sbitmap: randomize initial alloc_hint values
In order to get good cache behavior from a sbitmap, we want each CPU to
stick to its own cacheline(s) as much as possible. This might happen
naturally as the bitmap gets filled up and the alloc_hint values spread
out, but we really want this behavior from the start. blk-mq apparently
intended to do this, but the code to do this was never wired up. Get rid
of the dead code and make it part of the sbitmap library.
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 be55f74..928b82a 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -15,6 +15,7 @@
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
+#include <linux/random.h>
#include <linux/sbitmap.h>
int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
@@ -211,6 +212,11 @@
return -ENOMEM;
}
+ if (depth && !round_robin) {
+ for_each_possible_cpu(i)
+ *per_cpu_ptr(sbq->alloc_hint, i) = prandom_u32() % depth;
+ }
+
sbq->wake_batch = sbq_calc_wake_batch(depth);
atomic_set(&sbq->wake_index, 0);