Red Hat Bugzilla – Bug 1959
procinfo -d -n1 gets FP exceptions
Last modified: 2008-05-01 11:37:49 EDT
procinfo-16-3's procinfo gets FP exceptions sometimes
when run with '-d -n1' on a dual-CPU machine. What seems
to do it is running something with heavy CPU usage on
both CPUs (such as the distributed.net rc5des client).
If you run it for a while and wait, boom, there it goes.
I have not tested this behavior on a UP kernel.
This is running with the latest Rawhide (although it
happened both under the Rawhide SMP kernel and under
The root cause of this seems to be that raw idle time number
in /proc/stat occasionally blips backwards (!), with a ripple
through effect on procinfo, which assumes it always goes forward.
I'm not sure how this is possible, but the idle number is not
a kstat variable but is instead worked out by subtracting all
the other time ticks (kernel, user, nice) from elapsed jiffies
(times the number of cpus). Possibly there is some update ordering
issues there, especially with multiple cpus and IRQs coming in at
just the wrong time on the 'other' cpu.
I will be asking the gurus. In the mean time, there is an obvious
patch that can be made to the code in procinfo that reads /proc/stat
(just check if new.cpu_idle < old.cpu_idle and make them equal if
fixed in procinfo-17-1, available in the next release. However, as
this seems to be a kernel bug, I also certainly hope that it has been
corrected in a later release of the kernel as well.