Re: [PATCH tip/core/rcu 03/17] rcu/tree: Skip entry into the page allocator for PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2020-07-02 09:48:26 [-0700], Paul E. McKenney wrote:
> On Thu, Jul 02, 2020 at 04:12:16PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2020-06-30 11:35:34 [-0700], Paul E. McKenney wrote:
> > > > This is not going to work together with the "wait context validator"
> > > > (CONFIG_PROVE_RAW_LOCK_NESTING). As of -rc3 it should complain about
> > > > printk() which is why it is still disabled by default.
> > > 
> > > Fixing that should be "interesting".  In particular, RCU CPU stall
> > > warnings rely on the raw spin lock to reduce false positives due
> > > to race conditions.  Some thought will be required here.
> > 
> > I don't get this part. Can you explain/give me an example where to look
> > at?
> 
> Starting from the scheduler-clock interrupt's call into RCU,
> we have rcu_sched_clock_irq() which calls rcu_pending() which
> calls check_cpu_stall() which calls either print_cpu_stall() or
> print_other_cpu_stall(), depending on whether the stall is happening on
> the current CPU or on some other CPU, respectively.
> 
> Both of these last functions acquire the rcu_node structure's raw ->lock
> and expect to do printk()s while holding it.

…
> Thoughts?

Okay. So in the RT queue there is a printk() rewrite which fixes this
kind of things. Upstream the printk() interface is still broken in this
regard and therefore CONFIG_PROVE_RAW_LOCK_NESTING is disabled.
[Earlier the workqueue would also trigger a warning but this has been
fixed as of v5.8-rc1.]
This was just me explaining why this bad, what debug function would
report it and why it is not enabled by default.

> > > > So assume that this is fixed and enabled then on !PREEMPT_RT it will
> > > > complain that you have a raw_spinlock_t acquired (the one from patch
> > > > 02/17) and attempt to acquire a spinlock_t in the memory allocator.
> > > 
> > > Given that the slab allocator doesn't acquire any locks until it gets
> > > a fair way in, wouldn't it make sense to allow a "shallow" allocation
> > > while a raw spinlock is held?  This would require yet another GFP_ flag,
> > > but that won't make all that much of a difference in the total.  ;-)
> > 
> > That would be one way of dealing with. But we could go back to
> > spinlock_t and keep the memory allocation even for RT as is. I don't see
> > a downside of this. And we would worry about kfree_rcu() from real
> > IRQ-off region once we get to it.
> 
> Once we get to it, your thought would be to do per-CPU queuing of
> memory from IRQ-off kfree_rcu(), and have IRQ work or some such clean
> up after it?  Or did you have some other trick in mind?

So for now I would very much like to revert the raw_spinlock_t back to
the spinlock_t and add a migrate_disable() just avoid the tiny
possible migration between obtaining the CPU-ptr and acquiring the lock
(I think Joel was afraid of performance hit).

Should we get to a *real* use case where someone must invoke kfree_rcu()
from a hard-IRQ-off region then we can think what makes sense. per-CPU
queues and IRQ-work would be one way of dealing with it.

> 							Thanx, Paul
> 
Sebastian