From: Omar Sandoval <osan...@fb.com>
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 <osan...@fb.com>
---
lib/sbitmap.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 928b82a..f736c52 100644
--- 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);
}
--
2.9.3
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