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

Namhyung Kim
2025-10-26T00:58:29Z

Hi Peter,

On Fri, Oct 24, 2025 at 02:58:41PM +0200, Peter Zijlstra wrote:
>
> 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.

Right, I don't remember why. I think there's no reason doing it for
system wide mode only.

Adrian, do you have any idea? I have a vague memory that you worked on
this in the past.

Thanks,
Namhyung