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From: Frederic Weisbecker <fweisbec@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC PATCH 2/4] sched: Use traced preempt count operations to toggle PREEMPT_ACTIVE
Date: Wed, 28 Jan 2015 14:59:43 +0100	[thread overview]
Message-ID: <20150128135941.GA29870@lerouge> (raw)
In-Reply-To: <20150127204239.3d3bc2e4@gandalf.local.home>

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@gmail.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
> 

  reply	other threads:[~2015-01-28 21:13 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-01-28  0:24 [PATCH 0/4] sched: schedule/preempt optimizations and cleanups Frederic Weisbecker
2015-01-28  0:24 ` [PATCH 1/4] sched: Pull resched loop to __schedule() callers Frederic Weisbecker
2015-02-04 14:36   ` [tip:sched/core] " tip-bot for Frederic Weisbecker
2015-01-28  0:24 ` [RFC PATCH 2/4] sched: Use traced preempt count operations to toggle PREEMPT_ACTIVE Frederic Weisbecker
2015-01-28  1:42   ` Steven Rostedt
2015-01-28 13:59     ` Frederic Weisbecker [this message]
2015-01-28 15:04       ` Steven Rostedt
2015-01-28 15:42   ` Peter Zijlstra
2015-02-02 17:22     ` Frederic Weisbecker
2015-01-28  0:24 ` [PATCH 3/4] sched: Pull preemption disablement to __schedule() caller Frederic Weisbecker
2015-01-28 15:50   ` Peter Zijlstra
2015-02-02 17:53     ` Frederic Weisbecker
2015-02-03 10:53       ` Peter Zijlstra
2015-02-04 17:31         ` Frederic Weisbecker
2015-02-04 17:48           ` Peter Zijlstra
2015-01-28  0:24 ` [RFC PATCH 4/4] sched: Account PREEMPT_ACTIVE context as atomic Frederic Weisbecker
2015-01-28 15:46   ` Peter Zijlstra
2015-02-02 17:29     ` Frederic Weisbecker

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