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Message-ID: <56E711C3.8020008@akamai.com> Date: Mon, 14 Mar 2016 15:32:19 -0400 From: Jason Baron <jbaron@...mai.com> To: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>, Andrew Morton <akpm@...ux-foundation.org> Cc: mingo@...nel.org, peterz@...radead.org, viro@....linux.org.uk, normalperson@...t.net, m@...odev.com, corbet@....net, luto@...capital.net, torvalds@...ux-foundation.org, hagen@...u.net, linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org Subject: Re: [PATCH] epoll: add exclusive wakeups flag On 03/14/2016 01:47 PM, Michael Kerrisk (man-pages) wrote: > [Restoring CC, which I see I accidentally dropped, one iteration back.] > > Hi Jason, > > Thanks for the review. I've tweaked one piece to respond to your > feedback. But I also have another new question below. > > On 03/15/2016 03:55 AM, Jason Baron wrote: >> On 03/11/2016 06:25 PM, Michael Kerrisk (man-pages) wrote: >>> On 03/11/2016 09:51 PM, Jason Baron wrote: >>>> On 03/11/2016 03:30 PM, Michael Kerrisk (man-pages) wrote: > > [...] > >> Hi Michael, >> >> Looks good. One comment below. >> >> Thanks, >> >>> EPOLLEXCLUSIVE (since Linux 4.5) >>> Sets an exclusive wakeup mode for the epoll file >>> descriptor that is being attached to the target file >>> descriptor, fd. When a wakeup event occurs and multiple >>> epoll file descriptors are attached to the same target >>> file using EPOLLEXCLUSIVE, one or more of the epoll file >>> descriptors will receive an event with epoll_wait(2). >>> The default in this scenario (when EPOLLEXCLUSIVE is not >>> set) is for all epoll file descriptors to receive an >>> event. EPOLLEXCLUSIVE is thus useful for avoiding thun‐ >>> dering herd problems in certain scenarios. >>> >>> If the same file descriptor is in multiple epoll >>> instances, some with the EPOLLEXCLUSIVE flag, and others >>> without, then events will provided to all epoll >>> instances that did not specify EPOLLEXCLUSIVE, and at >>> least one of the epoll instances that did specify >>> EPOLLEXCLUSIVE. >>> >>> The following values may be specified in conjunction >>> with EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and >>> EPOLLET. EPOLLHUP and EPOLLERR can also be specified, >>> but are ignored (as usual). Attempts to specify other >> >> I'm not sure 'ignored' is the right wording here. 'EPOLLHUP' and >> 'EPOLERR' are always included in the set of events when something is >> added as EPOLLEXCLUSIVE. This is consistent with the non-EPOLLEXCLUSIVE >> add case. > > Yes. > >> So 'EPOLLHUP' and 'EPOLERR' may be specified but will be >> included in the set of events on an add, whether they are specified or not. > > Yes. I understand your discomfort with the work "ignored", but the > problem was that, because it made special mention of EPOLLHUP and EPOLLERR, > your proposed text made it sound as though EPOLLEXCLUSIVE somehow was > special with respect to these two flags. I wanted to clarify that it is not. > How about this: > > The following values may be specified in conjunction > with EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and > EPOLLET. EPOLLHUP and EPOLLERR can also be specified, > but this is not required: as usual, these events are > always reported if they occur, regardless of whether > they are specified in events. > ? Yes, nothing special here with respect to EPOLLHUP and EPOLLERR. So this looks fine to me. > >>> values in events yield an error. EPOLLEXCLUSIVE may be >>> used only in an EPOLL_CTL_ADD operation; attempts to >>> employ it with EPOLL_CTL_MOD yield an error. If >>> EPOLLEXCLUSIVE has set using epoll_ctl(2), then a subse‐ >>> quent EPOLL_CTL_MOD on the same epfd, fd pair yields an > b>> error. An epoll_ctl(2) that specifies EPOLLEXCLUSIVE in >>> events and specifies the target file descriptor fd as an >>> epoll instance will likewise fail. The error in all of >>> these cases is EINVAL. >>> >>> ERRORS >>> EINVAL An invalid event type was specified along with EPOLLEX‐ >>> CLUSIVE in events. >>> >>> EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE. >>> >>> EINVAL op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has >>> previously been applied to this epfd, fd pair. >>> >>> EINVAL EPOLLEXCLUSIVE was specified in event and fd is refers >>> to an epoll instance. > > Returning to the second sentence in this description: > > When a wakeup event occurs and multiple epoll file descrip‐ > tors are attached to the same target file using EPOLLEXCLU‐ > SIVE, one or more of the epoll file descriptors will > receive an event with epoll_wait(2). > > There is a point that is unclear to me: what does "target file" refer to? > Is it an open file description (aka open file table entry) or an inode? > I suspect the former, but it was not clear in your original text. > So from epoll's perspective, the wakeups are associated with a 'wait queue'. So if the open() and subsequent EPOLL_CTL_ADD (which is done via file->poll()) results in adding to the same 'wait queue' then we will get 'exclusive' wakeup behavior. So in general, I think the answer here is that its associated with the inode (I coudn't say with 100% certainty without really looking at all file->poll() implementations). Certainly, with the 'FIFO' example below, the two scenarios will have the same behavior with respect to EPOLLEXCLUSIVE. Also, the 'non-exclusive' mode would be subject to the same question of which wait queue is the epfd is associated with... Thanks, -Jason > To make this point even clearer, here are two scenarios I'm thinking of. > In each case, we're talking of monitoring the read end of a FIFO. > > === > > Scenario 1: > > We have three processes each of which > 1. Creates an epoll instance > 2. Opens the read end of the FIFO > 3. Adds the read end of the FIFO to the epoll instance, specifying > EPOLLEXCLUSIVE > > When input becomes available on the FIFO, how many processes > get a wakeup? > > === > > Scenario 3 > > A parent process opens the read end of a FIFO and then calls > fork() three times to create three children. Each child then: > > 1. Creates an epoll instance > 2. Adds the read end of the FIFO to the epoll instance, specifying > EPOLLEXCLUSIVE > > When input becomes available on the FIFO, how many processes > get a wakeup? > > === > > Cheers, > > Michael >
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