Re: [PATCH tip/core/rcu 02/19] rcu: Defer reporting RCU-preempt quiescent states when disabled

[Joel Fernandes]

On Tue, Oct 30, 2018 at 05:58:00AM -0700, Paul E. McKenney wrote:
> On Mon, Oct 29, 2018 at 08:44:52PM -0700, Joel Fernandes wrote:
> > On Mon, Oct 29, 2018 at 07:27:35AM -0700, Paul E. McKenney wrote:
> > > On Mon, Oct 29, 2018 at 11:24:42AM +0000, Ran Rozenstein wrote:
> > > > Hi Paul and all,
> > > > 
> > > > > -----Original Message-----
> > > > > From: linux-kernel-owner@xxxxxxxxxxxxxxx [mailto:linux-kernel-
> > > > > owner@xxxxxxxxxxxxxxx] On Behalf Of Paul E. McKenney
> > > > > Sent: Thursday, August 30, 2018 01:21
> > > > > To: linux-kernel@xxxxxxxxxxxxxxx
> > > > > Cc: mingo@xxxxxxxxxx; jiangshanlai@xxxxxxxxx; dipankar@xxxxxxxxxx;
> > > > > akpm@xxxxxxxxxxxxxxxxxxxx; mathieu.desnoyers@xxxxxxxxxxxx;
> > > > > josh@xxxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx; peterz@xxxxxxxxxxxxx;
> > > > > rostedt@xxxxxxxxxxx; dhowells@xxxxxxxxxx; edumazet@xxxxxxxxxx;
> > > > > fweisbec@xxxxxxxxx; oleg@xxxxxxxxxx; joel@xxxxxxxxxxxxxxxxx; Paul E.
> > > > > McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > > Subject: [PATCH tip/core/rcu 02/19] rcu: Defer reporting RCU-preempt
> > > > > quiescent states when disabled
> > > > > 
> > > > > This commit defers reporting of RCU-preempt quiescent states at
> > > > > rcu_read_unlock_special() time when any of interrupts, softirq, or
> > > > > preemption are disabled.  These deferred quiescent states are reported at a
> > > > > later RCU_SOFTIRQ, context switch, idle entry, or CPU-hotplug offline
> > > > > operation.  Of course, if another RCU read-side critical section has started in
> > > > > the meantime, the reporting of the quiescent state will be further deferred.
> > > > > 
> > > > > This also means that disabling preemption, interrupts, and/or softirqs will act
> > > > > as an RCU-preempt read-side critical section.
> > > > > This is enforced by checking preempt_count() as needed.
> > > > > 
> > > > > Some special cases must be handled on an ad-hoc basis, for example,
> > > > > context switch is a quiescent state even though both the scheduler and
> > > > > do_exit() disable preemption.  In these cases, additional calls to
> > > > > rcu_preempt_deferred_qs() override the preemption disabling.  Similar logic
> > > > > overrides disabled interrupts in rcu_preempt_check_callbacks() because in
> > > > > this case the quiescent state happened just before the corresponding
> > > > > scheduling-clock interrupt.
> > > > > 
> > > > > In theory, this change lifts a long-standing restriction that required that if
> > > > > interrupts were disabled across a call to rcu_read_unlock() that the matching
> > > > > rcu_read_lock() also be contained within that interrupts-disabled region of
> > > > > code.  Because the reporting of the corresponding RCU-preempt quiescent
> > > > > state is now deferred until after interrupts have been enabled, it is no longer
> > > > > possible for this situation to result in deadlocks involving the scheduler's
> > > > > runqueue and priority-inheritance locks.  This may allow some code
> > > > > simplification that might reduce interrupt latency a bit.  Unfortunately, in
> > > > > practice this would also defer deboosting a low-priority task that had been
> > > > > subjected to RCU priority boosting, so real-time-response considerations
> > > > > might well force this restriction to remain in place.
> > > > > 
> > > > > Because RCU-preempt grace periods are now blocked not only by RCU read-
> > > > > side critical sections, but also by disabling of interrupts, preemption, and
> > > > > softirqs, it will be possible to eliminate RCU-bh and RCU-sched in favor of
> > > > > RCU-preempt in CONFIG_PREEMPT=y kernels.  This may require some
> > > > > additional plumbing to provide the network denial-of-service guarantees
> > > > > that have been traditionally provided by RCU-bh.  Once these are in place,
> > > > > CONFIG_PREEMPT=n kernels will be able to fold RCU-bh into RCU-sched.
> > > > > This would mean that all kernels would have but one flavor of RCU, which
> > > > > would open the door to significant code cleanup.
> > > > > 
> > > > > Moving to a single flavor of RCU would also have the beneficial effect of
> > > > > reducing the NOCB kthreads by at least a factor of two.
> > > > > 
> > > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> [ paulmck:
> > > > > Apply rcu_read_unlock_special() preempt_count() feedback
> > > > >   from Joel Fernandes. ]
> > > > > [ paulmck: Adjust rcu_eqs_enter() call to rcu_preempt_deferred_qs() in
> > > > >   response to bug reports from kbuild test robot. ] [ paulmck: Fix bug located
> > > > > by kbuild test robot involving recursion
> > > > >   via rcu_preempt_deferred_qs(). ]
> > > > > ---
> > > > >  .../RCU/Design/Requirements/Requirements.html |  50 +++---
> > > > >  include/linux/rcutiny.h                       |   5 +
> > > > >  kernel/rcu/tree.c                             |   9 ++
> > > > >  kernel/rcu/tree.h                             |   3 +
> > > > >  kernel/rcu/tree_exp.h                         |  71 +++++++--
> > > > >  kernel/rcu/tree_plugin.h                      | 144 +++++++++++++-----
> > > > >  6 files changed, 205 insertions(+), 77 deletions(-)
> > > > > 
> > > > 
> > > > We started seeing the trace below in our regression system, after I bisected I found this is the offending commit.
> > > > This appears immediately on boot. 
> > > > Please let me know if you need any additional details.
> > > 
> > > Interesting.  Here is the offending function:
> > > 
> > > 	static void rcu_preempt_deferred_qs(struct task_struct *t)
> > > 	{
> > > 		unsigned long flags;
> > > 		bool couldrecurse = t->rcu_read_lock_nesting >= 0;
> > > 
> > > 		if (!rcu_preempt_need_deferred_qs(t))
> > > 			return;
> > > 		if (couldrecurse)
> > > 			t->rcu_read_lock_nesting -= INT_MIN;
> > > 		local_irq_save(flags);
> > > 		rcu_preempt_deferred_qs_irqrestore(t, flags);
> > > 		if (couldrecurse)
> > > 			t->rcu_read_lock_nesting += INT_MIN;
> > > 	}
> > > 
> > > Using twos-complement arithmetic (which the kernel build gcc arguments
> > > enforce, last I checked) this does work.  But as UBSAN says, subtracting
> > > INT_MIN is unconditionally undefined behavior according to the C standard.
> > > 
> > > Good catch!!!
> > > 
> > > So how do I make the above code not simply function, but rather meet
> > > the C standard?
> > > 
> > > One approach to add INT_MIN going in, then add INT_MAX and then add 1
> > > coming out.
> > > 
> > > Another approach is to sacrifice the INT_MAX value (should be plenty
> > > safe), thus subtract INT_MAX going in and add INT_MAX coming out.
> > > For consistency, I suppose that I should change the INT_MIN in
> > > __rcu_read_unlock() to -INT_MAX.
> > > 
> > > I could also leave __rcu_read_unlock() alone and XOR the top
> > > bit of t->rcu_read_lock_nesting on entry and exit to/from
> > > rcu_preempt_deferred_qs().
> > > 
> > > Sacrificing the INT_MIN value seems most maintainable, as in the following
> > > patch.  Thoughts?
> > 
> > The INT_MAX naming could be very confusing for nesting levels, could we not
> > instead just define something like:
> > #define RCU_NESTING_MIN (INT_MIN - 1)
> > #define RCU_NESTING_MAX (INT_MAX)
> > 
> > and just use that? also one more comment below:
> 
> Hmmm...  There is currently no use for RCU_NESTING_MAX, but if the check
> at the end of __rcu_read_unlock() were to be extended to check for
> too-deep positive nesting, it would need to check for something like
> INT_MAX/2.  You could of course argue that the current check against
> INT_MIN/2 should instead be against -INT_MAX/2, but there really isn't
> much difference between the two.
> 
> Another approach would be to convert to unsigned in order to avoid the
> overflow problem completely.
> 
> For the moment, anyway, I am inclined to leave it as is.

Both the unsigned and INT_MIN/2 options sound good to me, but if you want
leave it as is, that would be fine as well. thanks,

- Joel