Re: Device hang when offlining a CPU due to IRQ misrouting

["Siddha, Suresh B" <suresh.b.siddha@intel.com>]

On Thu, May 31, 2007 at 05:44:27PM -0700, Darrick J. Wong wrote:
> Hi there,
> 
> I'm seeing a driver hang with 2.6.22-rc3 while being slightly stupid
> about offlining CPUs.  I suspect that this problem extends beyond a
> particular machine, as I've been able to replicate it with an IBM x3650
> and an IBM x3755.  This is what I'm doing:
> 
> 1) I tie an IRQ to a particular CPU via /proc/irq/XXX/smp_affinity (IRQ
> 4341 is the network card and we're picking on CPU1 in this example):
> echo 2 > /proc/irq/4341/smp_affinity

Darrick, I see a kernel bug in this area(which is already filled with bugs,
and I am looking into ways to fix them). Are you making sure that
between step-1 and step-2, that interrupts actually started arriving at cpu1?

i.e., do step-1 and wait till the irq's start hitting at cpu1. At this point
do step-2 and let us know if you still hit this bug?

> 
> 2) I then take CPU1 offline:
> echo 0 > /sys/devices/system/cpu/cpu1/online
> 
> 3) The kernel prints this:
> [ 1101.968040] Breaking affinity for irq 4341
> [ 1102.074019] CPU 1 is now offline
> [ 1102.081593] lockdep: not fixing up alternatives.
> [ 1112.886919] nfs: server 9.47.66.169 not responding, still trying
> 
> After step 2 the system never sees interrupts from the network card and
> remains hung like that until CPU1 is brought back up.  It looks as
> though the kernel is trying to reroute the IRQ (or so I'm assuming from
> the "Breaking affinity" message), but this doesn't ever happen, so the
> the kernel stops seeing interrupts from the device.
> 
> Granted, one should not be offlining the CPU that is currently
> designated to handle an IRQ, but I suspect that the kernel ought at a
> minimum to reject the offlining or route the IRQ to any online CPU
> instead of screwing things up.
> 
> There exists a similar scenario.  Set the IRQ affinity to a bunch of
> CPUs, watch /proc/interrupts to see which CPU is actually servicing the
> interrupts, then offline that CPU.  The kernel does not reroute the IRQ
> to any of the other CPUs and the device also hangs.

Is this a theory or did you observe this problem happening?

thanks,
suresh

> 
> The furthest that I've dug is that it works on 2.6.17 and is broken in
> 2.6.22-rc3 and 2.6.21.  Will git-bisect further, but I wanted to know if
> anyone else has seen this sort of problem.  afaik, this seems to happen
> with both IOAPIC and MSI interrupts, possibly more.
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