[PATCH v2 0/3] keepalive bugfixes

[PATCH v2 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 since v1 (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 | 38 +++++++++++++++++++++++++++++++++++---
 drivers/nvme/host/nvme.h |  4 ++--
 2 files changed, 37 insertions(+), 5 deletions(-)

-- 
2.25.1

[PATCH v2 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>
---
 drivers/nvme/host/core.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 6c1e7d6709e0..1298c7b9bffb 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1150,10 +1150,16 @@ 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 [..].
+ * 
+ * When TBKAS is on, we need to run nvme_keep_alive_work at twice this
+ * frequency, as one command completion can postpone sending a keep alive
+ * command by up to twice the delay between runs.
  */
 static void nvme_queue_keep_alive_work(struct nvme_ctrl *ctrl)
 {
-	queue_delayed_work(nvme_wq, &ctrl->ka_work, ctrl->kato * HZ / 2);
+	unsigned long delay = (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS) ?
+		ctrl->kato * HZ / 4 : ctrl->kato * HZ / 2;
+	queue_delayed_work(nvme_wq, &ctrl->ka_work, delay);
 }
 
 static enum rq_end_io_ret nvme_keep_alive_end_io(struct request *rq,
-- 
2.25.1

Re: [PATCH v2 1/3] nvme: double KA polling frequency to avoid KATO with TBKAS on

[Sagi Grimberg]

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>

[PATCH v2 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>
---
 drivers/nvme/host/core.c | 14 +++++++++++++-
 drivers/nvme/host/nvme.h |  4 ++--
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 1298c7b9bffb..0215dcf643ef 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -397,7 +397,14 @@ 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 && !ctrl->comp_seen &&
+	    nvme_req(req)->start_time >= ctrl->ka_last_check_time)
 		ctrl->comp_seen = true;
 
 	switch (nvme_decide_disposition(req)) {
@@ -1178,6 +1185,8 @@ static enum rq_end_io_ret nvme_keep_alive_end_io(struct request *rq,
 		return RQ_END_IO_NONE;
 	}
 
+	WRITE_ONCE(ctrl->ka_last_check_time, jiffies);
+	smp_wmb();
 	ctrl->comp_seen = false;
 	spin_lock_irqsave(&ctrl->lock, flags);
 	if (ctrl->state == NVME_CTRL_LIVE ||
@@ -1196,6 +1205,9 @@ static void nvme_keep_alive_work(struct work_struct *work)
 	bool comp_seen = ctrl->comp_seen;
 	struct request *rq;
 
+	WRITE_ONCE(ctrl->ka_last_check_time, jiffies);
+	smp_wmb();
+
 	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 bf46f122e9e1..b7f9a93b2d33 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;
 
@@ -1028,6 +1027,7 @@ 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 v2 2/3] nvme: check IO start time when deciding to defer KA

[Sagi Grimberg]

This makes sense to me.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>

[PATCH v2 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 | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 0215dcf643ef..2028fa050bcf 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1162,10 +1162,15 @@ EXPORT_SYMBOL_NS_GPL(nvme_passthru_end, NVME_TARGET_PASSTHRU);
  * frequency, as one command completion can postpone sending a keep alive
  * command by up to twice the delay between runs.
  */
+static unsigned long nvme_keep_alive_work_period(struct nvme_ctrl *ctrl)
+{
+	return (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS) ?
+		(ctrl->kato * HZ / 4) : (ctrl->kato * HZ / 2);
+}
+
 static void nvme_queue_keep_alive_work(struct nvme_ctrl *ctrl)
 {
-	unsigned long delay = (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS) ?
-		ctrl->kato * HZ / 4 : ctrl->kato * HZ / 2;
+	unsigned long delay = nvme_keep_alive_work_period(ctrl);
 	queue_delayed_work(nvme_wq, &ctrl->ka_work, delay);
 }
 
@@ -1175,6 +1180,15 @@ 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
+		delay = 0;
 
 	blk_mq_free_request(rq);
 
@@ -1194,7 +1208,7 @@ 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

Re: [PATCH v2 3/3] nvme: improve handling of long keep alives

[Sagi Grimberg]

> 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 | 20 +++++++++++++++++---
>   1 file changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 0215dcf643ef..2028fa050bcf 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -1162,10 +1162,15 @@ EXPORT_SYMBOL_NS_GPL(nvme_passthru_end, NVME_TARGET_PASSTHRU);
>    * frequency, as one command completion can postpone sending a keep alive
>    * command by up to twice the delay between runs.
>    */
> +static unsigned long nvme_keep_alive_work_period(struct nvme_ctrl *ctrl)
> +{
> +	return (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS) ?
> +		(ctrl->kato * HZ / 4) : (ctrl->kato * HZ / 2);
> +}
> +
>   static void nvme_queue_keep_alive_work(struct nvme_ctrl *ctrl)
>   {
> -	unsigned long delay = (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS) ?
> -		ctrl->kato * HZ / 4 : ctrl->kato * HZ / 2;
> +	unsigned long delay = nvme_keep_alive_work_period(ctrl);
>   	queue_delayed_work(nvme_wq, &ctrl->ka_work, delay);
>   }
>   
> @@ -1175,6 +1180,15 @@ 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
> +		delay = 0;
This should probably log a warning or something... The controller has
such a slow response time to keep-alive that it exceeds the delay...

Re: [PATCH v2 3/3] nvme: improve handling of long keep alives

[Uday Shankar]

On Thu, May 18, 2023 at 12:01:33PM +0300, Sagi Grimberg wrote:
> > +	/* Subtract off the keepalive RTT so nvme_keep_alive_work runs
> > +	 * at the desired frequency. */
> > +	if (rtt <= delay)
> > +		delay -= rtt;
> > +	else
> > +		delay = 0;
> This should probably log a warning or something... The controller has
> such a slow response time to keep-alive that it exceeds the delay...

Good idea. Added it in v3.

Re: [PATCH v2 0/3] keepalive bugfixes

[Uday Shankar]

Ping?