linux-kernel - Re: [PATCH v7 2/6] perf record: Get the first sample time and last sample time

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Date:   Fri, 5 Jan 2018 09:15:03 +0800
From:   "Jin, Yao" <yao.jin@...ux.intel.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     jolsa@...nel.org, peterz@...radead.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, Linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, kan.liang@...el.com, yao.jin@...el.com
Subject: Re: [PATCH v7 2/6] perf record: Get the first sample time and last
 sample time



On 1/5/2018 3:09 AM, Arnaldo Carvalho de Melo wrote:
> Em Fri, Dec 08, 2017 at 09:13:42PM +0800, Jin Yao escreveu:
>> In the default 'perf record' configuration, all samples are processed,
>> to create the HEADER_BUILD_ID table. So it's very easy to get the
>> first/last samples and save the time to perf file header via the
>> function write_sample_time().
>>
>> Later, at post processing time, perf report/script will fetch
>> the time from perf file header.
> 
> So, at this point I was expecting that that record would be present on
> the perf.data file:
> 
> [acme@...et perf]$ perf record --timestamp-boundary sleep 1
> Cannot read kernel map
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.001 MB perf.data (7 samples) ]
> [acme@...et perf]$ perf report -D | grep PERF_RECORD_SAMPLE | wc -l
> 7
> [acme@...et perf]$ perf report -D | grep PERF_RECORD_SAMPLE_TIME
> [acme@...et perf]$
> 
> What am I doing wrong?
> 
> To clarify, this is with just the first two patches in this series
> applied.
> 
> - Arnaldo
>   

Hi Arnaldo,

The timestamp boundary information is saved in perf file header.

So if we want to look at them, we need to add '--header' in perf report.

For example,

root@skl:/tmp# perf report -D --header | grep 'time of'
# time of first sample : 248333.706656
# time of last sample : 248357.215328

Thanks
Jin Yao