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Date: Wed, 09 Jul 2014 13:14:35 +0200
From: Hannes Reinecke <hare@...e.de>
To: Christoph Hellwig <hch@....de>,
James Bottomley <James.Bottomley@...senPartnership.com>
CC: Jens Axboe <axboe@...nel.dk>,
Bart Van Assche <bvanassche@...ionio.com>,
Robert Elliott <Elliott@...com>, linux-scsi@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/14] scsi: push host_lock down into scsi_{host,target}_queue_ready
On 06/25/2014 06:51 PM, Christoph Hellwig wrote:
> Prepare for not taking a host-wide lock in the dispatch path by pushing
> the lock down into the places that actually need it. Note that this
> patch is just a preparation step, as it will actually increase lock
> roundtrips and thus decrease performance on its own.
>
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
> drivers/scsi/scsi_lib.c | 75 ++++++++++++++++++++++++-----------------------
> 1 file changed, 39 insertions(+), 36 deletions(-)
>
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 6989b6f..18e6449 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -1300,18 +1300,18 @@ static inline int scsi_dev_queue_ready(struct request_queue *q,
> /*
> * scsi_target_queue_ready: checks if there we can send commands to target
> * @sdev: scsi device on starget to check.
> - *
> - * Called with the host lock held.
> */
> static inline int scsi_target_queue_ready(struct Scsi_Host *shost,
> struct scsi_device *sdev)
> {
> struct scsi_target *starget = scsi_target(sdev);
> + int ret = 0;
>
> + spin_lock_irq(shost->host_lock);
> if (starget->single_lun) {
> if (starget->starget_sdev_user &&
> starget->starget_sdev_user != sdev)
> - return 0;
> + goto out;
> starget->starget_sdev_user = sdev;
> }
>
> @@ -1319,57 +1319,66 @@ static inline int scsi_target_queue_ready(struct Scsi_Host *shost,
> /*
> * unblock after target_blocked iterates to zero
> */
> - if (--starget->target_blocked == 0) {
> - SCSI_LOG_MLQUEUE(3, starget_printk(KERN_INFO, starget,
> - "unblocking target at zero depth\n"));
> - } else
> - return 0;
> + if (--starget->target_blocked != 0)
> + goto out;
> +
> + SCSI_LOG_MLQUEUE(3, starget_printk(KERN_INFO, starget,
> + "unblocking target at zero depth\n"));
> }
>
> if (scsi_target_is_busy(starget)) {
> list_move_tail(&sdev->starved_entry, &shost->starved_list);
> - return 0;
> + goto out;
> }
>
> - return 1;
> + scsi_target(sdev)->target_busy++;
> + ret = 1;
> +out:
> + spin_unlock_irq(shost->host_lock);
> + return ret;
> }
>
> /*
> * scsi_host_queue_ready: if we can send requests to shost, return 1 else
> * return 0. We must end up running the queue again whenever 0 is
> * returned, else IO can hang.
> - *
> - * Called with host_lock held.
> */
> static inline int scsi_host_queue_ready(struct request_queue *q,
> struct Scsi_Host *shost,
> struct scsi_device *sdev)
> {
> + int ret = 0;
> +
> + spin_lock_irq(shost->host_lock);
> +
> if (scsi_host_in_recovery(shost))
> - return 0;
> + goto out;
> if (shost->host_busy == 0 && shost->host_blocked) {
> /*
> * unblock after host_blocked iterates to zero
> */
> - if (--shost->host_blocked == 0) {
> - SCSI_LOG_MLQUEUE(3,
> - shost_printk(KERN_INFO, shost,
> - "unblocking host at zero depth\n"));
> - } else {
> - return 0;
> - }
> + if (--shost->host_blocked != 0)
> + goto out;
> +
> + SCSI_LOG_MLQUEUE(3,
> + shost_printk(KERN_INFO, shost,
> + "unblocking host at zero depth\n"));
> }
> if (scsi_host_is_busy(shost)) {
> if (list_empty(&sdev->starved_entry))
> list_add_tail(&sdev->starved_entry, &shost->starved_list);
> - return 0;
> + goto out;
> }
>
> /* We're OK to process the command, so we can't be starved */
> if (!list_empty(&sdev->starved_entry))
> list_del_init(&sdev->starved_entry);
>
> - return 1;
> + shost->host_busy++;
> + ret = 1;
> +out:
> + spin_unlock_irq(shost->host_lock);
> + return ret;
> }
>
> /*
> @@ -1550,7 +1559,7 @@ static void scsi_request_fn(struct request_queue *q)
> blk_start_request(req);
> sdev->device_busy++;
>
> - spin_unlock(q->queue_lock);
> + spin_unlock_irq(q->queue_lock);
> cmd = req->special;
> if (unlikely(cmd == NULL)) {
> printk(KERN_CRIT "impossible request in %s.\n"
> @@ -1560,7 +1569,6 @@ static void scsi_request_fn(struct request_queue *q)
> blk_dump_rq_flags(req, "foo");
> BUG();
> }
> - spin_lock(shost->host_lock);
>
> /*
> * We hit this when the driver is using a host wide
> @@ -1571,9 +1579,11 @@ static void scsi_request_fn(struct request_queue *q)
> * a run when a tag is freed.
> */
> if (blk_queue_tagged(q) && !blk_rq_tagged(req)) {
> + spin_lock_irq(shost->host_lock);
> if (list_empty(&sdev->starved_entry))
> list_add_tail(&sdev->starved_entry,
> &shost->starved_list);
> + spin_unlock_irq(shost->host_lock);
> goto not_ready;
> }
>
> @@ -1581,16 +1591,7 @@ static void scsi_request_fn(struct request_queue *q)
> goto not_ready;
>
> if (!scsi_host_queue_ready(q, shost, sdev))
> - goto not_ready;
> -
> - scsi_target(sdev)->target_busy++;
> - shost->host_busy++;
> -
> - /*
> - * XXX(hch): This is rather suboptimal, scsi_dispatch_cmd will
> - * take the lock again.
> - */
> - spin_unlock_irq(shost->host_lock);
> + goto host_not_ready;
>
> /*
> * Finally, initialize any error handling parameters, and set up
> @@ -1613,9 +1614,11 @@ static void scsi_request_fn(struct request_queue *q)
>
> return;
>
> - not_ready:
> + host_not_ready:
> + spin_lock_irq(shost->host_lock);
> + scsi_target(sdev)->target_busy--;
> spin_unlock_irq(shost->host_lock);
> -
> + not_ready:
> /*
> * lock q, handle tag, requeue req, and decrement device_busy. We
> * must return with queue_lock held.
>
Reviewed-by: Hannes Reinecke <hare@...e.de>
Cheers,
Hannes
--
Dr. Hannes Reinecke zSeries & Storage
hare@...e.de +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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