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Message-ID: <CAP-5=fVOXjjcusjv858SOGrnNgE2w2sb7zS=0sZUpdFfR1T_GA@mail.gmail.com>
Date: Wed, 14 Jun 2023 15:03:43 -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 9:18 AM Ian Rogers <irogers@...gle.com> wrote:
>
> 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?
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.
Thanks,
Ian
> 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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