Subject: Re: [RFC PATCH 03/10] ftrace: Drop the ftrace_profile_enabled
 checks in tracing hot path
From: Steven Rostedt <rostedt@goodmis.org>
Reply-To: rostedt@goodmis.org
To: Mathieu Desnoyers <compudj@krystal.dyndns.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, 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>
In-Reply-To: <20100122022858.GA2604@Krystal>
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	 <1264125917.31321.312.camel@gandalf.stny.rr.com>
	 <20100122022858.GA2604@Krystal>
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Organization: Kihon Technologies Inc.
Date: Thu, 21 Jan 2010 22:12:58 -0500
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On Thu, 2010-01-21 at 21:28 -0500, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@goodmis.org) wrote:

> > Now for the reason I Cc'd Paul and Mathieu...
> > 
> > If we had a synchronize_sched() like function that would wait and return
> > when all preempted tasks have been scheduled again and went to either
> > userspace or called schedule directly, then we could actually do this.
> > 
> > After unregistering the function graph trace, you call this
> > "synchronize_tasks()" and it will guarantee that all currently preempted
> > tasks have either went to userspace or have called schedule() directly.
> > Then it would be safe to remove this check.
> 
> OK, so basically you need to know when you reach a quiescent state, but
> preemption is enabled and there is no RCU read lock taken around these
> code paths, am I correct ?
> 
> With tracepoints, life is easy because I disable preemption around the
> calls, so I can use synchronize_sched() to know when quiescent state is
> reached.
> 
> I recommend looking at kernel/kprobes.c:check_safety(). It uses
> thaw_processes() and synchronize_sched() for this purpose. Basically, it
> rely on the "refrigeration" points to detect such quiescent state. This
> trick should do the job for the function graph tracer too.
> 
> I'm adding Masami in CC. He is the one who implemented check_safety(),
> and I remember discussing it with him in the past.

Hmm, interesting. Maybe something like that might work. But what if
CONFIG_PREEMPT is enabled but CONFIG_FREEZER is not?

-- Steve


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