Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

On Tue, 29 Oct 2024 19:20:32 +0100
Peter Zijlstra <[email protected]> wrote:

> The 48:16 bit split gives you uniqueness for around 78 hours at 1GHz.

Are you saying that there will be one system call per nanosecond? This
number is incremented only when a task enters the kernel from user
spaces *and* requests a stack trace. If that happens 1000 times a
second, that would still be around 9000 years.

> 
> But seriously, perf doesn't need this. It really only needs a sequence
> number if you care to stitch over a LOST packet (and I can't say I care
> about that case much) -- and doing that right doesn't really take much
> at all.

Perf may not care because it has a unique descriptor per task, right?
Where it can already know what events are associated to a task. But
that's just a unique characteristic of perf. The unwinder should give a
identifier for every user space stack trace that it will produce and
pass that back to the tracer when it requests a stack trace but it
cannot yet be performed. This identifier is what we are calling a
context cookie. Then when it wants the stack trace, the unwinder will
give the tracer the stack trace along with the identifier
(context-cookie) that this stack trace was for in the past.

It definitely belongs with the undwinder logic.

-- Steve