On 2022/05/11 22:08, Jan Kara wrote:
On Tue 10-05-22 21:16:29, Yu Kuai wrote:
bfq_has_work() is using busy_queues currently, which is not accurate
because bfq_queue is busy doesn't represent that it has requests. Since
bfqd aready has a counter 'queued' to record how many requests are in
bfq, use it instead of busy_queues.
Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the
lock can't be held in bfq_has_work() to protect 'bfqd->queued'.
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
So did you find this causing any real problem? Because bfq queue is
accounted among busy queues once bfq_add_bfqq_busy() is called. And that
happens once a new request is inserted into the queue so it should be very
similar to bfqd->queued.
Honza
Hi,
The related problem is described here:
https://lore.kernel.org/all/20220510112302.1215092-1-yukuai3@xxxxxxxxxx/
The root cause of the panic is a linux-block problem, however, it can
be bypassed if bfq_has_work() is accurate. On the other hand,
unnecessary run_work will be triggered if bfqq stays busy:
__blk_mq_run_hw_queue
__blk_mq_sched_dispatch_requests
__blk_mq_do_dispatch_sched
if (!bfq_has_work())
break;
blk_mq_delay_run_hw_queues -> run again after 3ms
Thanks,
Kuai
---
block/bfq-iosched.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 61750696e87f..1d2f8110c26b 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5063,11 +5063,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx)
struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
/*
- * Avoiding lock: a race on bfqd->busy_queues should cause at
+ * Avoiding lock: a race on bfqd->queued should cause at
* most a call to dispatch for nothing
*/
return !list_empty_careful(&bfqd->dispatch) ||
- bfq_tot_busy_queues(bfqd) > 0;
+ READ_ONCE(bfqd->queued);
}
static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
--
2.31.1
