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Subject: Re: [PATCH] perf top & record: Fix segfault when default cycles event
 is not supported
To:     Ian Rogers <irogers@google.com>
CC:     <peterz@infradead.org>, <mingo@redhat.com>, <acme@kernel.org>,
        <mark.rutland@arm.com>, <alexander.shishkin@linux.intel.com>,
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        <adrian.hunter@intel.com>, <kan.liang@linux.intel.com>,
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References: <20230614151625.2077-1-yangjihong1@huawei.com>
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From:   Yang Jihong <yangjihong1@huawei.com>
Message-ID: <325b3a4c-b17e-67a8-da31-d5033878a157@huawei.com>
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