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From: Dennis Zhou <dennisszhou@gmail.com>
To: Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Josef Bacik <josef@toxicpanda.com>
Cc: kernel-team@fb.com, linux-block@vger.kernel.org,
	cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
	"Dennis Zhou (Facebook)" <dennisszhou@gmail.com>
Subject: [PATCH 12/15] blkcg: cleanup and make blk_get_rl use blkg_lookup_create
Date: Thu, 30 Aug 2018 21:53:53 -0400
Message-ID: <20180831015356.69796-13-dennisszhou@gmail.com> (raw)
In-Reply-To: <20180831015356.69796-1-dennisszhou@gmail.com>

From: "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>

blk_get_rl is responsible for identifying which request_list a request
should be allocated to. Try get logic was added earlier, but
semantically the logic was not changed.

This patch makes better use of the bio already having a reference to the
blkg in the hot path. The cold path uses a better fallback of
blkg_lookup_create rather than just blkg_lookup and then falling back to
the q->root_rl. If lookup_create fails with anything but -ENODEV, it
falls back to q->root_rl.

A clarifying comment is added to explain why q->root_rl is used rather
than the root blkg's rl.

Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>
---
 include/linux/blk-cgroup.h | 53 ++++++++++++++++++++++++++------------
 1 file changed, 37 insertions(+), 16 deletions(-)

diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 3eed491e4daa..97cb82029b18 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -537,28 +537,49 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
 
 	rcu_read_lock();
 
-	blkcg = bio_blkcg(bio);
-	if (!blkcg)
+	blkg = bio->bi_blkg;
+	if (blkg) {
+		blkcg = bio->bi_blkg->blkcg;
+		if (blkcg == &blkcg_root)
+			goto rl_use_root;
+
+		blkg_get(blkg);
+		return &blkg->rl;
+	}
+
+	while (true) {
 		blkcg = css_to_blkcg(blkcg_css());
+		if (blkcg == &blkcg_root)
+			goto rl_use_root;
 
-	/* bypass blkg lookup and use @q->root_rl directly for root */
-	if (blkcg == &blkcg_root)
-		goto root_rl;
+		blkg = blkg_lookup(blkcg, q);
 
-	/*
-	 * Try to use blkg->rl.  blkg lookup may fail under memory pressure
-	 * or if either the blkcg or queue is going away.  Fall back to
-	 * root_rl in such cases.
-	 */
-	blkg = blkg_lookup(blkcg, q);
-	if (unlikely(!blkg))
-		goto root_rl;
+		if (unlikely(!blkg))
+			blkg = __blkg_lookup_create(blkcg, q);
+
+		if (IS_ERR(blkg)) {
+			if (PTR_ERR(blkg) == -ENODEV) {
+				cpu_relax();
+				continue;
+			} else {
+				goto rl_use_root;
+			}
+		}
+
+		if (blkg_try_get(blkg))
+			break;
+		cpu_relax();
+	}
 
-	if (!blkg_try_get(blkg))
-		goto root_rl;
 	rcu_read_unlock();
 	return &blkg->rl;
-root_rl:
+
+	/*
+	 * Each blkg has its own request_list, however, the root blkcg
+	 * uses the request_queue's root_rl.  This is to avoid most
+	 * overhead for the root blkcg.
+	 */
+rl_use_root:
 	rcu_read_unlock();
 	return &q->root_rl;
 }
-- 
2.17.1


  parent reply index

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-31  1:53 [PATCH 00/15] blkcg ref count refactor/cleanup + blkcg avg_lat Dennis Zhou
2018-08-31  1:53 ` [PATCH 01/15] Revert "blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()" Dennis Zhou
2018-08-31  1:53 ` [PATCH 02/15] blkcg: delay blkg destruction until after writeback has finished Dennis Zhou
2018-08-31 15:27   ` Josef Bacik
2018-08-31 20:19     ` Dennis Zhou
2018-08-31  1:53 ` [PATCH 03/15] blkcg: use tryget logic when associating a blkg with a bio Dennis Zhou
2018-08-31 15:30   ` Josef Bacik
2018-08-31 20:20     ` Dennis Zhou
2018-08-31  1:53 ` [PATCH 04/15] blkcg: fix ref count issue with bio_blkcg using task_css Dennis Zhou
2018-08-31 15:35   ` Josef Bacik
2018-08-31 23:04     ` Tejun Heo
2018-09-06 15:21     ` Dennis Zhou
2018-08-31  1:53 ` [PATCH 05/15] blkcg: update blkg_lookup_create to do locking Dennis Zhou
2018-08-31 15:37   ` Josef Bacik
2018-08-31 23:09   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 06/15] blkcg: always associate a bio with a blkg Dennis Zhou
2018-08-31  9:01   ` kbuild test robot
2018-08-31 10:02   ` kbuild test robot
2018-08-31 23:16   ` Tejun Heo
2018-09-06 20:41     ` Dennis Zhou
2018-09-07  3:03   ` [LKP] [blkcg] c02c58dab2: WARNING:at_block/blk-throttle.c:#blk_throtl_bio kernel test robot
2018-08-31  1:53 ` [PATCH 07/15] blkcg: consolidate bio_issue_init and blkg association Dennis Zhou
2018-08-31  9:19   ` kbuild test robot
2018-08-31 11:11   ` kbuild test robot
2018-08-31 15:42   ` Josef Bacik
2018-09-06 20:43     ` Dennis Zhou
2018-08-31 23:45   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 08/15] blkcg: associate a blkg for pages being evicted by swap Dennis Zhou
2018-08-31 15:44   ` Josef Bacik
2018-08-31 23:47   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 09/15] blkcg: associate writeback bios with a blkg Dennis Zhou
2018-08-31 15:45   ` Josef Bacik
2018-08-31 23:53   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 10/15] blkcg: remove bio->bi_css and instead use bio->bi_blkg Dennis Zhou
2018-08-31 15:46   ` Josef Bacik
2018-09-01  0:13   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 11/15] blkcg: remove additional reference to the css Dennis Zhou
2018-09-01  0:26   ` Tejun Heo
2018-09-06 20:45     ` Dennis Zhou
2018-08-31  1:53 ` Dennis Zhou [this message]
2018-09-01  0:29   ` [PATCH 12/15] blkcg: cleanup and make blk_get_rl use blkg_lookup_create Tejun Heo
2018-09-11  2:37   ` [LKP] [blkcg] 22f657e287: general_protection_fault:#[##] kernel test robot
2018-08-31  1:53 ` [PATCH 13/15] blkcg: change blkg reference counting to use percpu_ref Dennis Zhou
2018-08-31 15:49   ` Josef Bacik
2018-09-01  0:31   ` Tejun Heo
2018-09-06 20:46     ` Dennis Zhou
2018-09-07  3:08   ` [LKP] [blkcg] 6ef69a3a0b: WARNING:suspicious_RCU_usage kernel test robot
2018-08-31  1:53 ` [PATCH 14/15] blkcg: rename blkg_try_get to blkg_tryget Dennis Zhou
2018-08-31 15:50   ` Josef Bacik
2018-09-01  0:32   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 15/15] blkcg: add average latency tracking to blk-cgroup Dennis Zhou
2018-08-31 10:22   ` kbuild test robot
2018-08-31 11:38   ` kbuild test robot
2018-09-01  0:35 ` [PATCH 00/15] blkcg ref count refactor/cleanup + blkcg avg_lat Tejun Heo

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