Re: [PATCH v12 06/14] unwind_user/deferred: Add deferred unwinding interface

[Mathieu Desnoyers]

On 2025-07-02 15:40, Steven Rostedt wrote:
> On Wed, 2 Jul 2025 15:36:00 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > union unwind_task_id {
> > 	struct {
> > 		u32		task_id;
> > 		u32		cnt;
> > 	}
> > 	u64 id;
> > };
> >
> > static u64 get_cookie(struct unwind_task_info *info)
> > {
> > 	u32 cnt = READ_ONCE(info->id.cnt);
> > 	u32 new_cnt;
> >
> > 	if (cnt & 1)
> > 		return info->id;
> >
> > 	if (unlikely(!info->id.task_id)) {
> > 		u32 task_id = local_clock();
> >
> > 		cnt = 0;
> > 		if (try_cmpxchg(&info->id.task_id, &cnt, task_id))
> > 			task_id = cnt;
> > 	}
> >
> > 	new_cnt = cnt + 3;
> > 	if (try_cmpxchg(&info->id, &cnt, new_cnt))
> > 		new_cnt = cnt; // try_cmpxchg() expects something
> >
> > 	return info->id;
> > }
>
> Honestly I think this is way overkill. What I would do, is to have the
> cookie saved in the event be 64 bit, but we can start with the
> simple 32 bit solution keeping the top 32 bits zeros. If this does
> indeed become an issue in the future, we could fix it with a 64 bit
> number. By making sure all the exposed "cookies" are 64 bit, it should
> not break anything. The cookie is just supposed to be a random unique
> number that associates a request with its deferred user space stack
> trace.
>
> With any exposed cookies to user space being 64 bits, this should not
> be an issue to address in the future.

FWIW, I liked your idea of making the cookie 64-bit with:

- 32-bit cpu number,
- 32-bit per-CPU free running counter.

This is simple, works on 32-bit archs, and it does not overflow as often
as time LSB because it counts execution contexts.

Thanks,

Mathieu


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com