blk-iolatency: only call ktime_get() if needed

ktime_to_ns(ktime_get()), which is expensive, does not need to be called
if blk_iolatency_enabled() return false in blkcg_iolatency_done_bio().
Postponing ktime_to_ns(ktime_get()) execution reduces the CPU usage when
blk_iolatency is disabled.

Signed-off-by: Hongnan Li <hongnan.li@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index c128d50..f90429c 100644 (file)
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -591,7 +591,7 @@ static void blkcg_iolatency_done_bio(struct rq_qos *rqos, struct bio *bio)
        struct rq_wait *rqw;
        struct iolatency_grp *iolat;
        u64 window_start;
-       u64 now = ktime_to_ns(ktime_get());
+       u64 now;
        bool issue_as_root = bio_issue_as_root_blkg(bio);
        bool enabled = false;
        int inflight = 0;
@@ -608,6 +608,7 @@ static void blkcg_iolatency_done_bio(struct rq_qos *rqos, struct bio *bio)
        if (!enabled)
                return;
 
+       now = ktime_to_ns(ktime_get());
        while (blkg && blkg->parent) {
                iolat = blkg_to_lat(blkg);
                if (!iolat) {