linux-kernel - Re: [PATCH 1/1] perf symbols: Use the buildids if present

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Message-ID: <20091105082733.GA18011@elte.hu>
Date:	Thu, 5 Nov 2009 09:27:33 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Arnaldo Carvalho de Melo <acme@...radead.org>,
	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	"K. Prasad" <prasad@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Roland McGrath <roland@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 1/1] perf symbols: Use the buildids if present


* Masami Hiramatsu <mhiramat@...hat.com> wrote:

> Arnaldo Carvalho de Melo wrote:
>> From: Arnaldo Carvalho de Melo<acme@...hat.com>
>>
>> Now 'perf record' will intercept PERF_RECORD_MMAP calls, creating a
>> linked list of DSOs, then when the session finishes, it will traverse
>> this list and read the buildids, stashing them at the end of the file
>> and will set up a new feature bit in the header bitmask.
>>
>> 'perf report' will then notice this feature and populate the 'dsos' list
>> and set the build ids.
>>
>> When reading the symtabs it will refuse to load from a file that doesn't
>> have the same build id.
>>
>> Example:
>>
>>   [root@...pio ~]# perf report | head
>>   /home/acme/bin/perf with build id b1ea544ac3746e7538972548a09aadecc5753868 not found, continuing without symbols
>>    # Samples: 2621434559
>>    #
>>    # Overhead          Command                  Shared Object  Symbol
>>    # ........  ...............  .............................  ......
>>    #
>>         7.91%             init  [kernel]        [k] read_hpet
>>         7.64%             init  [kernel]        [k] mwait_idle_with_hints
>>         7.60%          swapper  [kernel]        [k] read_hpet
>>         7.60%          swapper  [kernel]        [k] mwait_idle_with_hints
>>         3.65%             init  [kernel]        [k] 0xffffffffa02339d9
>> [root@...pio ~]#
>>
>> In this case the 'perf' binary was an older one, vanished, so it the symbols
>> probably wouldn't match.
>>
>> Next patches will support the kernel as well, reading the build id notes for it
>> and the modules from /sys.
>
> Great! then I can use it on 'perf probe' to check the dwarf binary is
> same as running kernel.
>
>> Another patch should also introduce a new plumbing command:
>>
>> 'perf list-buildids'
>>
>> that will then be used in porcelain that is distro specific to
>> fetch -debuginfo packages where such buildids are present. This will in turn
>> allow for one to run 'perf record' in one machine and 'perf report' in another.
>
> Hmm, so, will this command list up all debuginfo files with buildids?
> If so, can it also find a kernel binary built locally?

Arnaldo, how about adding the kernel binary's build path to 
/sys/kernel/notes, during the kernel build? (With perhaps a .config 
override as well, for package builds.)

That object might or might not exist, and if it does not exist, or if 
there is a buildid mismatch, we can fall back to 'well known' places for 
kernel/module binaries.

	Ingo
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