Re: [RFC PATCH 2/2] perf stat: Use event group to simulate PMI on PMI-less hardware counter

[Lin Ming <ming.m.lin@intel.com>]

On Wed, 2010-11-10 at 22:53 +0800, Peter Zijlstra wrote:
> On Wed, 2010-11-10 at 22:45 +0800, Lin Ming wrote:
> > On Wed, 2010-11-10 at 20:21 +0800, Peter Zijlstra wrote:
> > > On Wed, 2010-11-10 at 14:15 +0800, Lin Ming wrote:
> > > > Some hardware counters(for example, Intel RAPL) can't generate interrupt
> > > > when overflow. So we need to simulate the interrupt to periodically
> > > > record the counter values. Otherwise, the counter may overflow and the
> > > > wrong value is read.
> > > > 
> > > > This patch uses event group to simulate PMI as suggested by Peter
> > > > Zijlstra, http://marc.info/?l=linux-kernel&m=128220854801819&w=2
> > > > 
> > > > create_group_counters() will create a group with 2 events, one hrtimer
> > > > based event as the group leader, and the other event to count. The
> > > > hrtimer is fired periodically, so the sibling event can record its
> > > > counter value periodically as well.
> > > 
> > > I'm terribly confused here....
> > > 
> > >  - you introduce perf_event_attr:pmi_simulate, but then you never
> > > implement it -- nor do we need it afaict.
> > 
> > Someone need to simluate pmi will use it in future.
> 
> Maybe, but simply adding an ABI just in case doesn't seem like a good
> idea. The proposed idea was to group with a software hrtimer-based event
> and use the hrtimer's sample to read the hardware group sibling using
> PERF_SAMPLE_READ.

So this is usefull in perf top/record on PMI-less counter?

> 
> That should be possible using today's interface.
> 
> > > 
> > > 
> > >  - you use grouped counters for perf-stat, perf-stat doesn't use
> > > sampling so I don't see a need to group events to simulate the PMI.
> > > 
> > 
> > Aha, sorry, actually, I mean to periodically read the PMI-less counter
> > and reset it to zero each time to avoid overflow.
> > 
> > Well, seems I have done this in the wrong way.
> > Let me re-think about it.
> 
> Right, so you're wanting to avoid overflowing the hardware counter? This

Yes.

> is only a problem for short hardware counters without a pmi, SH and the
> like currently cascade 2 32bit counters to create 64bit hardware
> counters and avoid the overflow case that way.
> 
> Another thing they can do is simply use the system tick to fold the
> 32bit counters into a the 64bit counter.
> 
> Again, this doesn't need any changes to the ABI and generic code.

Thanks for the explanation.