[PATCH v4 0/2] Prevent non-NCQ command starvation

[PATCH v4 0/2] Prevent non-NCQ command starvation

[Damien Le Moal]

This small patch series addresses potential command starvation issues
with non-NCQ commands issued to a device accessed through a multi-queue
libsas HBA. In such setup, a non-NCQ command issued to a device may
suffer long delays and even complete starvation if the device is under
a constant workload of NCQ commands issued from multiple queues.

This issue is avoided by limiting requeing request through the block
layer and instead explicitly waiting for the device command queue to
drain before issuing the non-NCQ command. This is done reliably by not
trying to second guess ata EH in case of error or device reset, by
always requeuing a deferred command whenever EH is scheduled for the
device port.

When a non-ncq command is deferred waiting for the device queue to
drain, all new incoming commands are always requeued through the regular
busy mechanism, thus avoiding the need to manage an internal command
queue.

With this, the long latencies observed when executing non-NCQ commands
under a heavy workload of NCQ commands at high queue depth is resolved:
the non-NCQ command latency only depends on how long it takes for all
in-flight NCQ commands to complete.

Changes from v3:
 - Added review and test tags
 - Improved patch 2 commit message
 - Added a comment in ata_scsi_translate() to clarify the handling of
   ata_scsi_defer() return value.

Changes from v2:
 - Improved comment about ata_scsi_qc_new() behavior in patch 1
   (suggested by Garry).
 - Removed some lockdep annotations from patch 2 as they have little
   value.
 - In patch 2, change ata_scsi_requeue_deferred_qc() and
   ata_scsi_schedule_deferred_qc() to return early as suggested by
   Garry.
 - Fixed comments typo in patch 2.

Changes from v1:
 - Fixed ac leak in ata_scsi_defer() in patch 2.
 - Added Hannes review tag

Damien Le Moal (2):
  ata: libata-scsi: refactor ata_scsi_translate()
  ata: libata-scsi: avoid Non-NCQ command starvation

 drivers/ata/libata-core.c |   5 ++
 drivers/ata/libata-eh.c   |   6 ++
 drivers/ata/libata-scsi.c | 163 ++++++++++++++++++++++++++++++++------
 drivers/ata/libata.h      |   2 +
 include/linux/libata.h    |   3 +
 5 files changed, 154 insertions(+), 25 deletions(-)

-- 
2.52.0

[PATCH v4 1/2] ata: libata-scsi: refactor ata_scsi_translate()

[Damien Le Moal]

Factor out of ata_scsi_translate() the code handling queued command
deferral using the port qc_defer callback into the new function
ata_scsi_defer(), and simplify the goto used in ata_scsi_translate().
While at it, also add a lockdep annotation to check that the port lock
is held when ata_scsi_translate() is called.

No functional changes.

Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Igor Pylypiv <ipylypiv@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
---
 drivers/ata/libata-scsi.c | 66 ++++++++++++++++++++++++---------------
 1 file changed, 40 insertions(+), 26 deletions(-)

diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
index 721d3f270c8e..8e04fc173ea3 100644
--- a/drivers/ata/libata-scsi.c
+++ b/drivers/ata/libata-scsi.c
@@ -1691,6 +1691,30 @@ static void ata_scsi_qc_complete(struct ata_queued_cmd *qc)
 	ata_qc_done(qc);
 }
 
+static int ata_scsi_defer(struct ata_port *ap, struct ata_queued_cmd *qc)
+{
+	int ret;
+
+	if (!ap->ops->qc_defer)
+		return 0;
+
+	ret = ap->ops->qc_defer(qc);
+	if (!ret)
+		return 0;
+
+	ata_qc_free(qc);
+
+	switch (ret) {
+	case ATA_DEFER_LINK:
+		return SCSI_MLQUEUE_DEVICE_BUSY;
+	case ATA_DEFER_PORT:
+		return SCSI_MLQUEUE_HOST_BUSY;
+	default:
+		WARN_ON_ONCE(1);
+		return SCSI_MLQUEUE_HOST_BUSY;
+	}
+}
+
 /**
  *	ata_scsi_translate - Translate then issue SCSI command to ATA device
  *	@dev: ATA device to which the command is addressed
@@ -1714,8 +1738,8 @@ static void ata_scsi_qc_complete(struct ata_queued_cmd *qc)
  *	spin_lock_irqsave(host lock)
  *
  *	RETURNS:
- *	0 on success, SCSI_ML_QUEUE_DEVICE_BUSY if the command
- *	needs to be deferred.
+ *	0 on success, SCSI_ML_QUEUE_DEVICE_BUSY or SCSI_MLQUEUE_HOST_BUSY if the
+ *	command needs to be deferred.
  */
 static int ata_scsi_translate(struct ata_device *dev, struct scsi_cmnd *cmd,
 			      ata_xlat_func_t xlat_func)
@@ -1724,56 +1748,46 @@ static int ata_scsi_translate(struct ata_device *dev, struct scsi_cmnd *cmd,
 	struct ata_queued_cmd *qc;
 	int rc;
 
+	lockdep_assert_held(ap->lock);
+
+	/*
+	 * ata_scsi_qc_new() calls scsi_done(cmd) in case of failure. So we
+	 * have nothing further to do here when allocating a qc fails.
+	 */
 	qc = ata_scsi_qc_new(dev, cmd);
 	if (!qc)
-		goto err_mem;
+		return 0;
 
 	/* data is present; dma-map it */
 	if (cmd->sc_data_direction == DMA_FROM_DEVICE ||
 	    cmd->sc_data_direction == DMA_TO_DEVICE) {
 		if (unlikely(scsi_bufflen(cmd) < 1)) {
 			ata_dev_warn(dev, "WARNING: zero len r/w req\n");
-			goto err_did;
+			cmd->result = (DID_ERROR << 16);
+			goto done;
 		}
 
 		ata_sg_init(qc, scsi_sglist(cmd), scsi_sg_count(cmd));
-
 		qc->dma_dir = cmd->sc_data_direction;
 	}
 
 	qc->complete_fn = ata_scsi_qc_complete;
 
 	if (xlat_func(qc))
-		goto early_finish;
+		goto done;
 
-	if (ap->ops->qc_defer) {
-		if ((rc = ap->ops->qc_defer(qc)))
-			goto defer;
-	}
+	rc = ata_scsi_defer(ap, qc);
+	if (rc)
+		return rc;
 
-	/* select device, send command to hardware */
 	ata_qc_issue(qc);
 
 	return 0;
 
-early_finish:
-	ata_qc_free(qc);
-	scsi_done(cmd);
-	return 0;
-
-err_did:
+done:
 	ata_qc_free(qc);
-	cmd->result = (DID_ERROR << 16);
 	scsi_done(cmd);
-err_mem:
 	return 0;
-
-defer:
-	ata_qc_free(qc);
-	if (rc == ATA_DEFER_LINK)
-		return SCSI_MLQUEUE_DEVICE_BUSY;
-	else
-		return SCSI_MLQUEUE_HOST_BUSY;
 }
 
 /**
-- 
2.52.0

[PATCH v4 2/2] ata: libata-scsi: avoid Non-NCQ command starvation

[Damien Le Moal]

When a non-NCQ command is issued while NCQ commands are being executed,
ata_scsi_defer() indicates to ata_scsi_translate() that ata_qc_issue()
should not be called for the command, and instead should return
SCSI_MLQUEUE_XXX_BUSY to defer the command execution.  This command
deferring is correct and as mandated by the ACS specifications since
NCQ and non-NCQ commands cannot be mixed.

However, in the case of a host adapter using multiple submission queues,
when the target device is under a constant load of NCQ commands, there
are no guarantees that requeueing the non-NCQ command will be executed
later and it may be deferred again repeatedly as other submission queues
can constantly issue NCQ commands from different CPUs ahead of the
non-NCQ command. This can lead to very long delays for the execution of
non-NCQ commands, and even complete starvation for these commands in the
worst case scenario.

Since the block layer and the SCSI layer do not distinguish between
queueable (NCQ) and non queueable (non-NCQ) commands, libata-scsi SAT
implementation must ensure forward progress for non-NCQ commands in the
presence of NCQ command traffic. This is similar to what SAS HBAs with a
hardware/firmware based SAT implementation do.

Implement such forward progress guarantee by limiting requeueing of
non-NCQ commands: when a non-NCQ command is received and NCQ commands
are in-flight, do not force a requeue of the non-NCQ command by
returning SCSI_MLQUEUE_XXX_BUSY in ata_scsi_translate() and instead
return 0 to indicate that the command was accepted but hold on to the qc
using the new deferred_qc field of struct ata_port.

This deferred qc will be issued using the work item deferred_qc_work
running the function ata_scsi_deferred_qc_work() once all in-flight
commands complete, which is checked with the port qc_defer() callback
return value indicating that no further delay is necessary. This check
is done using the helper function ata_scsi_schedule_deferred_qc() which
is called from ata_scsi_qc_complete(). This thus excludes this mechanism
from all internal non-NCQ commands issued by ATA EH.

When a port deferred_qc is non NULL, that is, the port has a command
waiting for the device queue to drain, the issuing of all incoming
commands (both NCQ and non-NCQ) is deferred using the regular busy
mechanism. This simplifies the code and also avoids potential denial of
service problems if a user issues too many non-NCQ commands.

Finally, whenever ata EH is scheduled, regardless of the reason, a
deferred qc is always requeued so that it can be retried once EH
completes. This is done by calling the function
ata_scsi_requeue_deferred_qc() from ata_eh_set_pending(). This avoids
the need for any special processing for the deferred qc in case of NCQ
error, link or device reset, or device timeout.

Reported-by: Xingui Yang <yangxingui@huawei.com>
Reported-by: Igor Pylypiv <ipylypiv@google.com>
Fixes: 42f22fe36d51 ("scsi: pm8001: Expose hardware queues for pm80xx")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Tested-by: Igor Pylypiv <ipylypiv@google.com>
Tested-by: Xingui Yang <yangxingui@huawei.com>
---
 drivers/ata/libata-core.c |   5 ++
 drivers/ata/libata-eh.c   |   6 +++
 drivers/ata/libata-scsi.c | 101 +++++++++++++++++++++++++++++++++++++-
 drivers/ata/libata.h      |   2 +
 include/linux/libata.h    |   3 ++
 5 files changed, 116 insertions(+), 1 deletion(-)

diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index 09d8c035fcdf..447d8dc666a4 100644
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -5561,6 +5561,7 @@ struct ata_port *ata_port_alloc(struct ata_host *host)
 	mutex_init(&ap->scsi_scan_mutex);
 	INIT_DELAYED_WORK(&ap->hotplug_task, ata_scsi_hotplug);
 	INIT_DELAYED_WORK(&ap->scsi_rescan_task, ata_scsi_dev_rescan);
+	INIT_WORK(&ap->deferred_qc_work, ata_scsi_deferred_qc_work);
 	INIT_LIST_HEAD(&ap->eh_done_q);
 	init_waitqueue_head(&ap->eh_wait_q);
 	init_completion(&ap->park_req_pending);
@@ -6173,6 +6174,10 @@ static void ata_port_detach(struct ata_port *ap)
 		}
 	}
 
+	/* Make sure the deferred qc work finished. */
+	cancel_work_sync(&ap->deferred_qc_work);
+	WARN_ON(ap->deferred_qc);
+
 	/* Tell EH to disable all devices */
 	ap->pflags |= ATA_PFLAG_UNLOADING;
 	ata_port_schedule_eh(ap);
diff --git a/drivers/ata/libata-eh.c b/drivers/ata/libata-eh.c
index 2586e77ebf45..b90b17f680f8 100644
--- a/drivers/ata/libata-eh.c
+++ b/drivers/ata/libata-eh.c
@@ -917,6 +917,12 @@ static void ata_eh_set_pending(struct ata_port *ap, bool fastdrain)
 
 	ap->pflags |= ATA_PFLAG_EH_PENDING;
 
+	/*
+	 * If we have a deferred qc, requeue it so that it is retried once EH
+	 * completes.
+	 */
+	ata_scsi_requeue_deferred_qc(ap);
+
 	if (!fastdrain)
 		return;
 
diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
index 8e04fc173ea3..126cf2de6b1c 100644
--- a/drivers/ata/libata-scsi.c
+++ b/drivers/ata/libata-scsi.c
@@ -1658,8 +1658,73 @@ static void ata_qc_done(struct ata_queued_cmd *qc)
 	done(cmd);
 }
 
+void ata_scsi_deferred_qc_work(struct work_struct *work)
+{
+	struct ata_port *ap =
+		container_of(work, struct ata_port, deferred_qc_work);
+	struct ata_queued_cmd *qc;
+	unsigned long flags;
+
+	spin_lock_irqsave(ap->lock, flags);
+
+	/*
+	 * If we still have a deferred qc and we are not in EH, issue it. In
+	 * such case, we should not need any more deferring the qc, so warn if
+	 * qc_defer() says otherwise.
+	 */
+	qc = ap->deferred_qc;
+	if (qc && !ata_port_eh_scheduled(ap)) {
+		WARN_ON_ONCE(ap->ops->qc_defer(qc));
+		ap->deferred_qc = NULL;
+		ata_qc_issue(qc);
+	}
+
+	spin_unlock_irqrestore(ap->lock, flags);
+}
+
+void ata_scsi_requeue_deferred_qc(struct ata_port *ap)
+{
+	struct ata_queued_cmd *qc = ap->deferred_qc;
+	struct scsi_cmnd *scmd;
+
+	/*
+	 * If we have a deferred qc when a reset occurs or NCQ commands fail,
+	 * do not try to be smart about what to do with this deferred command
+	 * and simply retry it by completing it with DID_SOFT_ERROR.
+	 */
+	if (!qc)
+		return;
+
+	scmd = qc->scsicmd;
+	ap->deferred_qc = NULL;
+	ata_qc_free(qc);
+	scmd->result = (DID_SOFT_ERROR << 16);
+	scsi_done(scmd);
+}
+
+static void ata_scsi_schedule_deferred_qc(struct ata_port *ap)
+{
+	struct ata_queued_cmd *qc = ap->deferred_qc;
+
+	/*
+	 * If we have a deferred qc, then qc_defer() is defined and we can use
+	 * this callback to determine if this qc is good to go, unless EH has
+	 * been scheduled.
+	 */
+	if (!qc)
+		return;
+
+	if (ata_port_eh_scheduled(ap)) {
+		ata_scsi_requeue_deferred_qc(ap);
+		return;
+	}
+	if (!ap->ops->qc_defer(qc))
+		queue_work(system_highpri_wq, &ap->deferred_qc_work);
+}
+
 static void ata_scsi_qc_complete(struct ata_queued_cmd *qc)
 {
+	struct ata_port *ap = qc->ap;
 	struct scsi_cmnd *cmd = qc->scsicmd;
 	u8 *cdb = cmd->cmnd;
 	bool have_sense = qc->flags & ATA_QCFLAG_SENSE_VALID;
@@ -1689,12 +1754,24 @@ static void ata_scsi_qc_complete(struct ata_queued_cmd *qc)
 	}
 
 	ata_qc_done(qc);
+
+	ata_scsi_schedule_deferred_qc(ap);
 }
 
 static int ata_scsi_defer(struct ata_port *ap, struct ata_queued_cmd *qc)
 {
 	int ret;
 
+	/*
+	 * If we already have a deferred qc, then rely on the SCSI layer to
+	 * requeue and defer all incoming commands until the deferred qc is
+	 * processed, once all on-going commands are completed.
+	 */
+	if (ap->deferred_qc) {
+		ata_qc_free(qc);
+		return SCSI_MLQUEUE_DEVICE_BUSY;
+	}
+
 	if (!ap->ops->qc_defer)
 		return 0;
 
@@ -1702,6 +1779,17 @@ static int ata_scsi_defer(struct ata_port *ap, struct ata_queued_cmd *qc)
 	if (!ret)
 		return 0;
 
+	/*
+	 * We must defer this qc: if this is not a NCQ command, keep this qc
+	 * as a deferred one and wait for all on-going NCQ commands to complete
+	 * before issuing it with the deferred qc work.
+	 */
+	if (!ata_is_ncq(qc->tf.protocol)) {
+		ap->deferred_qc = qc;
+		return SCSI_MLQUEUE_DEVICE_BUSY;
+	}
+
+	/* Use the SCSI layer to requeue and defer the command. */
 	ata_qc_free(qc);
 
 	switch (ret) {
@@ -1776,9 +1864,20 @@ static int ata_scsi_translate(struct ata_device *dev, struct scsi_cmnd *cmd,
 	if (xlat_func(qc))
 		goto done;
 
+	/*
+	 * Check if the qc must be deferred. If yes, do not report the deferral
+	 * for the qc that we retained for processing using the port deferred
+	 * qc work by returning 0 without issuing the qc. This signals to the
+	 * SCSI layer that we * accepted the command and do not need a requeue
+	 * for this deferred qc. This deferred qc will be issued from the port
+	 * deferred qc work context, once the device queue drains.
+	 */
 	rc = ata_scsi_defer(ap, qc);
-	if (rc)
+	if (rc) {
+		if (qc == ap->deferred_qc)
+			return 0;
 		return rc;
+	}
 
 	ata_qc_issue(qc);
 
diff --git a/drivers/ata/libata.h b/drivers/ata/libata.h
index 0e7ecac73680..60a675df61dc 100644
--- a/drivers/ata/libata.h
+++ b/drivers/ata/libata.h
@@ -165,6 +165,8 @@ void ata_scsi_sdev_config(struct scsi_device *sdev);
 int ata_scsi_dev_config(struct scsi_device *sdev, struct queue_limits *lim,
 		struct ata_device *dev);
 int __ata_scsi_queuecmd(struct scsi_cmnd *scmd, struct ata_device *dev);
+void ata_scsi_deferred_qc_work(struct work_struct *work);
+void ata_scsi_requeue_deferred_qc(struct ata_port *ap);
 
 /* libata-eh.c */
 extern unsigned int ata_internal_cmd_timeout(struct ata_device *dev, u8 cmd);
diff --git a/include/linux/libata.h b/include/linux/libata.h
index 39534fafa36a..c5b27d97dfaf 100644
--- a/include/linux/libata.h
+++ b/include/linux/libata.h
@@ -903,6 +903,9 @@ struct ata_port {
 	u64			qc_active;
 	int			nr_active_links; /* #links with active qcs */
 
+	struct work_struct	deferred_qc_work;
+	struct ata_queued_cmd	*deferred_qc;
+
 	struct ata_link		link;		/* host default link */
 	struct ata_link		*slave_link;	/* see ata_slave_link_init() */
 
-- 
2.52.0

Re: [PATCH v4 2/2] ata: libata-scsi: avoid Non-NCQ command starvation

[Niklas Cassel]

On Fri, Jan 02, 2026 at 10:35:33AM +0900, Damien Le Moal wrote:
> @@ -1776,9 +1864,20 @@ static int ata_scsi_translate(struct ata_device *dev, struct scsi_cmnd *cmd,
>  	if (xlat_func(qc))
>  		goto done;
>  
> +	/*
> +	 * Check if the qc must be deferred. If yes, do not report the deferral
> +	 * for the qc that we retained for processing using the port deferred
> +	 * qc work by returning 0 without issuing the qc. This signals to the
> +	 * SCSI layer that we * accepted the command and do not need a requeue

s/we */we/ ?