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

Uladzislau Rezki
2020-07-02T19:45:14Z

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.
>
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);
+
/*
* NOTE: For one argument of kvfree_rcu() we can
* drop the lock and get the page in sleepable
@@ -3315,6 +3320,12 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
*/
bnode = (struct kvfree_rcu_bulk_data *)
__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
+
+ tmp = krc_this_cpu_lock(flags);
+ migrate_enable();
+
+ /* Sanity check, just in case. */
+ WARN_ON(tmp != krcp);
}

/* Switch to emergency path. */
@@ -3386,7 +3397,7 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
* Under high memory pressure GFP_NOWAIT can fail,
* in that case the emergency path is maintained.
*/
- success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr);
+ success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr, &flags);
if (!success) {
if (head == NULL)
// Inline if kvfree_rcu(one_arg) call.
<snip>

--
Vlad Rezki