linux-kernel - Re: [PATCH V3 1/4] perf: Add PERF_RECORD

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Message-ID: <559D28AB.4080201@intel.com>
Date:	Wed, 08 Jul 2015 16:42:03 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Andi Kleen <ak@...ux.intel.com>, Ingo Molnar <mingo@...nel.org>,
	linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...hat.com>,
	Stephane Eranian <eranian@...gle.com>,
	mathieu.poirier@...aro.org, Pawel Moll <pawel.moll@....com>
Subject: Re: [PATCH V3 1/4] perf: Add PERF_RECORD_SWITCH to indicate context
 switches

On 08/07/15 16:28, Arnaldo Carvalho de Melo wrote:
> Em Wed, Jul 08, 2015 at 12:52:40AM +0200, Peter Zijlstra escreveu:
>> On Tue, Jul 07, 2015 at 01:13:59PM -0300, Arnaldo Carvalho de Melo wrote:
>>> Em Tue, Jul 07, 2015 at 05:36:14PM +0200, Peter Zijlstra escreveu:
>>>>> To help userspace in places where all it has is the union perf_event, we
>>>>> can reuse one bit in misc to state that, i.e.
> 
>>>>>   #define PERF_RECORD_MISC_SWITCH_NEXT_PREV_PID 14
>  
>>>>> For instance.
> 
>>>> The other option would be a separate RECORD type, which might be
>>>> simpler.
> 
>>> Humm, do we really need it?
> 
>>> I think this is just us wanting to, since we are going to add a new
>>> record, to make it more useful for other, not right now needed,
>>> situations, i.e. if the user is priviledged, there are two other options
>>> to get his info, right?
>  
>> I was just thinking that 2 records, each with a fixed layout would be
>> easier to parse than 1 record with variable layout.
>  
>> The record space is immense, so from that point it really doesn't
>> matter.
> 
> We could do a land grab at some point there, if/when we find some reason
> for that... :-)
>  
>> Do whatever is easiest, less mistakes get made etc. :-)
>  
>> No real preference either way, as long we we've thought about it.
> 
> Right, I just don't want to have two u32 carrying -1 for no reason.

So you'd be OK with 2 RECORD types?

I will see what is involved.

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