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

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Message-ID: <20140528085538.GR11096@twins.programming.kicks-ass.net>
Date:	Wed, 28 May 2014 10:55:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
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 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?

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