Messages in this thread | | | Subject | Re: [PATCH] perf top & record: Fix segfault when default cycles event is not supported | From | Yang Jihong <> | Date | Thu, 15 Jun 2023 14:57:56 +0800 |
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Hello,
On 2023/6/15 10:04, Ian Rogers wrote: > On Wed, Jun 14, 2023 at 6:55 PM Yang Jihong <yangjihong1@huawei.com> wrote: >> >> Hello, >> >> On 2023/6/15 6:03, Ian Rogers wrote: >>> On Wed, Jun 14, 2023 at 9:18 AM Ian Rogers <irogers@google.com> wrote: >>>> >>>> On Wed, Jun 14, 2023 at 8:18 AM Yang Jihong <yangjihong1@huawei.com> 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@huawei.com> >>>> >>>> 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. >>> >> >> Yes, it's a bit of the same logic as evsel__fallback, or we can call >> evlist__add_default() as before, simply create an evsel of hardware >> cycles and add it directly to evlist. >> >> Please confirm whether this solution is feasible. If it is feasible, I >> will send a v2 version. > > The previous evlist__add_default logic didn't handle wildcard PMUs for > cycles, hence wanting to reuse the parse events logic. The error is > that the logic now isn't doing the fallback properly. I think an > evlist__add_cycles which uses evsel__fallback makes sense matching the > previous logic. I'd be happy if you took a look. I'll write a patch so > that the perf_pmus list of core PMUs is never empty.
Do you mean to do fallback when parsing events? Instead of doing it at evsel__open?
Okay, I'll re-patch it with this solution, and it looks like it should work.
Thanks, Yang
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