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 2020-06-30 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?

> > 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.

> 							Thanx, Paul
> 
> > >  			bnode = (struct kfree_rcu_bulk_data *)
> > >  				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
> > >  		}

Sebastian