[PATCH v4 0/3] keepalive bugfixes

[PATCH v4 0/3] keepalive bugfixes

[Uday Shankar]

While reviewing the Linux KATO implementation in an attempt to better
understand the current NVMe Keep Alive specification, we found a
few issues in the host implementation that could contribute to spurious
Keep Alive timeouts being detected by controllers.

Changes v3-v4 (https://lore.kernel.org/linux-nvme/20230518183311.3224326-1-ushankar@purestorage.com/):
- Patch 1: fix style, improve readability, include helper from patch 3
- Patch 2: refactor to remove need for WRITE_ONCE/barriers
- Patch 3: fix comment style, move helper to patch 1

Changes v2-v3 (https://lore.kernel.org/linux-nvme/20230424232225.1975793-1-ushankar@purestorage.com/):
- Patch 3: add a warning log line for long keepalive RTT

Changes v1-v2 (https://lore.kernel.org/linux-nvme/20230417225558.2890062-1-ushankar@purestorage.com/):
- Patch 2: fix indentation, set start_time even when stats disabled

Uday Shankar (3):
  nvme: double KA polling frequency to avoid KATO with TBKAS on
  nvme: check IO start time when deciding to defer KA
  nvme: improve handling of long keep alives

 drivers/nvme/host/core.c | 46 +++++++++++++++++++++++++++++++++++++---
 drivers/nvme/host/nvme.h |  4 ++--
 2 files changed, 45 insertions(+), 5 deletions(-)

-- 
2.25.1

[PATCH v4 1/3] nvme: double KA polling frequency to avoid KATO with TBKAS on

[Uday Shankar]

With TBKAS on, the completion of one command can defer sending a
keep alive for up to twice the delay between successive runs of
nvme_keep_alive_work. The current delay of KATO / 2 thus makes it
possible for one command to defer sending a keep alive for up to
KATO, which can result in the controller detecting a KATO. The following
trace demonstrates the issue, taking KATO = 8 for simplicity:

1. t = 0: run nvme_keep_alive_work, no keep-alive sent
2. t = ε: I/O completion seen, set comp_seen = true
3. t = 4: run nvme_keep_alive_work, see comp_seen == true,
          skip sending keep-alive, set comp_seen = false
4. t = 8: run nvme_keep_alive_work, see comp_seen == false,
          send a keep-alive command.

Here, there is a delay of 8 - ε between receiving a command completion
and sending the next command. With ε small, the controller is likely to
detect a keep alive timeout.

Fix this by running nvme_keep_alive_work with a delay of KATO / 4
whenever TBKAS is on. Going through the above trace now gives us a
worst-case delay of 4 - ε, which is in line with the recommendation of
sending a command every KATO / 2 in the NVMe specification.

Reported-by: Costa Sapuntzakis <costa@purestorage.com>
Reported-by: Randy Jennings <randyj@purestorage.com>
Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
---
 drivers/nvme/host/core.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index f586a4808e6e..5ad2b9b0d3b9 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1161,9 +1161,25 @@ EXPORT_SYMBOL_NS_GPL(nvme_passthru_end, NVME_TARGET_PASSTHRU);
  *   The host should send Keep Alive commands at half of the Keep Alive Timeout
  *   accounting for transport roundtrip times [..].
  */
+static unsigned long nvme_keep_alive_work_period(struct nvme_ctrl *ctrl)
+{
+	unsigned long delay = ctrl->kato * HZ / 2;
+
+	/*
+	 * When using Traffic Based Keep Alive, we need to run
+	 * nvme_keep_alive_work at twice the normal frequency, as one
+	 * command completion can postpone sending a keep alive command
+	 * by up to twice the delay between runs.
+	 */
+	if (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS)
+		delay /= 2;
+	return delay;
+}
+
 static void nvme_queue_keep_alive_work(struct nvme_ctrl *ctrl)
 {
-	queue_delayed_work(nvme_wq, &ctrl->ka_work, ctrl->kato * HZ / 2);
+	queue_delayed_work(nvme_wq, &ctrl->ka_work,
+			   nvme_keep_alive_work_period(ctrl));
 }
 
 static enum rq_end_io_ret nvme_keep_alive_end_io(struct request *rq,
-- 
2.25.1

[PATCH v4 2/3] nvme: check IO start time when deciding to defer KA

[Uday Shankar]

When a command completes, we set a flag which will skip sending a
keep alive at the next run of nvme_keep_alive_work when TBKAS is on.
However, if the command was submitted long ago, it's possible that
the controller may have also restarted its keep alive timer (as a
result of receiving the command) long ago. The following trace
demonstrates the issue, assuming TBKAS is on and KATO = 8 for
simplicity:

1. t = 0: submit I/O commands A, B, C, D, E
2. t = 0.5: commands A, B, C, D, E reach controller, restart its keep
            alive timer
3. t = 1: A completes
4. t = 2: run nvme_keep_alive_work, see recent completion, do nothing
5. t = 3: B completes
6. t = 4: run nvme_keep_alive_work, see recent completion, do nothing
7. t = 5: C completes
8. t = 6: run nvme_keep_alive_work, see recent completion, do nothing
9. t = 7: D completes
10. t = 8: run nvme_keep_alive_work, see recent completion, do nothing
11. t = 9: E completes

At this point, 8.5 seconds have passed without restarting the
controller's keep alive timer, so the controller will detect a keep
alive timeout.

Fix this by checking the IO start time when deciding to defer sending a
keep alive command. Only set comp_seen if the command started after the
most recent run of nvme_keep_alive_work. With this change, the
completions of B, C, and D will not set comp_seen and the run of
nvme_keep_alive_work at t = 4 will send a keep alive.

Reported-by: Costa Sapuntzakis <costa@purestorage.com>
Reported-by: Randy Jennings <randyj@purestorage.com>
Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
---
 drivers/nvme/host/core.c | 12 +++++++++++-
 drivers/nvme/host/nvme.h |  4 ++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 5ad2b9b0d3b9..54f5440a0b83 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -397,7 +397,13 @@ void nvme_complete_rq(struct request *req)
 	trace_nvme_complete_rq(req);
 	nvme_cleanup_cmd(req);
 
-	if (ctrl->kas)
+	/*
+	 * Completions of long-running commands should not be able to
+	 * defer sending of periodic keep alives, since the controller
+	 * may have completed processing such commands a long time ago
+	 * (arbitrarily close to command submission time).
+	 */
+	if (ctrl->kas && nvme_req(req)->start_time >= ctrl->ka_last_check_time)
 		ctrl->comp_seen = true;
 
 	switch (nvme_decide_disposition(req)) {
@@ -1173,6 +1179,7 @@ static unsigned long nvme_keep_alive_work_period(struct nvme_ctrl *ctrl)
 	 */
 	if (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS)
 		delay /= 2;
+
 	return delay;
 }
 
@@ -1198,6 +1205,7 @@ static enum rq_end_io_ret nvme_keep_alive_end_io(struct request *rq,
 		return RQ_END_IO_NONE;
 	}
 
+	ctrl->ka_last_check_time = jiffies;
 	ctrl->comp_seen = false;
 	spin_lock_irqsave(&ctrl->lock, flags);
 	if (ctrl->state == NVME_CTRL_LIVE ||
@@ -1216,6 +1224,8 @@ static void nvme_keep_alive_work(struct work_struct *work)
 	bool comp_seen = ctrl->comp_seen;
 	struct request *rq;
 
+	ctrl->ka_last_check_time = jiffies;
+
 	if ((ctrl->ctratt & NVME_CTRL_ATTR_TBKAS) && comp_seen) {
 		dev_dbg(ctrl->device,
 			"reschedule traffic based keep-alive timer\n");
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index f0a84e390a55..07ed39ea43e3 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -162,9 +162,7 @@ struct nvme_request {
 	u8			retries;
 	u8			flags;
 	u16			status;
-#ifdef CONFIG_NVME_MULTIPATH
 	unsigned long		start_time;
-#endif
 	struct nvme_ctrl	*ctrl;
 };
 
@@ -323,6 +321,7 @@ struct nvme_ctrl {
 	struct delayed_work ka_work;
 	struct delayed_work failfast_work;
 	struct nvme_command ka_cmd;
+	unsigned long ka_last_check_time;
 	struct work_struct fw_act_work;
 	unsigned long events;
 
@@ -1032,6 +1031,7 @@ static inline void nvme_start_request(struct request *rq)
 {
 	if (rq->cmd_flags & REQ_NVME_MPATH)
 		nvme_mpath_start_request(rq);
+	nvme_req(rq)->start_time = jiffies;
 	blk_mq_start_request(rq);
 }
 
-- 
2.25.1

Re: [PATCH v4 2/3] nvme: check IO start time when deciding to defer KA

[Keith Busch]

On Wed, May 24, 2023 at 01:38:08PM -0600, Uday Shankar wrote:
> +	/*
> +	 * Completions of long-running commands should not be able to
> +	 * defer sending of periodic keep alives, since the controller
> +	 * may have completed processing such commands a long time ago
> +	 * (arbitrarily close to command submission time).
> +	 */
> +	if (ctrl->kas && nvme_req(req)->start_time >= ctrl->ka_last_check_time)
>  		ctrl->comp_seen = true;

Instead of saving start_time, 'req->deadline - req->timeout' should get
the same result. And you won't have to set nvme's start_time twice for
the mpath case.

[PATCH v4 3/3] nvme: improve handling of long keep alives

[Uday Shankar]

Upon keep alive completion, nvme_keep_alive_work is scheduled with the
same delay every time. If keep alive commands are completing slowly,
this may cause a keep alive timeout. The following trace illustrates the
issue, taking KATO = 8 and TBKAS off for simplicity:

1. t = 0: run nvme_keep_alive_work, send keep alive
2. t = ε: keep alive reaches controller, controller restarts its keep
          alive timer
3. t = 4: host receives keep alive completion, schedules
          nvme_keep_alive_work with delay 4
4. t = 8: run nvme_keep_alive_work, send keep alive

Here, a keep alive having RTT of 4 causes a delay of at least 8 - ε
between the controller receiving successive keep alives. With ε small,
the controller is likely to detect a keep alive timeout.

Fix this by calculating the RTT of the keep alive command, and adjusting
the scheduling delay of the next keep alive work accordingly.

Reported-by: Costa Sapuntzakis <costa@purestorage.com>
Reported-by: Randy Jennings <randyj@purestorage.com>
Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
---
 drivers/nvme/host/core.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 54f5440a0b83..a4287ae967b2 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1195,6 +1195,20 @@ static enum rq_end_io_ret nvme_keep_alive_end_io(struct request *rq,
 	struct nvme_ctrl *ctrl = rq->end_io_data;
 	unsigned long flags;
 	bool startka = false;
+	unsigned long rtt = jiffies - nvme_req(rq)->start_time;
+	unsigned long delay = nvme_keep_alive_work_period(ctrl);
+
+	/*
+	 * Subtract off the keepalive RTT so nvme_keep_alive_work runs
+	 * at the desired frequency.
+	 */
+	if (rtt <= delay) {
+		delay -= rtt;
+	} else {
+		dev_warn(ctrl->device, "long keepalive RTT (%u ms)\n",
+			 jiffies_to_msecs(rtt));
+		delay = 0;
+	}
 
 	blk_mq_free_request(rq);
 
@@ -1213,7 +1227,7 @@ static enum rq_end_io_ret nvme_keep_alive_end_io(struct request *rq,
 		startka = true;
 	spin_unlock_irqrestore(&ctrl->lock, flags);
 	if (startka)
-		nvme_queue_keep_alive_work(ctrl);
+		queue_delayed_work(nvme_wq, &ctrl->ka_work, delay);
 	return RQ_END_IO_NONE;
 }
 
-- 
2.25.1