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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Mon, 6 Jul 2020 21:55:57 +0200
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
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        oleg@redhat.com
Subject: Re: [PATCH tip/core/rcu 03/17] rcu/tree: Skip entry into the page
 allocator for PREEMPT_RT
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On Mon, Jul 06, 2020 at 03:42:32PM -0400, Joel Fernandes wrote:
> On Thu, Jul 02, 2020 at 09:45:06PM +0200, Uladzislau Rezki 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?
> > > 
> > > > > 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.
> > > 
> 
> Sorry for my late reply as the day job and family demanded a lot last week...
> 
> > Another way of fixing it is just dropping the lock letting the page
> > allocator to do an allocation without our "upper/local" lock. I did a
> > proposal like that once upon a time, so maybe it is a time to highlight
> > it again:
> > <snip>
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 21c2fa5bd8c3..249f10a89bb9 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3278,9 +3278,11 @@ static void kfree_rcu_monitor(struct work_struct *work)
> >  }
> > 
> >  static inline bool
> > -kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> > +kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
> > +       void *ptr, unsigned long *flags)
> >  {
> >         struct kvfree_rcu_bulk_data *bnode;
> > +       struct kfree_rcu_cpu *tmp;
> >         int idx;
> > 
> >         if (unlikely(!krcp->initialized))
> > @@ -3306,6 +3308,9 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> >                         if (IS_ENABLED(CONFIG_PREEMPT_RT))
> >                                 return false;
> > 
> > +                       migrate_disable();
> > +                       krc_this_cpu_unlock(krcp, *flags);
> 
> If I remember, the issue here is that migrate_disable is not implemented on a
> non-RT kernel due to issues with starvation.
> 
It is implemented. Please have a look linux/preempt.h for regular kernel.

--
Vlad Rezki