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

[Sagi Grimberg]

On 4/18/23 01:55, Uday Shankar wrote:
> 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 at purestorage.com>
> Reported-by: Randy Jennings <randyj at purestorage.com>
> Signed-off-by: Uday Shankar <ushankar at purestorage.com>
> Reviewed-by: Hannes Reinecke <hare at suse.de>
> ---
>   drivers/nvme/host/core.c | 14 +++++++++++++-
>   drivers/nvme/host/nvme.h |  5 +++--
>   2 files changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 1298c7b9bffb..8a63051d7b5e 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;

indentation is wrong here.

>   
>   	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..f044ffb9ce10 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 +1027,8 @@ static inline void nvme_start_request(struct request *rq)
>   {
>   	if (rq->cmd_flags & REQ_NVME_MPATH)
>   		nvme_mpath_start_request(rq);
> +	else
> +		nvme_req(rq)->start_time = jiffies;

nvme_mpath_start_request may not set the start_time if stats are
disabled...