Date: Thu, 21 Jan 2010 23:09:59 -0500
From: Mathieu Desnoyers <compudj@krystal.dyndns.org>
To: Steven Rostedt <rostedt@goodmis.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>
Subject: Re: [RFC PATCH 03/10] ftrace: Drop the ftrace_profile_enabled
	checks in tracing hot path
Message-ID: <20100122040959.GA11827@Krystal>
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>
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* Steven Rostedt (rostedt@goodmis.org) wrote:
> 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?

Then you may want to make the function tracer depend on CONFIG_FREEZER,
but maybe Masami has other ideas ?

Mathieu

> 
> -- Steve
> 
> 

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