Hi! Am 26.11.2015 um 10:49 schrieb Anton Ivanov: > Hi List, hi Richard, > > While working on the EPOLL I managed to consistently reproduce and get down > to the bottom of the process in D state bug which you occasionally see with > UML. I recall asking > Richard's help on this for the first time nearly 5 years ago ;-).
O_o > It is extremely rare with the POLL based controller, timers and the stock UBD > drivers. As you make things go faster (anywhere in UML) it rares its ugly > head. So improving the IRQs, > improving UBD itself, etc - all make it easier to trigger. > > It looks like it is possible to end up in a state where the restart list is > not empty (an earlier transaction to the disk io thread failed with EAGAIN), > but with no pending IO on > the UBD IPC thread fd. So the restart list is never re-triggered and the UBD > device ends up with a non-empty queue. The process that requested the IO ends > up in D state. Any other > processes trying IO to the same disk join it. As the requests to the same UBD > queue up, ultimately, UML goes belly up. > > Pinging the UML process with SIGIO does not help as there is no IO pending on > the fd. So it is not a lost interrupt. It somehow manages to race forming the > restart queue. > > If, however, you have more than one UBD device IO to the other one unstucks > it by re-running the restart queue out of the ubd interrupt handler. > > Once again - this is extremely rare at present, but possible (I have seen it > a few times over the last 5 years). > > So it needs a viable fix or a workaround. I will have to get this one out of > the door first as it constantly gets in the way in debugging both the Epoll > and the signals stuff. Okay, let's collect some facts first. Is a guest or a host process in state D? If it is a guest process, you can use "/proc/<pid>/stack" to find out where in the UML kernel it is blocking. 5 years ago UML didn't have this feature. Thanks, //richard ------------------------------------------------------------------------------ _______________________________________________ User-mode-linux-devel mailing list User-mode-linux-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/user-mode-linux-devel
