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Subject: Re: [PATCH] perf record: Fix continue profiling after draining the
 buffer
To:     Arnaldo Carvalho de Melo <acme@kernel.org>
CC:     Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
        <amistry@google.com>,
        Alexey Budankov <alexey.budankov@linux.intel.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        <zhangjinhao2@huawei.com>
References: <20210205065001.23252-1-yangjihong1@huawei.com>
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From:   Yang Jihong <yangjihong1@huawei.com>
Message-ID: <b28a2b0d-99a0-88e3-873d-9963640a6fdd@huawei.com>
Date:   Thu, 4 Mar 2021 09:19:32 +0800
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On 2021/3/4 0:43, Arnaldo Carvalho de Melo wrote:
> Em Mon, Feb 22, 2021 at 09:31:51AM +0800, Yang Jihong escreveu:
>> Hello,
>>
>> On 2021/2/19 1:09, Arnaldo Carvalho de Melo wrote:
>>> Em Thu, Feb 18, 2021 at 10:20:53AM -0300, Arnaldo Carvalho de Melo escreveu:
>>>> Em Fri, Feb 05, 2021 at 11:46:12AM +0100, Jiri Olsa escreveu:
>>>>> On Fri, Feb 05, 2021 at 07:35:22PM +0900, Namhyung Kim wrote:
>>>>>> Hello,
>>>>>>
>>>>>> On Fri, Feb 5, 2021 at 3:50 PM Yang Jihong <yangjihong1@huawei.com> wrote:
>>>>>>>
>>>>>>> commit da231338ec9c098707c8a1e4d8a50e2400e2fe17 uses eventfd to solve rare race
>>>>>>> where the setting and checking of 'done' which add done_fd to pollfd.
>>>>>>> When draining buffer, revents of done_fd is 0 and evlist__filter_pollfd
>>>>>>> function returns a non-zero value.
>>>>>>> As a result, perf record does not stop profiling.
>>>>>>>
>>>>>>> The following simple scenarios can trigger this condition:
>>>>>>>
>>>>>>> sleep 10 &
>>>>>>> perf record -p $!
>>>>>>>
>>>>>>> After the sleep process exits, perf record should stop profiling and exit.
>>>>>>> However, perf record keeps running.
>>>>>>>
>>>>>>> If pollfd revents contains only POLLERR or POLLHUP,
>>>>>>> perf record indicates that buffer is draining and need to stop profiling.
>>>>>>> Use fdarray_flag__nonfilterable to set done eventfd to nonfilterable objects,
>>>>>>> so that evlist__filter_pollfd does not filter and check done eventfd.
>>>>>>>
>>>>>>> Fixes: da231338ec9c (perf record: Use an eventfd to wakeup when done)
>>>>>>> Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
>>>>>>> ---
>>>>>>>    tools/perf/builtin-record.c | 2 +-
>>>>>>>    tools/perf/util/evlist.c    | 8 ++++++++
>>>>>>>    tools/perf/util/evlist.h    | 4 ++++
>>>>>>>    3 files changed, 13 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
>>>>>>> index fd3911650612..51e593e896ea 100644
>>>>>>> --- a/tools/perf/builtin-record.c
>>>>>>> +++ b/tools/perf/builtin-record.c
>>>>>>> @@ -1663,7 +1663,7 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
>>>>>>>                   status = -1;
>>>>>>>                   goto out_delete_session;
>>>>>>>           }
>>>>>>> -       err = evlist__add_pollfd(rec->evlist, done_fd);
>>>>>>> +       err = evlist__add_wakeup_eventfd(rec->evlist, done_fd);
>>>>>>>           if (err < 0) {
>>>>>>>                   pr_err("Failed to add wakeup eventfd to poll list\n");
>>>>>>>                   status = err;
>>>>>>> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
>>>>>>> index 05363a7247c4..fea4c1e8010d 100644
>>>>>>> --- a/tools/perf/util/evlist.c
>>>>>>> +++ b/tools/perf/util/evlist.c
>>>>>>> @@ -572,6 +572,14 @@ int evlist__filter_pollfd(struct evlist *evlist, short revents_and_mask)
>>>>>>>           return perf_evlist__filter_pollfd(&evlist->core, revents_and_mask);
>>>>>>>    }
>>>>>>>
>>>>>>> +#ifdef HAVE_EVENTFD_SUPPORT
>>>>>>> +int evlist__add_wakeup_eventfd(struct evlist *evlist, int fd)
>>>>>>> +{
>>>>>>> +       return perf_evlist__add_pollfd(&evlist->core, fd, NULL, POLLIN,
>>>>>>> +                                      fdarray_flag__nonfilterable);
>>>>>>> +}
>>>>>>> +#endif
>>>>>>
>>>>>> Does it build when HAVE_EVENTFD_SUPPORT is not defined?
>>>>>
>>>>> yea, I was wondering the same.. but it's called only from
>>>>> code within HAVE_EVENTFD_SUPPORT ifdef
>>>>
>>>> Yes, this can't work on systems without eventfd, it will simply not
>>>> build, and why do we have to make the definition of this function
>>>> conditional on HAVE_EVENTFD_SUPPORT?
>>>>
>>>> I'm missing something :-\
>>>>
>>>> Yeah, this whole call to evlist__add_pollfd is already surrounded by
>>>> #ifdef HAVE_EVENTFD_SUPPORT:
>>>>
>>>> 1656         if (zstd_init(&session->zstd_data, rec->opts.comp_level) < 0) {
>>>> 1657                 pr_err("Compression initialization failed.\n");
>>>> 1658                 return -1;
>>>> 1659         }
>>>> 1660 #ifdef HAVE_EVENTFD_SUPPORT
>>>> 1661         done_fd = eventfd(0, EFD_NONBLOCK);
>>>> 1662         if (done_fd < 0) {
>>>> 1663                 pr_err("Failed to create wakeup eventfd, error: %m\n");
>>>> 1664                 status = -1;
>>>> 1665                 goto out_delete_session;
>>>> 1666         }
>>>> 1667         err = evlist__add_pollfd(rec->evlist, done_fd);
>>>> 1668         if (err < 0) {
>>>> 1669                 pr_err("Failed to add wakeup eventfd to poll list\n");
>>>> 1670                 status = err;
>>>> 1671                 goto out_delete_session;
>>>> 1672         }
>>>> 1673 #endif // HAVE_EVENTFD_SUPPORT
>>>> 1674
>>>> 1675         session->header.env.comp_type  = PERF_COMP_ZSTD;
>>>> 1676         session->header.env.comp_level = rec->opts.comp_level;
>>>>
>>>> Jiri, does your Acked-by stands? Namhyung?
>>>
>>> Thanks tested and applied, together with Jiri's Tested-by,
>>>
>>> - Arnaldo
>>> .
>>>
>> Is this patch okay? Is there anything that needs to be modified?
> 
> It was merged:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/tools/perf/builtin-record.c?id=e16c2ce7c5ed5de881066c1fd10ba5c09af69559
> 
> - Arnaldo
> .
> Thanks.
Yang