Date:	Thu, 16 Apr 2009 20:06:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [PATCH 2/2] tracing/events/lockdep: move tracepoints within
 recursive protection

On Thu, 2009-04-16 at 13:58 -0400, Steven Rostedt wrote:
> 
> [ added Maitheu, since he likes things like this ]
> 
> On Thu, 16 Apr 2009, Peter Zijlstra wrote:
> 
> > On Thu, 2009-04-16 at 13:38 -0400, Steven Rostedt wrote:
> > 
> > > > > Note, that the ring buffer and events are made to be recursive. That is, 
> > > > > it allows one event to trace within another event.
> > > > 
> > > > But surely not in the same context. You could do a 4 level recursion
> > > > protection like I did in perf-counter, not allowing recursion in:
> > > > 
> > > >  nmi, irq, softirq, process - context.
> > > 
> > > Why not allow a nested interrupt to trace?
> > > 
> > > I don't want to add this logic to the lower levels, where only a few
> > > users need the protection. The protecting should be at the user level.
> > 
> > wouldn't you want to disable preemption/softirq/irqs in the tracer -- to
> > avoid such recursion to begin with (preemption isn't even strictly
> > needed if you put the recursion count in the task struct, as each task
> > has a new stack anyway).
> 
> No, we only disable preemption, nothing more. Interrupts and softirqs are 
> free to happen. Also, we allow tracing of NMIs.

Right.

> > I think having a recursion detection in place is far more valuable than
> > being able to recursively trace interrupts and the like, which are
> > exceedingly rare (on x86, and power and other arch with multiple
> > interrupt levels that each have their own stack can extend the recursion
> > levels too).
> 
> Is there any arch generic way to tell what level you are at?

No, on x86 there are a few broken ass pieces of hardware/drivers that
require interrupts enabled in the interrupt handler, and can cause
interrupt recursion -- these should be rare and IMHO can be ignored, esp
for a FTRACE_DEBUG option that detects recursion -- that is, simply
disable interrupts when entering the tracer in irq context.

IRQ level nesting like on power would need some arch support.

> That is, at thread context, you are at level 0, if an interrupt comes
> in, it sets you to level 1, if another interrupt comes in, it sets you to 
> level 2, and so on.
> 
> I guess we could add this into the irq_enter/exit sofirq_enter/exit and 
> nmi_enter/exit.
> 
> Thus we can have each task with a bitmask. When we start to trace, we set 
> the bit coresponding to the level the task is at.
> 
> Ie. in thread context, we set bit 0, if we are interrupted by a 
> softirq/irq/nmi, we set the level bit we are at. Hmm, we might be able to 
> do this via the preempt count already :-/
> 
> Just add the softirq/irq/nmi bits together.
> 
> The if the bit is already set we can dump out a warning.
> 
> I'll try that out.

static int *perf_swcounter_recursion_context(struct perf_cpu_context *cpuctx)
{
        if (in_nmi())
                return &cpuctx->recursion[3];

        if (in_irq())
                return &cpuctx->recursion[2];

        if (in_softirq())
                return &cpuctx->recursion[1];

        return &cpuctx->recursion[0];
}

Is what I use for perf-counters.

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