sbitmap: re-initialize allocation hints after resize

After a struct sbitmap_queue is resized smaller, the allocation hints
may still be set to bits beyond the new depth of the bitmap. This means
that, for example, if the number of blk-mq tags is reduced through
sysfs, more requests than the nominal queue depth may be in flight.

It's tempting to fix this at resize time by doing a one-time
reinitialization of the hints, but this can race with
__sbitmap_queue_get() updating the hint. Instead, check the hint before
we use it. This caused no measurable performance difference in my
synthetic benchmarks.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 928b82a..f736c52 100644 (file)
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -246,10 +246,15 @@ EXPORT_SYMBOL_GPL(sbitmap_queue_resize);
 
 int __sbitmap_queue_get(struct sbitmap_queue *sbq)
 {
-       unsigned int hint;
+       unsigned int hint, depth;
        int nr;
 
        hint = this_cpu_read(*sbq->alloc_hint);
+       depth = READ_ONCE(sbq->sb.depth);
+       if (unlikely(hint >= depth)) {
+               hint = depth ? prandom_u32() % depth : 0;
+               this_cpu_write(*sbq->alloc_hint, hint);
+       }
        nr = sbitmap_get(&sbq->sb, hint, sbq->round_robin);
 
        if (nr == -1) {
@@ -258,7 +263,7 @@ int __sbitmap_queue_get(struct sbitmap_queue *sbq)
        } else if (nr == hint || unlikely(sbq->round_robin)) {
                /* Only update the hint if we used it. */
                hint = nr + 1;
-               if (hint >= sbq->sb.depth - 1)
+               if (hint >= depth - 1)
                        hint = 0;
                this_cpu_write(*sbq->alloc_hint, hint);
        }