linux-kernel - Re: [RFC PATCH 03/10] ftrace: Drop the ftrace_profile

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Date:	Mon, 25 Jan 2010 21:58:16 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Masami Hiramatsu <mhiramat@...hat.com>
Subject: Re: [RFC PATCH 03/10] ftrace: Drop the ftrace_profile_enabled
	checks in tracing hot path

On Fri, Jan 22, 2010 at 07:34:51AM -0500, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@...dmis.org) wrote:
> > On Thu, 2010-01-21 at 23:09 -0500, Mathieu Desnoyers wrote:
> > > * Steven Rostedt (rostedt@...dmis.org) wrote:
> > 
> > > > Hmm, interesting. Maybe something like that might work. But what if
> > > > CONFIG_PREEMPT is enabled but CONFIG_FREEZER is not?
> > > 
> > > Then you may want to make the function tracer depend on CONFIG_FREEZER,
> > > but maybe Masami has other ideas ?
> > 
> > egad no! This is just to help add guarantees to those that use the
> > function tracer that when the tracing is disabled, it is guaranteed that
> > no more tracing will be called by the function tracer. Currently,
> > nothing relies on this. But we may add cases that might need this.
> 
> Yep, identifying tracer quiescent state can become handy.
> 
> > 
> > In fact, only those that need this requirement would need to do this
> > trick. Anyway, we could make those depend on CONFIG_FREEZER, but that
> > just seems to be a strange dependency.
> 
> This makes me wonder (question for Masami)...
> 
> static int __kprobes check_safety(void)
> {
>         int ret = 0;
> #if defined(CONFIG_PREEMPT) && defined(CONFIG_FREEZER)
>         ret = freeze_processes();
>         if (ret == 0) {
>                 struct task_struct *p, *q;
>                 do_each_thread(p, q) {
>                         if (p != current && p->state == TASK_RUNNING &&
>                             p->pid != 0) {
>                                 printk("Check failed: %s is running\n",p->comm);
>                                 ret = -1;
>                                 goto loop_end;
>                         }
>                 } while_each_thread(p, q);



How does that deal with kernel threads that don't freeze?

Also freezing every processes seems a bit of a heavy thing for that.
Looks like a synchronize_tasks() would be really useful.

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