Commit 05fd095d authored Sep 17, 2016 by Omar Sandoval Committed by Jens Axboe Sep 17, 2016

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>

parent 98d95416

lib/sbitmap.c

+7 −2

Original line number	Diff line number	Diff line
		@@ -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);
		}

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