Re: [PATCH] blk-flush: Queue through IO scheduler when flush not required

From: Jeff Moyer
Date: Mon Nov 16 2015 - 15:14:59 EST

Jan Kara <jack@xxxxxxxx> writes:

> Currently blk_insert_flush() just adds flush request to q->queue_head
> when flush is not required. That completely bypasses IO scheduler so
> e.g. CFQ can be idling waiting for new request to arrive and will idle
> through the whole window unnecessarily. Luckily this only happens in
> rare cases as usually checks in generic_make_request_checks() clear
> FLUSH and FUA flags early if they are not needed.

Right. I think the only way we'd even enter that 'if' block was if the
drive state changed (from WB cache to WT cache) between
generic_make_request_checks and blk_insert_flush.

> When no flushing is actually required, we can easily fix the problem by
> properly queueing the request through the IO scheduler. Ideally IO
> scheduler should be also made aware of requests queued via
> blk_flush_queue_rq(). However inserting flush request through IO
> scheduler can have unwanted side-effects since due to flush batching
> delaying the flush request in IO scheduler will delay all flush requests
> possibly coming from other processes. So we keep adding the request
> directly to q->queue_head.

Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx>

> Signed-off-by: Jan Kara <jack@xxxxxxxx>
> ---
> block/blk-flush.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/block/blk-flush.c b/block/blk-flush.c
> index 9c423e53324a..c81d56ec308f 100644
> --- a/block/blk-flush.c
> +++ b/block/blk-flush.c
> @@ -422,7 +422,7 @@ void blk_insert_flush(struct request *rq)
> if (q->mq_ops) {
> blk_mq_insert_request(rq, false, false, true);
> } else
> - list_add_tail(&rq->queuelist, &q->queue_head);
> + q->elevator->type->ops.elevator_add_req_fn(q, rq);
> return;
> }
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