Re: [PATCH] perf sched: kill time stamp discrepancy between script and latency

Joonwoo Park
2016-10-03T22:05:00Z

On 09/30/2016 10:15 PM, Namhyung Kim wrote:
> Hi Joonwoo,
>
> On Wed, Sep 28, 2016 at 07:25:26PM -0700, Joonwoo Park wrote:
>> Perf sched latency is handy to find out the maximum sched latency and
>> the time stamp of the event. After running sched latency, if a found
>> latency looks suspicious it's quite reasonable to run perf script
>> subsequently and search with the time stamp given by perf sched latency
>> to continue further debugging. However, at present, it's possible the
>> time stamp given by perf sched latency cannot be found in the trace
>> output by perf script because perf sched latency converts the event
>> time from ns to ms as double float and prints it with printf which
>> does banker's rounding as opposed to perf script doesn't.
>>
>> For example:
>>
>> 0x750ff0 [0x80]: event: 9
>> <snip>
>> 2 1858303049520 0x750ff0 [0x80]: PERF_RECORD_SAMPLE(IP, 0x1): 15281/15281: 0xffffffff8162a63a period: 1 addr: 0
>> ... thread: hackbench:15281
>> <snip>
>>
>> $ perf sched -i perf.data latency | grep hackbench
>> hackbench:(401) + 3539.283 ms | 23347 | avg: 7.286 ms | max: 829.998 ms | max at: 1858.303050 s
>>
>> $ perf script -i perf.data | grep "1858\.303050"
>>
>> $ perf script -i perf.data | grep "1858\.303049"
>> hackbench 15281 [002] 1858.303049: sched:sched_switch: prev_comm=hackbench prev_pid=15281 prev_prio=120 prev_state=D ==> next_comm=hackbench next_pid=15603 next_prio=120
>>
>> Fix perf latency to print out time stamp without rounding to avoid such
>> discrepancy.
>>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
>> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Cc: Namhyung Kim <namhyung@kernel.org>
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
>> ---
>>
>> I was tempted to get rid of all u64 to double casting in the function
>> output_lat_thread but didn't because there is no data loss as of
>> today. Double float gives at least 15 significant decimal digits
>> precision while the function requires only 14 significant digits precision.
>>
>> $ python -c "print(len(str(int(0xffffffffffffffff / 1e6))))"
>> 14
>>
>> tools/lib/traceevent/event-parse.h | 1 +
>> tools/perf/builtin-sched.c | 12 ++++++++++--
>> 2 files changed, 11 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/lib/traceevent/event-parse.h b/tools/lib/traceevent/event-parse.h
>> index 9ffde37..f42703c 100644
>> --- a/tools/lib/traceevent/event-parse.h
>> +++ b/tools/lib/traceevent/event-parse.h
>> @@ -174,6 +174,7 @@ struct pevent_plugin_option {
>>
>> #define NSECS_PER_SEC 1000000000ULL
>> #define NSECS_PER_USEC 1000ULL
>> +#define MSECS_PER_SEC 1000ULL
>>
>> enum format_flags {
>> FIELD_IS_ARRAY = 1,
>> diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
>> index afa0576..e5cf51a 100644
>> --- a/tools/perf/builtin-sched.c
>> +++ b/tools/perf/builtin-sched.c
>> @@ -1190,6 +1190,7 @@ static void output_lat_thread(struct perf_sched *sched, struct work_atoms *work_
>> int i;
>> int ret;
>> u64 avg;
>> + u64 max_lat_at_sec, max_lat_at_msec;
>
> Isn't it usec rathen than msec? :)

It's to contain three decimal digits which are msecs when 'max_lat_at'
is expressed in sec.
For example when max_lat_at = 1858303049520 which is 1858.3030495199998
sec, max_lat_at_msec is meant to be 303.

I must admit variable's name is bit misleading. Maybe just secs, msecs
are better?
Also just noticed u64 isn't needed for msecs. Will size down.

>
>>
>> if (!work_list->nb_atoms)
>> return;
>> @@ -1212,11 +1213,18 @@ static void output_lat_thread(struct perf_sched *sched, struct work_atoms *work_
>>
>> avg = work_list->total_lat / work_list->nb_atoms;
>>
>> - printf("|%11.3f ms |%9" PRIu64 " | avg:%9.3f ms | max:%9.3f ms | max at: %13.6f s\n",
>> + /*
>> + * Avoid round up with printf to prevent event time discrepency
>> + * between sched script and latency.
>> + */
>> + max_lat_at_sec = work_list->max_lat_at / NSECS_PER_SEC;
>> + max_lat_at_msec = (work_list->max_lat_at -
>> + max_lat_at_sec * NSECS_PER_SEC) / MSECS_PER_SEC;
>> + printf("+%11.3f ms |%9" PRIu64 " | avg:%9.3f ms | max:%9.3f ms | max at: %6lu.%06lu s\n",
>
> Maybe you'd better to be in sync with the script code:
>
> if (PRINT_FIELD(TIME)) {
> nsecs = sample->time;
> secs = nsecs / NSECS_PER_SEC;
> nsecs -= secs * NSECS_PER_SEC;
> usecs = nsecs / NSECS_PER_USEC;
> if (nanosecs)
> printf("%5lu.%09llu: ", secs, nsecs);
> else
> printf("%5lu.%06lu: ", secs, usecs);
> }

Apart from variable name, I'm not quite sure what to sync because sched
doesn't print in nsecs.
Maybe you just wanted for variable names in sync rather than logic?

Thanks!
Joonwoo

>
> Thanks,
> Namhyung
>
>
>> (double)work_list->total_runtime / 1e6,
>> work_list->nb_atoms, (double)avg / 1e6,
>> (double)work_list->max_lat / 1e6,
>> - (double)work_list->max_lat_at / 1e9);
>> + max_lat_at_sec, max_lat_at_msec);
>> }
>>
>> static int pid_cmp(struct work_atoms *l, struct work_atoms *r)
>> --
>> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
>> hosted by The Linux Foundation
>>