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Date: Mon, 25 Jan 2010 21:58:16 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Mathieu Desnoyers <compudj@krystal.dyndns.org>
Cc: Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@elte.hu>,
       LKML <linux-kernel@vger.kernel.org>, Li Zefan <lizf@cn.fujitsu.com>,
       Lai Jiangshan <laijs@cn.fujitsu.com>,
       Masami Hiramatsu <mhiramat@redhat.com>
Subject: Re: [RFC PATCH 03/10] ftrace: Drop the ftrace_profile_enabled
	checks in tracing hot path
Message-ID: <20100125205814.GD5087@nowhere>
References: <1264122982-1553-1-git-send-regression-fweisbec@gmail.com> <1264122982-1553-4-git-send-regression-fweisbec@gmail.com> <1264125917.31321.312.camel@gandalf.stny.rr.com> <20100122022858.GA2604@Krystal> <1264129978.31321.333.camel@gandalf.stny.rr.com> <20100122040959.GA11827@Krystal> <1264135966.31321.375.camel@gandalf.stny.rr.com> <20100122123450.GA19348@Krystal>
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On Fri, Jan 22, 2010 at 07:34:51AM -0500, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@goodmis.org) wrote:
> > On Thu, 2010-01-21 at 23:09 -0500, Mathieu Desnoyers wrote:
> > > * Steven Rostedt (rostedt@goodmis.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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