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.
