Commit 05fd095d authored by Omar Sandoval's avatar Omar Sandoval Committed by Jens Axboe

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: default avatarOmar Sandoval <>
Signed-off-by: default avatarJens Axboe <>
parent 98d95416
......@@ -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);
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment