Re: [RFC -tip] perf, x86: Add PERF_COUNT_HW_NMI_WATCHDOG event v2

From: Cyrill Gorcunov
Date: Tue Jun 21 2011 - 13:23:28 EST

On Tue, Jun 21, 2011 at 07:10:05PM +0200, Stephane Eranian wrote:
> >
> >  The key here is that we use that named non-sleeping ticks (as oprofile
> > did) for nmi-watchdog and it allows us to free "cpu-cycles" counter for
> > user needs. Of course we pick up one counter for this but it doesn't intersect
> > with "cpu-cycles" counter (because counters are grouped and can count only
> > specified events in each group). Stepane, should I post more details?
> >
> Ok, so you're using a different PMU event for the watchdog. But then, in this
> case why not simply change kernel/watchdog.c to hardcode something specific
> for P4, i.e, change the wd_hw_attr.config/type fields to match the
> event you want.

It doesn't change things much. Of course I can put some specifics into
.config but where is the guarantee some new generic event would not ever
intersect with it. I know (for example) we could reserve -1ULL for this
event but again where is the guarantee that it will never ever be used
system wide in future for some different event?

So I need some event-id in which I may be sure it will never be
used for different purpose.

> You could create a arch specific callback to setup config/type.
> I don't think it makes sense to expose yet another generic PMU event, especially
> given the name you gave to it and what it actually does. People might wonder
> what good is that for?

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