Re: [RFC PATCH RT 4/4] rcutorture: Avoid problematic critical section nesting

From: Scott Wood
Date: Fri Jun 28 2019 - 15:37:31 EST

On Thu, 2019-06-27 at 17:52 -0700, Paul E. McKenney wrote:
> On Thu, Jun 27, 2019 at 05:46:27PM -0500, Scott Wood wrote:
> > On Thu, 2019-06-27 at 13:50 -0700, Paul E. McKenney wrote:
> > > If by IPI-to-self you mean the IRQ work trick, that isn't implemented
> > > across all architectures yet, is it?
> >
> > Right... smp_send_reschedule() has wider coverage, but even then there's
> > some hardware that just can't do it reasonably (e.g. pre-APIC x86).
> Except that smp_send_reschedule() won't do anything unless the scheduler
> things something needs to be done, as it its wake list is non-empty.
> Which might explain why Peter Zijlstra didn't suggest it.

The wake list stuff is separate from the original purpose of the IPI, which
is to hit the need_resched check on IRQ exit. When that happens, the
scheduler will call into RCU, even if it doesn't change threads.

> > So I guess the options are:
> >
> > 1. Accept that such hardware might experience delayed grace period
> > completion in certain configurations,
> > 2. Have such hardware check for need_resched in local_irq_enable() (not
> > nice
> > if sharing a kernel build with hardware that doesn't need it), or
> > 3. Forbid the sequence (enforced by debug checks). Again, this would
> > only
> > prohibit rcu_read_lock()/local_irq_disable()/rcu_read_unlock()/
> > local_irq_enable() *without* preempt disabling around the IRQ-disabled
> > region.
> 4. If further testing continues to show it to be reliable, continue
> using the scheme in -rcu.

If the testing isn't done on machines that can't do the IPI then it's
basically option #1. FWIW I don't think option #1 is unreasonable given
that we're talking about very old and/or specialized hardware, and we're
only talking about delays, not a crash (maybe limit the ability to use
nohz_full on such hardware?). Of course if it turns out people are actually
trying to run (modern versions of) RT on such hardware, that might be
different. :-)

> 5. Use a short-duration hrtimer to get a clean environment in short
> order. Yes, the timer might fire while preemption and/or softirqs
> are disabled, but then the code can rely on the following
> preempt_enable(), local_bh_enable(), or whatever. This condition
> should be sufficiently rare to avoid issues with hrtimer overhead.

Yeah, I considered that but was hesitant due to overhead -- at least in the
case of the example I gave (pre-APIC x86), arming a oneshot timer is pretty
slow. Plus, some hardware might entirely lack one-shot timer capability.

> 6. Use smp_call_function_single() to IPI some other poor slob of a
> CPU, which then does the same back. Non-waiting version in both
> cases, of course.

I was assuming any hardware that can't do smp_send_reschedule() is not SMP.

> Probably others as well.
> > > Why not simply make rcutorture cyheck whether it is running in a
> > > PREEMPT_RT_FULL environment and avoid the PREEMPT_RT_FULL-unfriendly
> > > testing only in that case?
> > >
> > > And should we later get to a place where the PREEMPT_RT_FULL-
> > > unfriendly
> > > scenarios are prohibited across all kernel configurations, then the
> > > module
> > > parameter can be removed. Again, until we know (as opposed to
> > > suspect)
> > > that these scenarios really don't happen, mainline rcutorture must
> > > continue testing them.
> >
> > Yes, I already acknowledged that debug checks detecting the sequences
> > should
> > come before the test removal
> OK, good to hear. As you may have noticed, I was getting the impression
> that you might have changed your mind on this point. ;-)
> > (including this patch as an RFC at this
> > point
> > was mainly meant as a demonstration of what's needed to get rcutorture
> > to
> > pass), but it'd be nice to have some idea of whether there would be
> > opposition to the concept before coding up the checks. I'd rather not
> > continue the state of "these sequences can blow up on RT and we don't
> > know
> > if they exist or not" any longer than necessary. Plus, only one of the
> > sequences is exclusively an RT issue (though it's the one with the worst
> > consequences).
> Steve Rostedt's point about enlisting the aid of lockdep seems worth
> looking into.

Sure. I was just concerned by the "Linus was against enforcing this in the
past" comment and was hoping for more details.