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    SubjectRe: [-next regression] TCP window full with EPOLLWAKEUP
    Date
    On Monday, May 21, 2012, Jiri Slaby wrote:
    > On 05/20/2012 08:34 PM, Rafael J. Wysocki wrote:
    > > On Sunday, May 20, 2012, Rafael J. Wysocki wrote:
    > >> On Sunday, May 20, 2012, Rafael J. Wysocki wrote:
    > >>> On Sunday, May 20, 2012, Jiri Slaby wrote:
    > >>>> Hi,
    > >>>>
    > >>>> a bisection shows that with the following commit from -next:
    > >>>> commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
    > >>>> Author: Arve Hjønnevåg <arve@android.com>
    > >>>> Date: Tue May 1 21:33:34 2012 +0200
    > >>>>
    > >>>> epoll: Add a flag, EPOLLWAKEUP, to prevent suspend while epoll
    > >>>> events are ready
    > >>>>
    > >>>> ====
    > >>>>
    > >>>> one of mono programs I use stops receiving data from the network.
    > >>>> Wireshark shows that the TCP window of a connection is filled. This
    > >>>> means the program does not read the data fast enough after requesting
    > >>>> the data.
    > >>>>
    > >>>> If I revert that commit on the top of -next (20120518), everything works
    > >>>> as expected.
    > >>>
    > >>> Hmm. I suppose that the failing program doesn't set EPOLLWAKEUP by mistake,
    > >>> does it?
    > >>
    > >> If it doesn't, we can assume that epi-ws is always NULL and all of the added
    > >> overhead comes from the function calls. So, I wonder if the appended patch
    > >> makes any difference?
    > >
    > > Having thought more about this I have to say this doesn't seem to make much
    > > sense, because in that case you'd see some progress, although probably a bit
    > > slower than before.
    > >
    > > So, I think what happens is that the application tries to set EPOLLWAKEUP,
    > > but doesn't have the capability, so the entire operation fails for it, but
    > > it doesn't check the return value.
    > >
    > > I wonder if the following helps, then.
    > >
    > > Thanks,
    > > Rafael
    > >
    > >
    > > ---
    > > fs/eventpoll.c | 2 +-
    > > 1 file changed, 1 insertion(+), 1 deletion(-)
    > >
    > > Index: linux/fs/eventpoll.c
    > > ===================================================================
    > > --- linux.orig/fs/eventpoll.c
    > > +++ linux/fs/eventpoll.c
    > > @@ -1711,7 +1711,7 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, in
    > >
    > > /* Check if EPOLLWAKEUP is allowed */
    > > if ((epds.events & EPOLLWAKEUP) && !capable(CAP_EPOLLWAKEUP))
    > > - goto error_tgt_fput;
    > > + epds.events &= ~EPOLLWAKEUP;
    >
    > Yes, this fixed the issue.

    Cool, thanks for testing!

    I'll resend it in a while with a changelog and tags.

    Thanks,
    Rafael
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