Re: [PATCH v4 03/10] xen/blkfront: pseudo support for multi hardware queues/rings

Bob Liu
2015-11-04T01:01:28Z

On 11/04/2015 03:44 AM, Konrad Rzeszutek Wilk wrote:
> On Mon, Nov 02, 2015 at 12:21:39PM +0800, Bob Liu wrote:
>> Preparatory patch for multiple hardware queues (rings). The number of
>> rings is unconditionally set to 1, larger number will be enabled in next
>> patch so as to make every single patch small and readable.
>
> s/next patch/"xen/blkfront: negotiate number of queues/rings to be used with backend"
>
>>
>> Signed-off-by: Bob Liu <bob.liu@oracle.com>
>> ---
>> drivers/block/xen-blkfront.c | 327 +++++++++++++++++++++++++------------------
>> 1 file changed, 188 insertions(+), 139 deletions(-)
>>
>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>> index 2a557e4..eab78e7 100644
>> --- a/drivers/block/xen-blkfront.c
>> +++ b/drivers/block/xen-blkfront.c
>> @@ -145,6 +145,7 @@ struct blkfront_info
>> int vdevice;
>> blkif_vdev_t handle;
>> enum blkif_state connected;
>> + /* Number of pages per ring buffer */
>
> Missing full stop, aka '.'.
>
>> unsigned int nr_ring_pages;
>> struct request_queue *rq;
>> struct list_head grants;
>> @@ -158,7 +159,8 @@ struct blkfront_info
>> unsigned int max_indirect_segments;
>> int is_ready;
>> struct blk_mq_tag_set tag_set;
>> - struct blkfront_ring_info rinfo;
>> + struct blkfront_ring_info *rinfo;
>> + unsigned int nr_rings;
>> };
>>
>> static unsigned int nr_minors;
>> @@ -190,7 +192,7 @@ static DEFINE_SPINLOCK(minor_lock);
>> ((_segs + SEGS_PER_INDIRECT_FRAME - 1)/SEGS_PER_INDIRECT_FRAME)
>>
>> static int blkfront_setup_indirect(struct blkfront_ring_info *rinfo);
>> -static int blkfront_gather_backend_features(struct blkfront_info *info);
>> +static void blkfront_gather_backend_features(struct blkfront_info *info);
>>
>> static int get_id_from_freelist(struct blkfront_ring_info *rinfo)
>> {
>> @@ -443,12 +445,13 @@ static int blkif_queue_request(struct request *req, struct blkfront_ring_info *r
>> */
>> max_grefs += INDIRECT_GREFS(req->nr_phys_segments);
>>
>> - /* Check if we have enough grants to allocate a requests */
>> - if (info->persistent_gnts_c < max_grefs) {
>> + /* Check if we have enough grants to allocate a requests, we have to
>> + * reserve 'max_grefs' grants because persistent grants are shared by all
>> + * rings */
>
> Missing full stop.
>
>> + if (0 < max_grefs) {
>
> <blinks> ? 0!?
>
> max_grefs will at least be BLKIF_MAX_SEGMENTS_PER_REQUEST
> so this will always be true.
>

No, max_grefs = req->nr_phys_segments;

It's 0 in some cases(flush req?), and gnttable_alloc_grant_references() can not handle 0 as the parameter.

> In which ase why not just ..
>> new_persistent_gnts = 1;
>> if (gnttab_alloc_grant_references(
>> - max_grefs - info->persistent_gnts_c,
>> - &gref_head) < 0) {
>> + max_grefs, &gref_head) < 0) {
>> gnttab_request_free_callback(
>> &rinfo->callback,
>> blkif_restart_queue_callback,
>
> .. move this whole code down? And get rid of 'new_persistent_gnts'
> since it will always be true?
>

Unless we fix gnttable_alloc_grant_references(0).

> But more importantly, why do we not check for 'info->persistent_gnts_c' anymore?
>

Info->persistent_gnts_c is for per-device not per-ring, the persistent grants may be taken by other queues/rings after we checked the value here.
Which would make get_grant() fail, so we have to reserved enough grants in advance.
Those new-allocated grants will be freed if there are enough grants in persistent list.

Will fix all other comments for this patch.

Thanks,
Bob