Re: [RFC PATCH] perf_counter: Add support for pinned and exclusive counter groups

[Paul Mackerras <paulus@samba.org>]

Ingo Molnar writes:

> (btw., the percpu allocation code seems to have bitrotten a bit - the 
> logic around perf_reserved_percpu looks wrong and somewhat complicated.)

Yes, it has.  I don't think it was ever fixed to apply only to
hardware counters when software counters were added.

Side question - were you intending to make the various software
counters able to act as interrupting counters?  That will mean
e.g. that anything that increments current->maj_flt or
current->min_flt (i.e. do_page_fault, __get_user_pages) will need to
check if that causes any counter to overflow, which could be a bit
invasive.

> hm, i guess this could be improved: by queueing pinned counters in front 
> of the list and unpinned counters to the tail.

Yes - and there are other little optimizations we could do too, such
as recording the number of hardware counters in the group leader's
struct perf_counter.

> > What this doesn't handle is when a pinned counter gets inherited and 
> > goes into error state in the child.  The sensible thing would be to put 
> > the parent counter into error state in __perf_counter_exit_task, but 
> > that might mean taking it off the PMU on some other CPU, and I was 
> > nervous about doing anything substantial to parent_counter.  Maybe what 
> > we need instead of an error value for counter->state is a separate error 
> > flag that can be set atomically by exiting children. BTW, I think we 
> > need an smp_wmb after updating parent_counter->count, so that when the 
> > parent sees the child has exited it is guaranteed to see the updated 
> > ->count value.
> 
> Yeah. Such artifacts at inheritance stem from the reduction in utility 
> that comes from any exclusive-resource-usage scheme, and are expected.

Hmmm, I don't see pinning as being about exclusivity, and the reason
my patch didn't handle it was because I haven't completely got my head
around the relevant lifetime and locking rules.

> For example, right now it works just fine to nest 'timec' in itself [there 
> is a reduction in statistical value if we start round-robining, but 
> there's still full utility]. If it used exclusive or pinned counters that 
> might not work.

Should be fine with pinned counters, and even with exclusive counters
as long as the counters are in one group.

> Again, which restrictions users/developers are more willing to live with 
> will be shown in actual usage of these facilities.

Indeed.

Regards,
Paul.