linux-kernel - Re: [PATCH 3/3] ftrace: add ability to only trace swapper tasks

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Message-ID: <alpine.DEB.1.10.0812040953470.5240@gandalf.stny.rr.com>
Date:	Thu, 4 Dec 2008 09:57:40 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	containers@...ts.osdl.org,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Oleg Nesterov <oleg@...sign.ru>
Subject: Re: [PATCH 3/3] ftrace: add ability to only trace swapper tasks


On Thu, 4 Dec 2008, Eric W. Biederman wrote:

> Steven Rostedt <rostedt@...dmis.org> writes:
> >
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index 10b1d7c..eb57dc1 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -49,6 +49,7 @@ static int last_ftrace_enabled;
> >  
> >  /* set when tracing only a pid */
> >  struct pid *ftrace_pid_trace;
> > +static struct pid * const ftrace_swapper_pid = (struct pid *)1;
> 
> The initializer should be spelled &init_struct_pid instead of (struct pid *)1;
> 
> Except for the special case of finding this unhashed pid, by making the
> attach_pid(p, PIDTYPE_PID, pid) unconditional in copy_process, you can
> make the rest of the special cases go away.

Not that easy. It turns out that all idle tasks share the init_struct_pid, 
and if you add the attach_pid() outside the if statement, you only change 
the owner of the init_struct_pid to the last CPU to come on line.

I tried to allocate the pid, but by doing that it seems that the alloc_pid 
code will assign a number other than 0.  The changes needed to fork.c to 
make this work is quickly going beyond a trivial fix, where I do not want 
to become a pid container developer in order to implement it.

-- Steve

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