linux-kernel - [PATCH -next RFC 4/6] sbitmap: wake up the number of threads based on required tags

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Date:   Tue, 29 Mar 2022 17:40:46 +0800
From:   Yu Kuai <yukuai3@...wei.com>
To:     <axboe@...nel.dk>, <andriy.shevchenko@...ux.intel.com>,
        <john.garry@...wei.com>, <ming.lei@...hat.com>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yukuai3@...wei.com>, <yi.zhang@...wei.com>
Subject: [PATCH -next RFC 4/6] sbitmap: wake up the number of threads based on required tags

Currently, __sbq_wake_up() will wake up 'wake_batch' threads
unconditionally, for split io this will intensify competition and split
io won't be issued sequentially.

This modification can optimize the ratio of sequentially for split
big io, however, in order to gain optimal result, tag preemption still
need to be disabled, which will be done in later patches.

Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
 lib/sbitmap.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index ae4fd4de9ebe..9d04c0ecc8f7 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -597,6 +597,26 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
 	return NULL;
 }
 
+static unsigned int get_wake_nr(struct sbq_wait_state *ws, unsigned int nr_tags)
+{
+	struct sbq_wait *wait;
+	struct wait_queue_entry *entry;
+	unsigned int nr = 1;
+
+	spin_lock_irq(&ws->wait.lock);
+	list_for_each_entry(entry, &ws->wait.head, entry) {
+		wait = container_of(entry, struct sbq_wait, wait);
+		if (nr_tags <= wait->nr_tags)
+			break;
+
+		nr++;
+		nr_tags -= wait->nr_tags;
+	}
+	spin_unlock_irq(&ws->wait.lock);
+
+	return nr;
+}
+
 static bool __sbq_wake_up(struct sbitmap_queue *sbq)
 {
 	struct sbq_wait_state *ws;
@@ -628,7 +648,7 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
 		ret = atomic_cmpxchg(&ws->wait_cnt, wait_cnt, wake_batch);
 		if (ret == wait_cnt) {
 			sbq_index_atomic_inc(&sbq->wake_index);
-			wake_up_nr(&ws->wait, wake_batch);
+			wake_up_nr(&ws->wait, get_wake_nr(ws, wake_batch));
 			return false;
 		}
 
-- 
2.31.1