Re: [patch 1/2] x86_64 page fault NMI-safe

From: Peter Zijlstra
Date: Wed Aug 04 2010 - 10:50:54 EST

On Wed, 2010-08-04 at 10:06 -0400, Mathieu Desnoyers wrote:

> The first major gain is the ability to implement flight recorder tracing
> (overwrite mode), which Perf still lacks.

I've send out something like that several times, but nobody took it
(that is, tested it and provided a user). Note how it doesn't require
anything like sub-buffers.

> A second major gain: having these sub-buffers lets the trace analyzer seek in
> the trace very efficiently by allowing it to perform a binary search for time to
> find the appropriate sub-buffer. It becomes immensely useful with large traces.

You can add sync events with a specific magic cookie in. Once you find
the cookie you can sync and start reading it reliably -- the advantage
is that sync events are very easy to have as an option and don't
complicate the reserve path.

> The third major gain: for live streaming of traces, having sub-buffer lets you
> "package" the event data you send over the network into sub-buffers.

See the sync events. Also, a transport can rewrite the stream any which
way it pretty well wants to, as long as the kernel<->user interface is
reliable an unreliable user<->user transport can repackage it to suit
its needs.

> Making sure events don't cross sub-buffer boundaries simplify a lot of things,
> starting with dealing with "overwritten" sub-buffers in flight recorder mode.
> Trying to deal with a partially overwritten event is just insane.

See the above patch, simply parse the events and push the tail pointer
ahead of the reservation before you trample on it.

If you worry about the cost of parsing the events, you can amortize that
by things like keeping the offset of the first event in every page in
the pageframe, or the offset of the next sync event or whatever scheme
you want.

Again, no need for sub-buffers.

Also, not having sub-buffers makes reservation easier since you don't
need to worry about those empty tails.
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