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

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Message-ID: <20100122123450.GA19348@Krystal>
Date:	Fri, 22 Jan 2010 07:34:51 -0500
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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

* 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);
        }
loop_end:
        thaw_processes();
#else
        synchronize_sched();
#endif
        return ret;
}

How does that deal with CONFIG_PREEMPT && !CONFIG_FREEZER exactly ?

Thanks,

Mathieu

> 
> -- Steve
> 
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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