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

[Zhang Rui <rui.zhang@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.
> 
> 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
> 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.
> 
Well, the RAPL package energy perf event may need this piece of code.

 "MSR_PKG_ENERGY_STATUS is a read-only MSR. It reports the actual energy
use for the package domain. This MSR is updated every ~1msec. It has a
wraparound time of around 60 secs when power consumption is high, and
may be longer otherwise."

As it's an energy counter, we should show it in "perf stat", right?
As it doesn't have interrupt, I want to schedule a timer interrupt every
30s to update the event counter.

thanks,
rui
> 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.
> 
> 
>