On 8/24/22 22:14, Keith Busch wrote:
From: Keith Busch <kbusch@xxxxxxxxxx>
Batched completions can clear multiple bits, but we're only decrementing
the wait_cnt by one each time. This can cause waiters to never be woken,
stalling IO. Use the batched count instead.
Link: https://bugzilla.kernel.org/show_bug.cgi?id=215679
Signed-off-by: Keith Busch <kbusch@xxxxxxxxxx>
---
v1->v2:
Use atomic_try_cmpxchg instead of direct subtraction to prevent a race
between two batched completions.
Remove 'wait_cnt < 0' condition since it's not possible anymore.
Repeat wake up while waiters exist and more bits are unaccounted.
block/blk-mq-tag.c | 2 +-
include/linux/sbitmap.h | 3 ++-
lib/sbitmap.c | 32 ++++++++++++++++++--------------
3 files changed, 21 insertions(+), 16 deletions(-)
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 8e3b36d1cb57..9eb968e14d31 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -196,7 +196,7 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
* other allocations on previous queue won't be starved.
*/
if (bt != bt_prev)
- sbitmap_queue_wake_up(bt_prev);
+ sbitmap_queue_wake_up(bt_prev, 1);
ws = bt_wait_ptr(bt, data->hctx);
} while (1);
diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index 8f5a86e210b9..4d2d5205ab58 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -575,8 +575,9 @@ void sbitmap_queue_wake_all(struct sbitmap_queue *sbq);
* sbitmap_queue_wake_up() - Wake up some of waiters in one waitqueue
* on a &struct sbitmap_queue.
* @sbq: Bitmap queue to wake up.
+ * @nr: Number of bits cleared.
*/
-void sbitmap_queue_wake_up(struct sbitmap_queue *sbq);
+void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr);
/**
* sbitmap_queue_show() - Dump &struct sbitmap_queue information to a &struct
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 1f31147872e6..6fd3bc22c2e7 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -600,28 +600,33 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
return NULL;
}
-static bool __sbq_wake_up(struct sbitmap_queue *sbq)
+static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)
Why is '*nr' a pointer? It's always used as a value, so what does
promoting it to a point buys you?
{
struct sbq_wait_state *ws;
- unsigned int wake_batch;
- int wait_cnt;
+ int wake_batch, wait_cnt, sub, cur;
ws = sbq_wake_ptr(sbq);
if (!ws)
return false;
- wait_cnt = atomic_dec_return(&ws->wait_cnt);
+ wake_batch = READ_ONCE(sbq->wake_batch);
+ do {
+ cur = atomic_read(&ws->wait_cnt);
+ sub = min3(wake_batch, *nr, cur);
+ wait_cnt = cur - sub;
+ } while (!atomic_try_cmpxchg(&ws->wait_cnt, &cur, wait_cnt));
+
+ *nr -= sub;
+
/*
* For concurrent callers of this, callers should call this function
* again to wakeup a new batch on a different 'ws'.
*/
- if (wait_cnt < 0 || !waitqueue_active(&ws->wait))
+ if (!waitqueue_active(&ws->wait))
return true;
if (wait_cnt > 0)
- return false;
-
- wake_batch = READ_ONCE(sbq->wake_batch);
+ return *nr > 0;
/*
* Wake up first in case that concurrent callers decrease wait_cnt
@@ -649,15 +654,14 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
sbq_index_atomic_inc(&sbq->wake_index);
atomic_set(&ws->wait_cnt, wake_batch);
- return false;
+ return *nr > 0;
}
-void sbitmap_queue_wake_up(struct sbitmap_queue *sbq)
+void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
{
- while (__sbq_wake_up(sbq))
+ while (__sbq_wake_up(sbq, &nr))
;
}
-EXPORT_SYMBOL_GPL(sbitmap_queue_wake_up);
static inline void sbitmap_update_cpu_hint(struct sbitmap *sb, int cpu, int tag)
{
@@ -694,7 +698,7 @@ void sbitmap_queue_clear_batch(struct sbitmap_queue *sbq, int offset,
atomic_long_andnot(mask, (atomic_long_t *) addr);
smp_mb__after_atomic();
- sbitmap_queue_wake_up(sbq);
+ sbitmap_queue_wake_up(sbq, nr_tags);
sbitmap_update_cpu_hint(&sbq->sb, raw_smp_processor_id(),
tags[nr_tags - 1] - offset);
}
@@ -722,7 +726,7 @@ void sbitmap_queue_clear(struct sbitmap_queue *sbq, unsigned int nr,
* waiter. See the comment on waitqueue_active().
*/
smp_mb__after_atomic();
- sbitmap_queue_wake_up(sbq);
+ sbitmap_queue_wake_up(sbq, 1);
sbitmap_update_cpu_hint(&sbq->sb, cpu, nr);
}
EXPORT_SYMBOL_GPL(sbitmap_queue_clear);
Cheers,
Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
hare@xxxxxxx +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
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