Re: Unified tracing buffer

["Martin Bligh" <mbligh at google dot com>]

>> One thing that I think is still important is to have a unified timestamp
>> mechanism on everything, so we can co-ordinate different things back
>> together in userspace from different trace tools, so I intend to keep
>> that at a lower level, but I think you're right that the event id, etc can
>> move up into separate layers.
>
> I'm not so sure that the unified 'timestamp' must be required by all tracers.
> If you just need to merge and sort per-cpu data, you can use an atomic
> sequential number for it.
> IMHO, the unified 'timestamp' would better be an option, because some
> architectures can't support it. I think preparing timestamp-callback
> function will help us.

An atomic sequential number is:

(a) far less meaningful than a timestamp for the user
(b) more expensive to compute in many cases.

I think we came up with a way to approximate this, using a callback every
ms or so as the higher order bits, and a sequential counter in the lower
order for those broken platforms.

But perhaps it would be better if we started with a discussion of which
platforms can't do global timestamps, and why not? I know some of them
are fixable, but perhaps not all.