From 05fd095d53b979878f016c3a7080d3683cc89d72 Mon Sep 17 00:00:00 2001 From: Omar Sandoval Date: Sat, 17 Sep 2016 01:28:26 -0700 Subject: [PATCH] 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 Signed-off-by: Jens Axboe --- lib/sbitmap.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/lib/sbitmap.c b/lib/sbitmap.c index 928b82a733f2..f736c52a712c 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); } -- 2.20.1