Re: [patch 8/8] Add abilty to enable/disable nmi watchdog from sysfs

[Don Zickus <dzickus@redhat.com>]

On Wed, May 10, 2006 at 02:10:26AM -0700, Stephane Eranian wrote:
> Don,
> 
> Congratulations on the patch. I am glad to see that some of the SMP
> issues I reported a long time ago are now fixed.

Thanks and glad I could help.

> > Adds a new /proc/sys/kernel/nmi call that will enable/disable the nmi
> > watchdog.
> > 
> 
> This means you can at runtime enable/disbale nmi_watchdog, i.e., reserve
> some performance counters on the fly. This gets complicated because now
> the perfmon subsystem (and probably oprofile) cannot check register
> availability when they are first initialized. Basically each time,
> the /sys entry is modified, they would have to scan the list of available
> performance counters. I don't know exactly when Oprofile does this checking.
> For perfmon, this is done only once, when the PMU description table is loaded.
How often did you plan on enabling/disabling the nmi_watchdog?  My
understanding was you disable nmi_watchdog, run oprofile/perfmon,
re-enable nmi_watchdog.  I guess I don't understand what type of funky
scenarios you are dealing with.  

> 
> Also something that I did not see in this code is the error detection in
> case enable_lapic_nmi_watchdog() fails. Oprofile runs on all CPUs or none.
> Perfmon lets you monitor on subsets on CPUs. In case NMI was disabled and
> a monitoring session was active on some CPUs. The enable_lapic_nmi_watchdog()
> will fail on some CPUs. How is that handled?

It's not.  In fact I wouldn't know what to do in such situations.  Is it
really wrong to only have a subset of cpus being monitored by the
nmi_watchdog?  This seems to be wandering into the area where the user is
looking to do something complicated (profiling a subset of cpus) and as
such might be expected to make sure the nmi_watchdog is properly enabled
on all cpus when they are done.

Cheers,
Don