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

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Date:	Thu, 04 Dec 2008 13:43:24 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.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>
Subject: Re: [PATCH 3/3] ftrace: add ability to only trace swapper tasks

Steven Rostedt <rostedt@...dmis.org> writes:

> On Thu, 4 Dec 2008, Eric W. Biederman wrote:
>> >> >
>> >> >> > +static struct pid * const ftrace_swapper_pid = (struct pid *)1;
>> >> >> 
>> >> >> eh?
>> >> >
>> >> > all side-effects of getting rid of the integer based PID namespace and 
>> >> > replacing them with struct pid pointers.
>> >> 
>> >> Thanks for asking Andrew it looks like an unnecessary side effect.
>> >
>> > Well, it was necessary without hacking fork.c ;-)
>> 
>> The (struct pid *)1 has always been unnecessary.
>
> Well, I could set it to the &init_struct_pid as you said, but it will not 
> change any of the code below it. So it does not matter what 
> ftrace_swapper_pid is set to, as long as it is not set to something that 
> can be a legitimate pid struct for something not the swapper task.
>
> It will only matter when we fix the fork code.

Well that and if someone dereferences.  

>> As for fork.  It would be nice to remove most of the special cases
>> for the idle thread.  At least the counts are significant.  The rest
>> is pretty much a don't care at this point.
>
> Well, the swapper task should still have a pid of zero. That is probably 
> important.

Right.  I simply meant most of the
if (likely(p->pid)) conditional except for the counts is pretty much a don't
care.  Keeping the idle tasks off of the process list and out of the counts
is useful.

For this particular case what problem did you see with calling attach_pid
with PIDTYPE_PID on init_struct_pid?

Eric

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