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

[David Carrillo-Cisneros <davidcc@google.com>]

On Thu, Feb 2, 2017 at 11:33 AM, Luck, Tony <tony.luck@intel.com> wrote:
>>> Nice to have:
>>> 1)      Readout using "perf(1)" [subset of modes that make sense ... tying monitoring
>>> to resctrl file system will make most command line usage of perf(1) close to impossible.
>>
>>
>> We discussed this offline and I still disagree that it is close to
>> impossible to use perf and perf_event_open. In fact, I think it's very
>> simple :
>
> Maybe s/most/many/ ?
>
> The issue here is that we are going to define which tasks and cpus are being
> monitored *outside* of the perf command.  So usage like:
>
> # perf stat -I 1000 -e intel_cqm/llc_occupancy {command}
>
> are completely out of scope ... we aren't planning to change the perf(1)
> command to know about creating a CQM monitor group, assigning the
> PID of {command} to it, and then report on llc_occupancy.
>
> So perf(1) usage is only going to support modes where it attaches to some
> monitor group that was previously established.  The "-C 2" option to monitor
> CPU 2 is certainly plausible ... assuming you set up a monitor group to track
> what is happening on CPU 2 ... I just don't know how perf(1) would know the
> name of that group.

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'd argue is more stable and has less corner cases if the
task_mongroups get extra RMIDs for the task events attached to them
than having userspace tools create and destroy groups and move tasks
behind the scenes.

I provided implementation details on the write-up I shared offline on
Monday. If "easy monitoring" of stand-alone task becomes a
requirement, we can dig on the pros and cons of implementing in kernel
vs user space.

>
> Vikas is pushing for "-R rdtgroup" ... though our offline discussions included
> overloading "-g" and have perf(1) pick appropriately from cgroups or rdtgroups
> depending on event type.

I see it more like generalizing the -G option to represent a task
group that can be a cgroup or a PMU specific one.

Currently the perf(1) simply translates the argument of the -G option
into a file descriptor. My idea doesn't change that, just makes perf
tool to look for a "task_group_root" file in the PMU folder and use it
to find as base path for the file descriptor. If a PMU doesnt have
such file, then perf(1) uses the perf cgroup mounting point, as it
does now. That makes for a very simple implementation on the perf tool
side.