From: Jens Axboe <axboe@kernel.dk>
Date: Mon, 13 Jul 2020 15:48:16 -0600
Subject: [PATCH] block: relax jiffies rounding for timeouts
References: bsc#1175995,jsc#SLE-15608
Git-commit: 9054650fac24b784df8500aba2869ebf240d069a
Patch-mainline: v5.9-rc1

In doing high IOPS testing, blk-mq is generally pretty well optimized.
There are a few things that stuck out as using more CPU than what is
really warranted, and one thing is the round_jiffies_up() that we do
twice for each request. That accounts for about 0.8% of the CPU in
my testing.

We can make this cheaper by avoiding an integer division, by just adding
a rough HZ mask that we can AND with instead. The timeouts are only on a
second granularity already, we don't have to be that accurate here and
this patch barely changes that. All we care about is nice grouping.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Hannes Reinecke <hare@suse.de>
---
 block/blk-timeout.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/block/blk-timeout.c b/block/blk-timeout.c
index 8aa68fae96ad..164d5da7cd65 100644
--- a/block/blk-timeout.c
+++ b/block/blk-timeout.c
@@ -90,11 +90,29 @@ void blk_abort_request(struct request *req)
 }
 EXPORT_SYMBOL_GPL(blk_abort_request);
 
+static unsigned long blk_timeout_mask __read_mostly;
+
+int __init blk_timeout_init(void)
+{
+	blk_timeout_mask = roundup_pow_of_two(HZ) - 1;
+	return 0;
+}
+
+late_initcall(blk_timeout_init);
+
+/*
+ * Just a rough estimate, we don't care about specific values for timeouts.
+ */
+static inline unsigned long blk_round_jiffies(unsigned long j)
+{
+	return (j + blk_timeout_mask) + 1;
+}
+
 unsigned long blk_rq_timeout(unsigned long timeout)
 {
 	unsigned long maxt;
 
-	maxt = round_jiffies_up(jiffies + BLK_MAX_TIMEOUT);
+	maxt = blk_round_jiffies(jiffies + BLK_MAX_TIMEOUT);
 	if (time_after(timeout, maxt))
 		timeout = maxt;
 
@@ -131,7 +149,7 @@ void blk_add_timer(struct request *req)
 	 * than an existing one, modify the timer. Round up to next nearest
 	 * second.
 	 */
-	expiry = blk_rq_timeout(round_jiffies_up(expiry));
+	expiry = blk_rq_timeout(blk_round_jiffies(expiry));
 
 	if (!timer_pending(&q->timeout) ||
 	    time_before(expiry, q->timeout.expires)) {
-- 
2.16.4