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sbitmap: fix batched wait_cnt accounting 

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@kernel.org>
Link: https://lore.kernel.org/r/20220909184022.1709476-1-kbusch@fb.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Keith Busch 2022-09-09 11:40:22 -07:00 committed by Jens Axboe
parent c35227d4e8
commit 4acb83417c
3 changed files with 26 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
block/blk-mq-tag.c
View File

@ -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);

3
include/linux/sbitmap.h
View File

@ -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

37
lib/sbitmap.c
View File

@ -599,24 +599,31 @@ 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)
{
struct sbq_wait_state *ws;
unsigned int wake_batch;
int wait_cnt;
int wait_cnt, cur, sub;
bool ret;
if (*nr <= 0)
return false;
ws = sbq_wake_ptr(sbq);
if (!ws)
return false;
wait_cnt = atomic_dec_return(&ws->wait_cnt);
/*
* For concurrent callers of this, callers should call this function
* again to wakeup a new batch on a different 'ws'.
*/
if (wait_cnt < 0)
return true;
cur = atomic_read(&ws->wait_cnt);
do {
/*
* For concurrent callers of this, callers should call this
* function again to wakeup a new batch on a different 'ws'.
*/
if (cur == 0)
return true;
sub = min(*nr, cur);
wait_cnt = cur - sub;
} while (!atomic_try_cmpxchg(&ws->wait_cnt, &cur, wait_cnt));
/*
* If we decremented queue without waiters, retry to avoid lost
@ -625,6 +632,8 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
if (wait_cnt > 0)
return !waitqueue_active(&ws->wait);
*nr -= sub;
/*
* When wait_cnt == 0, we have to be particularly careful as we are
* responsible to reset wait_cnt regardless whether we've actually
@ -660,12 +669,12 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
sbq_index_atomic_inc(&sbq->wake_index);
atomic_set(&ws->wait_cnt, wake_batch);
return ret;
return ret || *nr;
}
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);
@ -705,7 +714,7 @@ 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);
}
@ -733,7 +742,7 @@ 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);