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

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Date:	Mon, 25 Jan 2010 17:14:16 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	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>
Subject: Re: [RFC PATCH 03/10] ftrace: Drop the ftrace_profile_enabled	checks
 in tracing hot path

Frederic Weisbecker wrote:
> 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?

Hmm, right. It can't handle non-freezable kernel threads.

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

Sure :-)
Maybe, I'd better remove booster support on preemptive kernel until then.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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