Messages in this thread | | | Date | Thu, 18 Feb 2010 19:45:47 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: Stupid futex question - 2.6.33-rc7-mmotm0210 |
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On Thu, 18 Feb 2010, Valdis.Kletnieks@vt.edu wrote: > (Adding some cc: to the list) > > On Thu, 18 Feb 2010 15:37:59 +0100, Peter Zijlstra said: > > On Thu, 2010-02-18 at 09:04 -0500, Valdis.Kletnieks@vt.edu wrote: > > > Kernel: x86_64 2.6.33-rc7-mmotm0210 > > > > > > I'm debugging a problem where pulseaudio is getting killed with a SIGKILL > > > out of the blue. It appears to be a problem where pulseaudio sets > > > RLIMIT_RTTIME and the bound gets exceeded. Analysis with 'top' shows > > > a short spike of 96% system time, and the tail end of strace shows this: > > > > > > [pid 25065] 01:50:20.371484 ioctl(28, USBDEVFS_CONTROL, 0x7fd3d76f630c) = 0 <0.000015> > > > [pid 25065] 01:50:20.371548 ioctl(28, 0x40045532, 0x7fd3d76f636c) = 0 <0.000016> > > > [pid 25065] 01:50:20.371611 open("/dev/snd/pcmC0D0p", O_RDWR|O_NONBLOCK|O_CLOEXEC <unfinished ...> > > > [pid 25064] 01:50:20.371678 <... write resumed> ) = 8 <0.002104> > > > [pid 25064] 01:50:20.371718 futex(0xc2ec00, FUTEX_WAIT_PRIVATE, 0, NULL <unfinished ...> > > > [pid 25066] 01:50:21.408392 +++ killed by SIGKILL +++ > > > PANIC: handle_group_exit: 25066 leader 25064 > > > [pid 25065] 01:50:21.408442 +++ killed by SIGKILL +++ > > > PANIC: handle_group_exit: 25065 leader 25064 > > > 01:50:21.420354 +++ killed by SIGKILL +++ > > > > > > thread 25064 apparently gets gunned down due to RTTIME because it spent a whole > > > second in a futex() call - is it reasonable for futex() to not return for that > > > long?
Well, it's in futex_wait(). If nothing unlocks the futex, then it stays there forever.
> > > In other words - kernel bug because futex() should return, or pulseaudio bug > > > for not understanding futex() can snooze a while? > > > > > > If a kernel bug, anybody got a better idea than nuking the RLIMIT_RTTIME call, > > > waiting for it to repeat (takes between 1 minute and 1 hour or so), and > > > whomping it a few times with sysrq-T? > > > > is that second spend in processing sysrq-t? > > No, currently that second is spent in a futex() syscall - I'm wondering: > > 1) should it get killed for RLIMIT_RTTIME because it's been in a futex() > for multiple seconds? It seems suspicious - docs say a blocking syscall > resets RTTIME - so if futex() blocks it shouldn't kill, and if it's in the > kernel for a second without blocking it's a bug too.
If it schedules out, then the RLIMIT_RTTIME should not be hit.
There are several possibilities why this happens:
- the futex code has a bug which causes it to busy loop - the rlimit code is wreckaged
> 2) Is sysrq-T my best bet here, or should I be trying something else first?
Can you enable the function tracer and check whether it reproduces with the function tracer. If yes, then we can put a tracing_off() into the code which handles the rlimit, so we can see in the trace what happened before it triggered.
Thanks,
tglx
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