Re: [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping due to a preempt_counter change

[Peter Zijlstra]

On Wed, May 29, 2019 at 03:19:57PM +0200, Peter Zijlstra wrote:
> On Wed, May 29, 2019 at 08:39:30AM -0400, Steven Rostedt wrote:
> > I believe I see what Daniel is talking about, but I hate the proposed
> > solution ;-)
> > 
> > First, if you care about real times that the CPU can't preempt
> > (preempt_count != 0 or interrupts disabled), then you want the
> > preempt_irqsoff tracer. The preempt_tracer is more academic where it
> > just shows you when we disable preemption via the counter. But even
> > with the preempt_irqsoff tracer you may not get the full length of time
> > due to the above explained race.
> 
> IOW, that tracer gives a completely 'make believe' number? What's the
> point? Just delete the pure preempt tracer.

Alternatively, fix the preempt tracer by having it completely disregard
IRQs.