linux-kernel - Re: [RFC PATCH v1 0/2] perf: Support for SDT markers

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Date:	Tue, 25 Feb 2014 21:27:07 +0530
From:	Hemant Kumar <hkshaw@...ux.vnet.ibm.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
CC:	linux-kernel@...r.kernel.org, srikar@...ux.vnet.ibm.com,
	peterz@...radead.org, oleg@...hat.com,
	hegdevasant@...ux.vnet.ibm.com, mingo@...hat.com, anton@...hat.com,
	systemtap@...rceware.org, namhyung@...nel.org,
	aravinda@...ux.vnet.ibm.com, penberg@....fi
Subject: Re: [RFC PATCH v1 0/2] perf: Support for SDT markers

On 02/25/2014 05:14 PM, Masami Hiramatsu wrote:
> (2014/02/24 18:14), Hemant Kumar wrote:
>> This patchset helps in listing dtrace style markers(SDT) present in user space
>> applications through perf.
>> Notes/markers are placed at important places by the
>> developers. They have a negligible overhead when not enabled.
>> We can enable them and probe at these places and find some important information
>> like the arguments' values, etc.
>>
>> We have lots of applications which use SDT markers today, like:
>> Postgresql, MySql, Mozilla, Perl, Python, Java, Ruby, libvirt, QEMU, glib
>>
>> To add SDT markers into user applications:
>> We need to have this header sys/sdt.h present.
>> sys/sdt.h used is version 3.
>> If not present, install systemtap-sdt-devel package (for fedora-18).
>>
>> Please refer to the Documentation patch to see how the SDT markers are added into
>> a program.
>>
>> With this patchset,
>> - Use perf to list the markers in the app:
>> # perf list sdt ./user_app
>>
>> ./user_app :
>> %user_app:foo_start
>> %user_app:fun_start
>>
>> - Also, we can see the SDT markers present in our system in the usual binaries.
>> These usual binaries are libraries (dsos) listed by ldconfig --print-cache and some
>> binaries present in PATH environment variable.
>>
>> First, scan the binaries using :
>> # perf list sdt --scan
>>
>> Creating a cache of SDT markers...
>> perf sdt cache created!
>>   Use : "perf list sdt"
>>   to see the SDT markers
> Hmm, in that case, I think you'd better introduce perf-sdt for scanning.
> e.g.
>
>   # perf sdt --scan app

Hmm, this seems a better idea :)

>   then you can add app to sdt cache, without app,
>
>   # perf sdt --scan
>
>   will just scans all binaries on the PATH and the libraries which listed
> by `ldconfig --print-caceh`
>
> And perf-list shows only the SDTs in the cache.

Well, what will be better? perf-list or perf-sdt or perf-list sdt??
If perf-list, then wouldn't it be a huge list!!

>
> If there is no SDTs in the cache, `perf-list sdt` will warn you to
> run the `perf sdt --scan`.

Right.

>
>> After the sdt cache file is created, use perf list to view the markers :
>> # perf list sdt
>>
>> %rtld : init_start
>> %rtld : init_complete
>> %rtld : map_failed
>> %rtld : map_start
>> %rtld : lll_futex_wake
>> ...
>> ...
>> %libgcc : unwind
>> %libvirt : rpc_server_client_auth_allow
>> %libvirt : rpc_server_client_auth_fail
>> %libvirt : rpc_server_client_auth_deny
>>
>> Alternatively, one can view the /var/cache/perf/perf-sdt.cache directly.
>>
>> This link shows an example of marker probing with Systemtap:
>> https://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps
>>
>> Also, this link provides important info regarding SDT notes:
>> http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
>>
>> - Markers in binaries :
>> These SDT markers are present in the ELF in the section named
>> ".note.stapsdt".
>> Here, the name of the marker, its provider, type, location, base
>> address, semaphore address.
>> We can retrieve these values using the members name_off and desc_off in
>> Nhdr structure. If these are not enabled, they are present in the ELF as nop.
>>
>> Changes since last series :
>> - Made the SDT markers present in a system more visible through perf list
>>    as suggested by Pekka Enberg.
>> - Less complex command line interface to access the SDT markers.
>> - Added a cache file to store most of the SDT markers of a system.
>>
>> TODO:
>> - Add support to probe these SDT markers and integrate with a previous patch
>>    (support to perf to probe SDT markers) posted in lkml.
>>    https://lkml.org/lkml/2013/10/23/10
> Yeah, but I think we'd better choose another way to integrate it.
> Since SDT is like markers(static events), setting each of them via perf-probe is
> not intuitive. :)  I'd like to use it as an event, e.g.
>
>   # perf top -e "%libgcc:unwind"
>
> And perf top internally calls perf-probe to add new uprobe event, and
> clean the new event at exit.
>
>
>

Yeah! Right :) Makes sense.

Will implement the suggestions in the next version asap!

-- 
Thanks
Hemant Kumar

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