Re: [PATCH 00/12] Cqm2: Intel Cache quality monitoring fixes

["Luck, Tony" <tony.luck@intel.com>]

On Thu, Feb 02, 2017 at 12:22:42PM -0800, David Carrillo-Cisneros wrote:
> There is no need to change perf(1) to support
>  # perf stat -I 1000 -e intel_cqm/llc_occupancy {command}
> 
> the PMU can work with resctrl to provide the support through
> perf_event_open, with the advantage that tools other than perf could
> also use it.

I agree it would be better to expose the counters through
a standard perf_event_open() interface ... but we don't seem
to have had much luck doing that so far.

That would need the requirements to be re-written with the
focus of what does resctrl need to do to support each of the
perf(1) command line modes of operation.  The fact that these
counters work rather differently from normal h/w counters
has resulted in massively complex volumes of code trying
to map them into what perf_event_open() expects.

The key points of weirdness seem to be:

1) We need to allocate an RMID for the duration of monitoring. While
   there are quite a lot of RMIDs, it is easy to envision scenarios
   where there are not enough.

2) We need to load that RMID into PQR_ASSOC on a logical CPU whenever a process
   of interest is running.

3) An RMID is shared by llc_occupancy, local_bytes and total_bytes events

4) For llc_occupancy the count can change even when none of the processes
   are running becauase cache lines are evicted

5) llc_occupancy measures the delta, not the absolute occupancy. To
   get a good result requires monitoring from process creation (or
   lots of patience, or the nuclear option "wbinvd").

6) RMID counters are package scoped


These result in all sorts of hard to resolve situations. E.g. you are
monitoring local bandwidth coming from logical CPU2 using RMID=22. I'm
looking at the cache occupancy of PID=234 using RMID=45. The scheduler
decides to run my proocess on your CPU.  We can only load one RMID, so
one of us will be disappointed (unless we have some crazy complex code
where your instance of perf borrows RMID=45 and reads out the local
byte count on sched_in() and sched_out() to add to the runing count
you were keeping against RMID=22).

How can we document such restrictions for people who haven't been
digging in this code for over a year?

I think a perf_event_open() interface would make some simple cases
work, but result in some swearing once people start running multiple
complex monitors at the same time.

-Tony