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SubjectRe: [patch] use cheaper elv_queue_empty when unplug a device
On Tue, Mar 29 2005, Nick Piggin wrote:
> @@ -2577,19 +2577,18 @@ static int __make_request(request_queue_
> spin_lock_prefetch(q->queue_lock);
>
> barrier = bio_barrier(bio);
> - if (barrier && (q->ordered == QUEUE_ORDERED_NONE)) {
> + if (unlikely(barrier) && (q->ordered == QUEUE_ORDERED_NONE)) {
> err = -EOPNOTSUPP;
> goto end_io;
> }
>
> -again:
> spin_lock_irq(q->queue_lock);
>
> if (elv_queue_empty(q)) {
> blk_plug_device(q);
> goto get_rq;
> }

This should just goto get_rq, the plug should happen only at the end
where you did add it:

> @@ -2693,10 +2675,11 @@ get_rq:
> req->rq_disk = bio->bi_bdev->bd_disk;
> req->start_time = jiffies;
>
> + spin_lock_irq(q->queue_lock);
> + if (elv_queue_empty(q))
> + blk_plug_device(q);
> add_request(q, req);
> out:
> - if (freereq)
> - __blk_put_request(q, freereq);
> if (bio_sync(bio))
> __generic_unplug_device(q);
>

--
Jens Axboe

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