linux-kernel - Re: [PATCH] tracing/ftrace: use preempt_enable_no_resched_notrace in ring_buffer_time

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Date:	Tue, 16 Dec 2008 22:50:33 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing/ftrace: use preempt_enable_no_resched_notrace
	in ring_buffer_time_stamp()


* Frédéric Weisbecker <fweisbec@...il.com> wrote:

> 2008/12/16 Ingo Molnar <mingo@...e.hu>:
> >
> > * Frederic Weisbecker <fweisbec@...il.com> wrote:
> >
> >> Impact: prevent a trace recursion
> >>
> >> After some tests with function graph tracer under x86-32, I saw some recursions
> >> caused by ring_buffer_time_stamp() that calls preempt_enable_no_notrace() which
> >> calls preempt_schedule() which is traced itself.
> >>
> >> This patch re-enables preemption without rescheduling.
> >>
> >> Cc: Steven Rostedt <rostedt@...dmis.org>
> >> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> >> ---
> >>  kernel/trace/ring_buffer.c |    2 +-
> >>  1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > applied to tip/tracing/ftrace, thanks Frederic!
> >
> > Does this explain+fix the weird crashes/reboots you were seeing?
> >
> >        Ingo
> >
> 
> No, actually the functions tracers are protected against recursion,
> but rescheduling attempts
> during all trace insertions (and moreover a call to
> prepare_ftrace_return with cancelled insertion)
> is a useless payload.
> 
> The hard reboots I've seen are related to x86-64 while 
> disabling/reenabling a CPU through /sys/device/system/cpu No tracer was 
> enabled at these times (the problem still remains with latest updates on 
> -tip for half an hour).
> 
> One other thing: I've seen these pointless calls to preempt_schedule 
> while testing the function graph tracer on VirtualBox. Since it provides 
> virtual serial lines, it was more easy to debug than usual (I haven't 
> any serial line on my box). The function graph tracer hangs on 
> VirtualBox with a x86-32 -tip. It seems that the tracing is too slow and 
> so hrtimer_interrupt() does an eternal loop, assuming that a new time 
> update has to be done (because too much time elapsed during th tracing) 
> after each iteration. I'm not sure what to do...disabling tracing for 
> hrtimers or....

reduce HZ?

	Ingo
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