linux-kernel - Re: [RFC PATCH 2/4] sched: Use traced preempt count operations to toggle PREEMPT

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Message-ID: <20150128135941.GA29870@lerouge>
Date:	Wed, 28 Jan 2015 14:59:43 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH 2/4] sched: Use traced preempt count operations to
 toggle PREEMPT_ACTIVE

On Tue, Jan 27, 2015 at 08:42:39PM -0500, Steven Rostedt wrote:
> On Wed, 28 Jan 2015 01:24:10 +0100
> Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > d1f74e20b5b064a130cd0743a256c2d3cfe84010 turned PREEMPT_ACTIVE modifiers
> > to use raw untraced preempt count operations. Meanwhile this prevents
> > from debugging and tracing preemption disabled if we pull that
> > responsibility to schedule() callers (see following patches).
> > 
> > Is there anything we can do about that?
> > 
> 
> I'm trying to understand how you solved the recursion issue with
> preempt_schedule()?

I don't solve it, I rather outline the issue to make sure it's still
a problem today. I can keep the non-traced API but we'll loose debuggability
and latency measurement in preempt_schedule(). But I think this was already
the case before my patchset.

> 
> Here's what happens:
> 
> preempt_schedule()
>   preempt_count_add() -> gets traced by function tracer
>      function_trace_call()
>          preempt_disable_notrace()
>          [...]
>          preempt_enable_notrace() -> sees NEED_RESCHED set
>              preempt_schedule()
>                 preempt_count_add() -> gets traced
>                    function_trace_call()
>                        preempt_disable_notrace()
>                        preempt_enable_notrace() -> sees NEED_RESCHED set
> 
>                           [etc etc until BOOM!]
> 
> Perhaps if you can find a way to clear NEED_RECHED before disabling
> preemption, then it would work. But I don't see that in the patch
> series.

Something like this in function tracing?

prev_resched = need_resched();
do_function_tracing()
    preempt_disable()
    .....
    preempt_enable_no_resched()

if (!prev_resched && need_resched())
    preempt_schedule()

Sounds like a good idea. More overhead but maybe more stability.

> 
> -- Steve
> 
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