Re: [PATCH RFC v3 1/3] sbitmap: fix that same waitqueue can be woken up continuously

[Yu Kuai]

Hi!

在 2022/07/11 22:20, Jan Kara 写道:
> On Sun 10-07-22 12:21:58, Yu Kuai wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > __sbq_wake_up		__sbq_wake_up
> >   sbq_wake_ptr -> assume	0
> > 			 sbq_wake_ptr -> 0
> >   atomic_dec_return
> > 			atomic_dec_return
> >   atomic_cmpxchg -> succeed
> > 			 atomic_cmpxchg -> failed
> > 			  return true
> >
> > 			__sbq_wake_up
> > 			 sbq_wake_ptr
> > 			  atomic_read(&sbq->wake_index) -> still 0
> >   sbq_index_atomic_inc -> inc to 1
> > 			  if (waitqueue_active(&ws->wait))
> > 			   if (wake_index != atomic_read(&sbq->wake_index))
> > 			    atomic_set -> reset from 1 to 0
> >   wake_up_nr -> wake up first waitqueue
> > 			    // continue to wake up in first waitqueue
> >
> > Fix the problem by using atomic_cmpxchg() instead of atomic_set()
> > to update 'wake_index'.
> >
> > Fixes: 417232880c8a ("sbitmap: Replace cmpxchg with xchg")
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> I don't think this patch is really needed after the following patches.  As
> I see it, wake_index is just a performance optimization (plus a fairness
> improvement) but in principle the code in sbq_wake_ptr() is always prone to
> races as the waitqueue it returns needn't have any waiters by the time we
> return. So for correctness the check-and-retry loop needs to happen at
> higher level than inside sbq_wake_ptr() and occasional wrong setting of
> wake_index will result only in a bit of unfairness and more scanning
> looking for suitable waitqueue but I don't think that really justifies the
> cost of atomic operations in cmpxchg loop...

It's right this patch just improve fairness. However, in hevyload tests
I found that the 'wrong setting of wake_index' can happen frequently,
for consequence, some waitqueue can be empty while some waitqueue have
a lot of waiters.

There shoud be lots of work to fix unfairness throughly, I can remove
this patch for now.

Thanks,
Kuai
> 								Honza
> > ---
> >   lib/sbitmap.c | 15 ++++++++++-----
> >   1 file changed, 10 insertions(+), 5 deletions(-)
> >
> > diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> > index 29eb0484215a..b46fce1beb3a 100644
> > --- a/lib/sbitmap.c
> > +++ b/lib/sbitmap.c
> > @@ -579,19 +579,24 @@ EXPORT_SYMBOL_GPL(sbitmap_queue_min_shallow_depth);
> >   
> >   static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
> >   {
> > -	int i, wake_index;
> > +	int i, wake_index, old_wake_index;
> >   
> > +again:
> >   	if (!atomic_read(&sbq->ws_active))
> >   		return NULL;
> >   
> > -	wake_index = atomic_read(&sbq->wake_index);
> > +	old_wake_index = wake_index = atomic_read(&sbq->wake_index);
> >   	for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
> >   		struct sbq_wait_state *ws = &sbq->ws[wake_index];
> >   
> >   		if (waitqueue_active(&ws->wait)) {
> > -			if (wake_index != atomic_read(&sbq->wake_index))
> > -				atomic_set(&sbq->wake_index, wake_index);
> > -			return ws;
> > +			if (wake_index == old_wake_index)
> > +				return ws;
> > +
> > +			if (atomic_cmpxchg(&sbq->wake_index, old_wake_index,
> > +					   wake_index) == old_wake_index)
> > +				return ws;
> > +			goto again;
> >   		}
> >   
> >   		wake_index = sbq_index_inc(wake_index);
> > --
> > 2.31.1