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Message-ID: <20120126183223.GB8630@elte.hu>
Date: Thu, 26 Jan 2012 19:32:23 +0100
From: Ingo Molnar <mingo@...e.hu>
To: Arnaldo Carvalho de Melo <acme@...radead.org>
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
* 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.
> 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
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