On 03/10/2016 07:53 PM, Jason Baron wrote: > 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. By the way, in the code you have case EPOLL_CTL_MOD: if (epi) { if (!(epi->event.events & EPOLLEXCLUSIVE)) { epds.events |= POLLERR | POLLHUP; error = ep_modify(ep, epi, &epds); } I think the "if" here is redundant. IIUC, earlier in the code you disallow EPOLL_CTL_MOD with EPOLLEXCLUSIVE. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- 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
