linux-kernel - Re: [PATCH] perf: Differentiate exec() and non-exec() comm events

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Message-ID: <5385A7A9.80702@intel.com>
Date:	Wed, 28 May 2014 12:08:57 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>, Dave Jones <davej@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH] perf: Differentiate exec() and non-exec() comm events

On 05/28/2014 11:55 AM, Peter Zijlstra wrote:
> On Wed, May 28, 2014 at 11:45:04AM +0300, Adrian Hunter wrote:
>> perf tools like 'perf report' can aggregate samples by comm
>> strings, which generally works.  However, there are other
>> potential use-cases.  For example, to pair up 'calls'
>> with 'returns' accurately (from branch events like Intel BTS)
>> it is necessary to identify whether the process has exec'd.
>> Although a comm event is generated when an 'exec' happens
>> it is also generated whenever the comm string is changed
>> on a whim (e.g. by prctl PR_SET_NAME).  This patch adds a
>> flag to the comm event to differentiate one case from the
>> other.
>>
>> In order to determine whether the kernel supports the new
>> flag, a selection bit named 'exec' is added to struct
>> perf_event_attr.  The bit does nothing but will cause
>> perf_event_open() to fail if the bit is set on kernels
>> that do not have it defined.
>>
> 
>> --- a/include/uapi/linux/perf_event.h
>> +++ b/include/uapi/linux/perf_event.h
>> @@ -302,8 +302,8 @@ struct perf_event_attr {
>>  				exclude_callchain_kernel : 1, /* exclude kernel callchains */
>>  				exclude_callchain_user   : 1, /* exclude user callchains */
>>  				mmap2          :  1, /* include mmap with inode data     */
>> -
>> -				__reserved_1   : 40;
>> +				exec           :  1, /* flag comm events that are due to an exec */
>> +				__reserved_1   : 39;
>>  
> 
> Yah.. that's just sad :-(
> 
> the only capabilities mask we have is in the mmap() page, so without
> mmap()ing we have no way to test that.
> 
> Would it make sense to call it comm_exec?

Yes, that is better.  Do you want me to resend the patch?

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