and the 'fd' argument is a epoll fd >> returned via epoll_create(). > > Okay -- that definitely belongs in the man page. > > I'll work up a text, but would like to get input about the "use case" > question above. > > Cheers, > > Michael > > > Ok, here's some updated text: EPOLLEXCLUSIVE Sets an exclusive wakeup mode for the epfd file descriptor that is being attached to the target file descriptor, fd. When a wakeup event occurs and multiple epfd file descriptors are attached to the same target file using EPOLLEXCLUSIVE, one or more epfds will receive an event with epoll_wait(2). The default in this scenario (when EPOLLEXCLUSIVE is not set) is for all epfds to receive an event. The events supported by EPOLLEXCLUSIVE are: EPOLLIN, EPOLLOUT, EPOLLERR, EPOLLHUP, EPOLLWAKEUP, and EPOLLET. epoll_wait(2) will always wait for EPOLLERR and EPOLLHUP; it is not necessary to set it in events. If EPOLLEXCLUSIVE is set using epoll_ctl(2), then a subsequent EPOLL_CTL_MOD on the same epfd, fd pair will retrun -EINVAL. An epoll_ctl(2) that specifies EPOLLEXCLUSIVE in events and specifies the target file descriptor fd as an epoll instance will return -EINVAL as well. Thanks, -Jason -- 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
