linux-kernel - Re: [RFC] perf-cache command interface design

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Date:	Mon, 10 Nov 2014 10:31:31 -0200
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Hagen Paul Pfeifer <hagen@...u.net>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Hemant Kumar <hemant@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	srikar@...ux.vnet.ibm.com, peterz@...radead.org, oleg@...hat.com,
	hegdevasant@...ux.vnet.ibm.com, mingo@...hat.com,
	systemtap@...rceware.org, aravinda@...ux.vnet.ibm.com,
	penberg@....fi, brendan.d.gregg@...il.com,
	"yrl.pp-manager.tt@...achi.com" <yrl.pp-manager.tt@...achi.com>
Subject: Re: [RFC] perf-cache command interface design

Em Mon, Nov 10, 2014 at 01:05:45PM +0100, Hagen Paul Pfeifer escreveu:
> On 7 November 2014 09:21, Masami Hiramatsu
> <masami.hiramatsu.pt@...achi.com> wrote:
> 
> > File Format
> > ===========
> > All the cache files are placed under ~/.debug/ by default.
> > The paths of buildid cache of binary/symbols are not changed.
> >
> > The SDT/probe caches are placed under the ~/.debug/.probes/path/to/bin/bu/ildid
> > and that is linked to ~/.debug/.probes/.buildid/bu/ildid
> > # To avoid conflict with files under /probes/*, I picked up .probes/.
> 
> A little bit late: but why ~/.debug? Why not $XDG_CACHE_HOME/perf/ as

Why "perf"?

> the root for all perf related files? debug is not unique nor is it sufficient

Probably should be some other name then, but ~/.debug/ is there since
the build-id cache was introduced, guess this is why it is being kept so
far in Masami's proposal.

> to meet file hierarchy: man file-hierarchy(8)

Humm,

It starts in $HOME/.debug/ because this is not supposed to be a system
wide cache, a developer can, for instance:

1. perf record myapp
2. edit myapp.c
3. rebuild it
4. perf record myapp
5. perf diff

And it will show the difference from the previous version, for which it
stored a copy of its binary on its private, keyed by build-id, cache.

So this is not merely a place where we will read stuff from.

If somebody decides to have it in a place accessible by multiple users
on the same system, then, yes, some more suitable place outside its
$HOME is needed, and then tooling should look there as well as on the
places it already looks for such files, i.e. $HOME/.debug/,
/usr/lib/debug (where foo-debuginfo packages store stuff, also keyed by
build-id), /lib/modules/$KVR/, etc.

- Arnaldo
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