linux-kernel - [PATCH v3 3/9] blk-throttle: ignore cgroup without io queued in blk_throtl_cancel

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Message-Id: <20221205115709.251489-4-shikemeng@huaweicloud.com>
Date:   Mon,  5 Dec 2022 19:57:03 +0800
From:   Kemeng Shi <shikemeng@...weicloud.com>
To:     tj@...nel.org, josef@...icpanda.com, axboe@...nel.dk
Cc:     cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, shikemeng@...wei.com,
        linfeilong@...wei.com, liuzhiqiang26@...wei.com
Subject: [PATCH v3 3/9] blk-throttle: ignore cgroup without io queued in blk_throtl_cancel_bios

From: Kemeng Shi <shikemeng@...wei.com>

Ignore cgroup without io queued in blk_throtl_cancel_bios for two
reasons:
1. Save cpu cycle for trying to dispatch cgroup which is no io queued.
2. Avoid non-consistent state that cgroup is inserted to service queue
without THROTL_TG_PENDING set as tg_update_disptime will unconditional
re-insert cgroup to service queue. If we are on the default hierarchy,
IO dispatched from child in tg_dispatch_one_bio will trigger inserting
cgroup to service queue without erase first and ruin the tree.

Signed-off-by: Kemeng Shi <shikemeng@...wei.com>
Acked-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>
---
 block/blk-throttle.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 2444ebf5f11d..75010110d481 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1738,7 +1738,18 @@ void blk_throtl_cancel_bios(struct gendisk *disk)
 		 * Set the flag to make sure throtl_pending_timer_fn() won't
 		 * stop until all throttled bios are dispatched.
 		 */
-		blkg_to_tg(blkg)->flags |= THROTL_TG_CANCELING;
+		tg->flags |= THROTL_TG_CANCELING;
+
+		/*
+		 * Do not dispatch cgroup without THROTL_TG_PENDING or cgroup
+		 * will be inserted to service queue without THROTL_TG_PENDING
+		 * set in tg_update_disptime below. Then IO dispatched from
+		 * child in tg_dispatch_one_bio will trigger double insertion
+		 * and corrupt the tree.
+		 */
+		if (!(tg->flags & THROTL_TG_PENDING))
+			continue;
+
 		/*
 		 * Update disptime after setting the above flag to make sure
 		 * throtl_select_dispatch() won't exit without dispatching.
-- 
2.30.0