snabbmark: Add PMU support to mp-ring benchmark

[lukego]

Following on from #808...

The command-line argment -e / --events already existed but now it is actually implemented.

The report is only printed by child process #0 based on the assumption that all children would tell the same story.

Just for a taste here is a little script to run the benchmark with different events and process counts:

#!/usr/bin/env bash
for processes in 1 2 3; do
    for events in 'l2_rqsts' 'l2_trans' 'mem_trans_retired[.]load_latency_gt_' 'cycle_activity'; do
        echo "@@@@ processes $processes"
        echo "@@@@ events $events"
        sudo ./snabb snabbmark mp-ring --packets 10e6 --processes "$processes" --events "$events"
    done
done

See this gist for the raw output of the first run.

[lukego]

cc @xrme @kbara this is hopefully a useful development tool.

[lukego]

Last touch for now: I created a new gist that runs the same test but in a different order, so that the results for 1/2/3 processes are shown together i.e. easier to look at how certain counters changed between the 1 process and 2 process scenarios.