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

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Date:	Wed, 04 Nov 2015 09:01:11 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
	roger.pau@...rix.com, felipe.franciosi@...rix.com, axboe@...com,
	avanzini.arianna@...il.com, rafal.mielniczuk@...rix.com,
	jonathan.davies@...rix.com, david.vrabel@...rix.com
Subject: Re: [PATCH v4 03/10] xen/blkfront: pseudo support for multi hardware
 queues/rings


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@...cle.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
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