linux-kernel - Re: [PATCH] perf tools: Exit recording if events have non matching sample type

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Message-ID: <4E85142C.101@gmail.com>
Date:	Thu, 29 Sep 2011 18:58:20 -0600
From:	David Ahern <dsahern@...il.com>
To:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
CC:	Jiri Olsa <jolsa@...hat.com>, a.p.zijlstra@...llo.nl,
	mingo@...e.hu, Stephane Eranian <eranian@...gle.com>,
	paulus@...ba.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf tools: Exit recording if events have non matching
 sample type



On 09/29/2011 06:40 PM, Arnaldo Carvalho de Melo wrote:
> Em Thu, Sep 29, 2011 at 05:33:42PM -0600, David Ahern escreveu:
>> On 09/29/2011 05:06 PM, Arnaldo Carvalho de Melo wrote:
>>>> >From what I can see sample_type has to be the same for all samples:
>>>> https://lkml.org/lkml/2011/8/15/6
> 
>>> We could add a PERF_SAMPLE_ID2 that if present would be guaranteed
>>> to be the last, or we can, as PeterZ prefers/suggests, to use one
>>> mmap per sample id, then we know that if we're getting it on that
>>> mmap, it has that sample_type.
> 
>>> For record that means we need to dump each per-sample_id mmap onto a
>>> separate file, i.e. use a directory, etc.
>  
>> That seems like a major re-write of perf. Why not keep the existing
>> restriction and have users who want different sample_types run
>> multiple perf commands?
>  
>> What's to be gained by refactoring the code to support multiple
>> sample_types in a single command invocation?
> 
> This comes from time to time:
> 
> http://us.generation-nt.com/answer/bug-perf-event-sampling-buffer-format-cannot-handle-multi-event-sampling-help-202985832.html
> 
> The whole point of sample_type is to ask for just what is needed for
> some specific event so that we reduce the per sample footprint.
> 
> So we allow for multiple types of events to be on the same stream, but
> not for them to have just what each need, its a limitation, one that we
> can solve. I see no reason not to solve it :-)
> 
> The evsel/evlist classes were designed to abstract away such details,
> i.e. perf_evlist__mmap should notice different sample_types and sort
> this out transparently.
> 
> And then perf_evlist__mmap_read(evlist, idx), evlist->nr_mmaps should be
> enough to process the events :-)
> 
> - Arnaldo

An ABI change fixes the root cause of the current problem -- put an id
in perf_event_header. The id maps to the evsel which has the
sample_type. Right now you have to parse the sample to get the id to get
the evsel.

With that change I have a patch that (mostly?) covers the rest of perf
userspace to handle sample_type per evsel.

David
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