Re: [PATCH v3 3/3] nvme: improve handling of long keep alives

[Christoph Hellwig <hch@lst.de>]

On Thu, May 18, 2023 at 12:33:11PM -0600, Uday Shankar wrote:
> 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 | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index a31c04b5f849..ce07218fc6eb 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -1172,10 +1172,15 @@ 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);
> +}

Please add this separate helper in patch 1 already instead of moving
things around.  Preferably in the style I mentioned there.

>  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);

.. and with this helper we don't really need the local variable here.

> +	/* Subtract off the keepalive RTT so nvme_keep_alive_work runs
> +	 * at the desired frequency. */

The kernel comment style is:

	/*
	 * Subtract off the keepalive RTT so nvme_keep_alive_work runs
	 * at the desired frequency.
	 */