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Message-ID: <CAP-5=fUf0+7HwZ+AHUR0nRD5QnfPn9_CPMEdJZP_5goPfrPB+Q@mail.gmail.com>
Date: Wed, 14 Jun 2023 09:18:31 -0700
From: Ian Rogers <irogers@...gle.com>
To: Yang Jihong <yangjihong1@...wei.com>
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
mark.rutland@....com, alexander.shishkin@...ux.intel.com,
jolsa@...nel.org, namhyung@...nel.org, adrian.hunter@...el.com,
kan.liang@...ux.intel.com, linux-perf-users@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf top & record: Fix segfault when default cycles event
is not supported
On Wed, Jun 14, 2023 at 8:18 AM Yang Jihong <yangjihong1@...wei.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@...wei.com>
Thanks, useful addition. The cpu-clock fall back wasn't present before
7b100989b4f6 so is the fixes tag correct?
Wrt segv, I'm beginning to think that we should always forcibly create
a core PMU even if we can't find one one in sysfs, my guess is that is
what triggers the segv.
evlist__add_default doesn't really explain what the function is doing
and default can have >1 meaning. Perhaps, evlist__add_cycles.
Thanks,
Ian
> ---
> tools/perf/builtin-record.c | 4 +---
> tools/perf/builtin-top.c | 3 +--
> tools/perf/util/evlist.c | 18 ++++++++++++++++++
> tools/perf/util/evlist.h | 1 +
> 4 files changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index aec18db7ff23..29ae2b84a63a 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -4161,9 +4161,7 @@ int cmd_record(int argc, const char **argv)
> record.opts.tail_synthesize = true;
>
> if (rec->evlist->core.nr_entries == 0) {
> - bool can_profile_kernel = perf_event_paranoid_check(1);
> -
> - err = parse_event(rec->evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
> + err = evlist__add_default(rec->evlist);
> if (err)
> goto out;
> }
> diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> index c363c04e16df..798cb9252a5f 100644
> --- a/tools/perf/builtin-top.c
> +++ b/tools/perf/builtin-top.c
> @@ -1665,8 +1665,7 @@ int cmd_top(int argc, const char **argv)
> goto out_delete_evlist;
>
> if (!top.evlist->core.nr_entries) {
> - bool can_profile_kernel = perf_event_paranoid_check(1);
> - int err = parse_event(top.evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
> + int err = evlist__add_default(top.evlist);
>
> if (err)
> goto out_delete_evlist;
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index 7ef43f72098e..60efa762405e 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -287,6 +287,24 @@ struct evsel *evlist__add_aux_dummy(struct evlist *evlist, bool system_wide)
> return evsel;
> }
>
> +int evlist__add_default(struct evlist *evlist)
> +{
> + bool can_profile_kernel;
> + int err;
> +
> + can_profile_kernel = perf_event_paranoid_check(1);
> + err = parse_event(evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
> + if (err)
> + return err;
> +
> + if (!evlist->core.nr_entries) {
> + pr_debug("The cycles event is not supported, trying to fall back to cpu-clock event\n");
> + return parse_event(evlist, "cpu-clock");
> + }
> +
> + return 0;
> +}
> +
> #ifdef HAVE_LIBTRACEEVENT
> struct evsel *evlist__add_sched_switch(struct evlist *evlist, bool system_wide)
> {
> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
> index 664c6bf7b3e0..47eea809ee91 100644
> --- a/tools/perf/util/evlist.h
> +++ b/tools/perf/util/evlist.h
> @@ -116,6 +116,7 @@ int arch_evlist__cmp(const struct evsel *lhs, const struct evsel *rhs);
>
> int evlist__add_dummy(struct evlist *evlist);
> struct evsel *evlist__add_aux_dummy(struct evlist *evlist, bool system_wide);
> +int evlist__add_default(struct evlist *evlist);
> static inline struct evsel *evlist__add_dummy_on_all_cpus(struct evlist *evlist)
> {
> return evlist__add_aux_dummy(evlist, true);
> --
> 2.30.GIT
>
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