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

[Peter Zijlstra]

On Mon, Oct 28, 2024 at 02:47:38PM -0700, Josh Poimboeuf wrote:

> + * The only exception is when the task has migrated to another CPU, *and* this
> + * is called while the task work is running (or has already run).  Then a new
> + * cookie will be generated and the callback will be called again for the new
> + * cookie.

So that's a bit crap. The user stack won't change for having been
migrated.

So perf can readily use the full u64 cookie value as a sequence number,
since the whole perf record will already have the TID of the task in.
Mixing in this CPU number for no good reason and causing trouble like
this just doesn't make sense to me.

If ftrace needs brain damage like this, can't we push this to the user?

That is, do away with the per-cpu sequence crap, and add a per-task
counter that is incremented for every return-to-userspace.