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

Peter Zijlstra
2025-10-24T12:58:56Z

Arnaldo, Namhyung,

On Fri, Oct 24, 2025 at 10:26:56AM +0200, Peter Zijlstra wrote:

> > 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.

When I use:

perf record -ag -e cycles -e instructions

I get:

# event : name = cycles, , id = { }, type = 0 (PERF_TYPE_HARDWARE), size = 136, config = 0 (PERF_COUNT_HW_CPU_CYCLES), { sample_period, sample_freq } = 2000, sample_type = IP|TID|TIME|CALLCHAIN|CPU|PERIOD|IDENTIFIER, read_format = ID|LOST, disabled = 1, freq = 1, sample_id_all = 1, defer_callchain = 1
# event : name = instructions, , id = { }, type = 0 (PERF_TYPE_HARDWARE), size = 136, config = 0x1 (PERF_COUNT_HW_INSTRUCTIONS), { sample_period, sample_freq } = 2000, sample_type = IP|TID|TIME|CALLCHAIN|CPU|PERIOD|IDENTIFIER, read_format = ID|LOST, disabled = 1, freq = 1, sample_id_all = 1, defer_callchain = 1
# event : name = dummy:u, , id = { }, type = 1 (PERF_TYPE_SOFTWARE), size = 136, config = 0x9 (PERF_COUNT_SW_DUMMY), { sample_period, sample_freq } = 1, sample_type = IP|TID|TIME|CPU|IDENTIFIER, read_format = ID|LOST, exclude_kernel = 1, exclude_hv = 1, mmap = 1, comm = 1, task = 1, sample_id_all = 1, exclude_guest = 1, mmap2 = 1, comm_exec = 1, ksymbol = 1, bpf_event = 1, build_id = 1, defer_output = 1

And we have this dummy event I spoke of above; and it has defer_output
set, none of the others do. This is what I expected.

*However*, when I use:

perf record -g -e cycles -e instruction

I get:

# event : name = cycles, , id = { }, type = 0 (PERF_TYPE_HARDWARE), size = 136, config = 0 (PERF_COUNT_HW_CPU_CYCLES), { sample_period, sample_freq } = 2000, sample_type = IP|TID|TIME|CALLCHAIN|ID|PERIOD, read_format = ID|LOST, disabled = 1, inherit = 1, mmap = 1, comm = 1, freq = 1, enable_on_exec = 1, task = 1, sample_id_all = 1, mmap2 = 1, comm_exec = 1, ksymbol = 1, bpf_event = 1, build_id = 1, defer_callchain = 1, defer_output = 1
# event : name = instructions, , id = { }, type = 0 (PERF_TYPE_HARDWARE), size = 136, config = 0x1 (PERF_COUNT_HW_INSTRUCTIONS), { sample_period, sample_freq } = 2000, sample_type = IP|TID|TIME|CALLCHAIN|ID|PERIOD, read_format = ID|LOST, disabled = 1, inherit = 1, freq = 1, enable_on_exec = 1, sample_id_all = 1, defer_callchain = 1

Which doesn't have a dummy event. Notably the first real event has
defer_output set (and all the other sideband stuff like mmap, comm,
etc.).

Is there a reason the !cpu mode doesn't have the dummy event? Anyway, it
should all work, just unexpected inconsistency that confused me.