Re-opening the FD on every poll is pretty expensive. So we should avoid
doing that as well re-use the read buffer (page-aligned, <=1 page) for
the sample data.
This will help make sense of vblank events in a multi-screen
environment (where there are two streams of vblanks), and hopefully
also useful for correlating to compositor events.
This should be useful to visualize system-wide events in the CPU
timeline, especially once we add some more events.
We may want to define some modes eventually, as not all captures will
care about GPU visualization. However, it's pretty low overhead (60Hz
* number of monitors), so maybe we can just filter it out at
visualization time.
We do a bit more setup work than necessary here (looking up the config
number for the tracepoint per CPU, rather than looking it up once at
startup), but I suspect I'll want to refactor the init anyway once we
add more tracepoint markers.
If we want to start watching other perf events, we need a way to look
up what perf event we got when a PERF_RECORD_SAMPLE comes in. The
sample identifier is what does that.
The lib/ directory was getting a bit out of hand, so this tries
to organize things a bit so it is easier going forward to locate
the code people want to patch.
