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Date: Thu, 26 Jan 2012 19:32:23 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: linux-kernel@vger.kernel.org, David Ahern <dsahern@gmail.com>,
        David Daney <david.daney@cavium.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Jan Beulich <jbeulich@suse.com>, Joerg Roedel <joerg.roedel@amd.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Mike Galbraith <efault@gmx.de>, Namhyung Kim <namhyung.kim@lge.com>,
        Paul Mackerras <paulus@samba.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Stephane Eranian <eranian@google.com>
Subject: Re: Fixing perf top --user shortcoming was: Re: [GIT PULL 0/9]
 perf/core improvements and fixes
Message-ID: <20120126183223.GB8630@elte.hu>
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* Arnaldo Carvalho de Melo <acme@infradead.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.

> 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.

> > If yes and if most of the 'real' tasks a user have can be 
> > profiled just fine then i think we should just skip the 
> > privileged tasks and not abort the profiling session?
> 
> Yeah, that can be done, while debugging I'll emit a warning 
> with the resulting thread_map of "special tasks" to figure out 
> what makes them special.

Ok, sounds great!

Thanks,

	Ingo