Re: [Crash-utility] [PATCH] Display online cpus value in preference to kt->cpus
On Mon, 2010-03-08 at 09:49 -0500, Dave Anderson wrote:
----- "Dave Anderson" <anderson(a)redhat.com> wrote:
> ----- "Luciano Chavez" <lnx1138(a)linux.vnet.ibm.com> wrote:
>
> > Hi Dave,
> >
> > Thinking about backward compatibility, would displaying "ONLINE
CPUS"
> > still seem OK for the case where kernel_init() finds the smp_num_cpus
> > symbol (as for a 2.4 kernel)? Before there were the various cpu maps, I
> > think smp_num_cpus was analogous to the possible cpus as opposed to
> > online. I can see this requiring some thought as to what CPUS in the
> > output means when you have various different maps now (online, possible,
> > and present). That being said, it would be good to leave no doubt and
> > explicitly state the count is for the present or online CPUS with the
> > latter being my suggestion.
> >
> > I forgot to mention that I suspect the problem I mentioned before would
> > get stranger for POWER7 which offers 4 threads per core. I didn't have
> > access to a POWER7 machine to see just what it would do if we tried
> > disabling SMT as before but it follows the same pattern the count
> > displayed would be way off from the online count.
>
> I just ran through a bunch of stashed dumpfiles I have on hand, and
> it gets even murkier when dealing with Xen or KVM kernels, because
> as part of the post-crash shutdown (or forced dump), all but one of
> the cpus may be taken "offline". So even though there may be 4 vcpus,
> and crash correctly shows 4 "CPUS", the cpu_online_map shows only one
> cpu bit. So if we went ahead and displayed a number based upon the
> cpu_online_map, it would completely misleading. Incorrect
> actually...
You can always dump the possible/present/online map information with
the "help -k" debug option.
So for example, taking a 2.6.9-era (RHEL4) xen kernel that crashed
on vcpu 3 due to a NULL reference, the hypervisor made a callback to
the other vcpus to shut them down prior to the core dumping procedure:
crash> help -k
...
cpu_possible_map: (does not exist)
cpu_present_map: 0 1 2 3
cpu_online_map: 3
...
So the online map cannot be used for the cpu count, and for that
matter, it wouldn't make any sense to even display the online map
count.
In any case, for now I prefer not to change things, at least for the
other architectures.
That being said, I defer machine-specific items for ppc64, s390
and s390x to the IBM maintainers, and to HP for ia64. (The ppc
and alpha architectures have no active "maintainers" any more,
so those arches are pretty much withering on the vine.)
So if you want to do something specifically for ppc64, please
re-post a patch for just that architecture.
Dave
Dave,
Thanks for taking a good look at all the many cases that would make a
general solution of using online cpu count messy. I originally did want
to make this change only applicable to ppc64. The thing was, only
ppc64_display_machine_stats() was possible to affect and to make the
value displayed consistent, changing display_sys_stats() and
dump_kernel_table() was necessary.
So, re-thinking this to be a ppc64 specific change to CPUS to be
displayed as the online count when possible and having everyone else do
what they do now, which is to display kt->cpus, I suggest the following:
1. Add a get_cpus_to_display as a machdep function
2. For ppc64, initialize machdep->get_cpus_to_display to
ppc64_get_cpus_to_display() which will attempt to use get_cpus_online()
or fallback to using kt->cpus
3. For all other architectures, have them initialize
machdep->get_cpus_to_display to generic_get_cpus_to_display() which
returns kt->cpus to maintain the status quo of the code as it is now
4. Replace kt->cpus in display_sys_stats() and dump_kernel_table() in
kernel.c to invoke machdep->get_cpus_to_display() when displaying CPUS
Let me know what you think. I think this solution allows for future
flexibility for other architectures if in the future they individually
need to change what they display for the cpu count.
regards,
--
Luciano Chavez <lnx1138(a)linux.vnet.ibm.com>
IBM Linux Technology Center