linux-kernel - Re: Fixing perf top --user shortcoming was: Re: [GIT PULL 0/9] perf/core improvements and fixes

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Message-ID: <20120126192455.GE9128@infradead.org>
Date:	Thu, 26 Jan 2012 17:24:55 -0200
From:	Arnaldo Carvalho de Melo <acme@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	David Daney <david.daney@...ium.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jan Beulich <jbeulich@...e.com>,
	Joerg Roedel <joerg.roedel@....com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung.kim@....com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: Fixing perf top --user shortcoming was: Re: [GIT PULL 0/9]
 perf/core improvements and fixes

Em Thu, Jan 26, 2012 at 07:32:23PM +0100, Ingo Molnar escreveu:
> 
> * Arnaldo Carvalho de Melo <acme@...radead.org> wrote:
> 
> > > > ptrace_may_access(task, PTRACE_MODE_READ) fails for some 
> > > > tasks owned by the user because, IIRC, in 
> > > > __ptrace_may_access:
> > 
> > > Which tasks are these, are they privileged in any sense?
> > 
> > IIRC one of them was a child of sshd, that runs as root and 
> > then changes the child ownership to the user logging in.
> 
> It's probably privileged then - or at least not sufficiently 
> deprivileged.
> 
> Skipping them ought to be the right solution - it's not like 
> such tasks tend to create a lot of overhead worth profiling. 
> They are also not debuggable via gdb so they are not part of the 
> user's development session and such.

Least convoluted way seems to be just to try to call sys_perf_event_open
on then when building the thread list in thread_new__by_uid(), will try
that after soccer :-)

/me vanishes

- Arnaldo

> > I'll continue investigation but probably for now the first 
> > thing to do is to just remove them from the thread_map when 
> > they return -EPERM.
> 
> Yeah. Maybe warn about them in verbose mode or such.
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