Re: [PATCH v2 3/3] xen-blkfront: dynamic configuration of per-vbd resources

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Jul 26, 2016 at 01:19:37PM +0800, Bob Liu wrote:
> The current VBD layer reserves buffer space for each attached device based on
> three statically configured settings which are read at boot time.
>  * max_indirect_segs: Maximum amount of segments.
>  * max_ring_page_order: Maximum order of pages to be used for the shared ring.
>  * max_queues: Maximum of queues(rings) to be used.
> 
> But the storage backend, workload, and guest memory result in very different
> tuning requirements. It's impossible to centrally predict application
> characteristics so it's best to leave allow the settings can be dynamiclly
> adjusted based on workload inside the Guest.
> 
> Usage:
> Show current values:
> cat /sys/devices/vbd-xxx/max_indirect_segs
> cat /sys/devices/vbd-xxx/max_ring_page_order
> cat /sys/devices/vbd-xxx/max_queues
> 
> Write new values:
> echo <new value> > /sys/devices/vbd-xxx/max_indirect_segs
> echo <new value> > /sys/devices/vbd-xxx/max_ring_page_order
> echo <new value> > /sys/devices/vbd-xxx/max_queues
> 
> Signed-off-by: Bob Liu <bob.liu@oracle.com>
> --
> v2: Rename to max_ring_page_order and rm the waiting code suggested by Roger.
> ---
>  drivers/block/xen-blkfront.c |  275 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 269 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 1b4c380..ff5ebe5 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -212,6 +212,11 @@ struct blkfront_info
>  	/* Save uncomplete reqs and bios for migration. */
>  	struct list_head requests;
>  	struct bio_list bio_list;
> +	/* For dynamic configuration. */
> +	unsigned int reconfiguring:1;
> +	int new_max_indirect_segments;

Can't you just use max_indirect_segments? Is it really needed to introduce a 
new struct member?

> +	int max_ring_page_order;
> +	int max_queues;
>  };
>  
>  static unsigned int nr_minors;
> @@ -1350,6 +1355,31 @@ static void blkif_free(struct blkfront_info *info, int suspend)
>  	for (i = 0; i < info->nr_rings; i++)
>  		blkif_free_ring(&info->rinfo[i]);
>  
> +	/* Remove old xenstore nodes. */
> +	if (info->nr_ring_pages > 1)
> +		xenbus_rm(XBT_NIL, info->xbdev->nodename, "ring-page-order");
> +
> +	if (info->nr_rings == 1) {
> +		if (info->nr_ring_pages == 1) {
> +			xenbus_rm(XBT_NIL, info->xbdev->nodename, "ring-ref");
> +		} else {
> +			for (i = 0; i < info->nr_ring_pages; i++) {
> +				char ring_ref_name[RINGREF_NAME_LEN];
> +
> +				snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
> +				xenbus_rm(XBT_NIL, info->xbdev->nodename, ring_ref_name);
> +			}
> +		}
> +	} else {
> +		xenbus_rm(XBT_NIL, info->xbdev->nodename, "multi-queue-num-queues");
> +
> +		for (i = 0; i < info->nr_rings; i++) {
> +			char queuename[QUEUE_NAME_LEN];
> +
> +			snprintf(queuename, QUEUE_NAME_LEN, "queue-%u", i);
> +			xenbus_rm(XBT_NIL, info->xbdev->nodename, queuename);
> +		}
> +	}
>  	kfree(info->rinfo);
>  	info->rinfo = NULL;
>  	info->nr_rings = 0;
> @@ -1763,15 +1793,21 @@ static int talk_to_blkback(struct xenbus_device *dev,
>  	const char *message = NULL;
>  	struct xenbus_transaction xbt;
>  	int err;
> -	unsigned int i, max_page_order = 0;
> +	unsigned int i, backend_max_order = 0;
>  	unsigned int ring_page_order = 0;
>  
>  	err = xenbus_scanf(XBT_NIL, info->xbdev->otherend,
> -			   "max-ring-page-order", "%u", &max_page_order);
> +			   "max-ring-page-order", "%u", &backend_max_order);
>  	if (err != 1)
>  		info->nr_ring_pages = 1;
>  	else {
> -		ring_page_order = min(xen_blkif_max_ring_order, max_page_order);
> +		if (info->max_ring_page_order) {
> +			/* Dynamic configured through /sys. */
> +			BUG_ON(info->max_ring_page_order > backend_max_order);

Maybe I'm missing something, but I don't think you can BUG here. The 
following flow for example will trigger this BUG:

 - Admin sets max_ring_page_order = 2 from sysfs, frontend reconfigures.
 - VM is migrated to a new host without multipage ring support.
 - BUG will trigger on reconnection (because backend_max_order == 1 and 
   max_ring_page_order == 2).

Roger.