On Wed, Jun 14, 2023 at 9:18 AM Ian Rogers <irogers@xxxxxxxxxx> wrote:
On Wed, Jun 14, 2023 at 8:18 AM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
The perf-record and perf-top call parse_event() to add a cycles event to
an empty evlist. For the system that does not support hardware cycles
event, such as QEMU, the evlist is empty due to the following code process:
parse_event(evlist, "cycles:P" or ""cycles:Pu")
parse_events(evlist, "cycles:P")
__parse_events
...
ret = parse_events__scanner(str, &parse_state);
// ret = 0
...
ret2 = parse_events__sort_events_and_fix_groups()
if (ret2 < 0)
return ret;
// The cycles event is not supported, here ret2 = -EINVAL,
// Here return 0.
...
evlist__splice_list_tail(evlist)
// The code here does not execute to, so the evlist is still empty.
A null pointer occurs when the content in the evlist is accessed later.
Before:
# perf list hw
List of pre-defined events (to be used in -e or -M):
# perf record true
libperf: Miscounted nr_mmaps 0 vs 1
WARNING: No sample_id_all support, falling back to unordered processing
perf: Segmentation fault
Obtained 1 stack frames.
[0xc5beff]
Segmentation fault
Solution:
If cycles event is not supported, try to fall back to cpu-clock event.
After:
# perf record true
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.006 MB perf.data ]
#
Fixes: 7b100989b4f6 ("perf evlist: Remove __evlist__add_default")
Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
Thanks, useful addition. The cpu-clock fall back wasn't present before
7b100989b4f6 so is the fixes tag correct?
Hmm... it should be coming from evsel__fallback:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/util/evsel.c?h=tmp.perf-tools-next#n2840
so we shouldn't duplicate that logic. The question is why we're not
doing the fallback.