Re: [PATCH v16 0/4] perf: Support the deferred unwinding infrastructure

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Oct 23, 2025 at 12:40:57PM -0400, Steven Rostedt wrote:
> On Thu, 23 Oct 2025 17:00:02 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > +/* Deferred unwinding callback for task specific events */
> > +static void perf_unwind_deferred_callback(struct unwind_work *work,
> > +					 struct unwind_stacktrace *trace, u64 cookie)
> > +{
> > +	struct perf_callchain_deferred_event deferred_event = {
> > +		.trace = trace,
> > +		.event = {
> > +			.header = {
> > +				.type = PERF_RECORD_CALLCHAIN_DEFERRED,
> > +				.misc = PERF_RECORD_MISC_USER,
> > +				.size = sizeof(deferred_event.event) +
> > +					(trace->nr * sizeof(u64)),
> > +			},
> > +			.cookie = cookie,
> > +			.nr = trace->nr,
> > +		},
> > +	};
> > +
> > +	perf_iterate_sb(perf_callchain_deferred_output, &deferred_event, NULL);
> > +}
> > +
> 
> So "perf_iterate_sb()" was the key point I was missing. I'm guessing it's
> basically a demultiplexer that distributes events to all the requestors?

A superset. Basically every event in the relevant context that 'wants'
it.

It is what we use for all traditional side-band events (hence the _sb
naming) like mmap, task creation/exit, etc.

I was under the impression the perf tool would create one software dummy
event to listen specifically for these events per buffer, but alas, when
I looked at the tool this does not appear to be the case.

As a result it is possible to receive these events multiple times. And
since that is a problem that needs to be solved anyway, I didn't think
it 'relevant' in this case.

> If I had know this, I would have done it completely different.

I did do mention it here:

  https://lkml.kernel.org/r/20250923103213.GD3419281@noisy.programming.kicks-ass.net

Anyway, no worries. Onwards to figuring out WTF the unwinder doesn't
seem to terminate properly.