linux-kernel - Re: [PATCH RFC v3 03/22] scsi: core: Implement reserved command handling

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Date:   Thu, 27 Oct 2022 09:51:00 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        John Garry <john.garry@...wei.com>, axboe@...nel.dk,
        jejb@...ux.ibm.com, martin.petersen@...cle.com,
        jinpu.wang@...ud.ionos.com, bvanassche@....org, hch@....de,
        ming.lei@...hat.com, niklas.cassel@....com
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ide@...r.kernel.org, linux-scsi@...r.kernel.org,
        linuxarm@...wei.com
Subject: Re: [PATCH RFC v3 03/22] scsi: core: Implement reserved command
 handling

On 10/27/22 03:18, Damien Le Moal wrote:
> On 10/25/22 19:17, John Garry wrote:
>> From: Hannes Reinecke <hare@...e.de>
>>
>> Quite some drivers are using management commands internally, which
>> typically use the same hardware tag pool (ie they are being allocated
>> from the same hardware resources) as the 'normal' I/O commands.
>> These commands are set aside before allocating the block-mq tag bitmap,
>> so they'll never show up as busy in the tag map.
>> The block-layer, OTOH, already has 'reserved_tags' to handle precisely
>> this situation.
>> So this patch adds a new field 'nr_reserved_cmds' to the SCSI host
>> template to instruct the block layer to set aside a tag space for these
>> management commands by using reserved tags.
>>
>> Signed-off-by: Hannes Reinecke <hare@...e.de>
>> #jpg: Set tag_set->queue_depth = shost->can_queue, and not
>> = shost->can_queue + shost->nr_reserved_cmds;
>> Signed-off-by: John Garry <john.garry@...wei.com>
>> ---
>>   drivers/scsi/hosts.c     |  3 +++
>>   drivers/scsi/scsi_lib.c  |  2 ++
>>   include/scsi/scsi_host.h | 15 ++++++++++++++-
>>   3 files changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
>> index 12346e2297fd..db89afc37bc9 100644
>> --- a/drivers/scsi/hosts.c
>> +++ b/drivers/scsi/hosts.c
>> @@ -489,6 +489,9 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
>>   	if (sht->virt_boundary_mask)
>>   		shost->virt_boundary_mask = sht->virt_boundary_mask;
>>   
>> +	if (sht->nr_reserved_cmds)
>> +		shost->nr_reserved_cmds = sht->nr_reserved_cmds;
>> +
> 
> Nit: the if is not really necessary I think. But it does not hurt.
> 
Yes, we do.
Not all HBAs are able to figure out the number of reserved commands 
upfront; some modify that based on the PCI device used etc.
So I'd keep it for now.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@...e.de                              +49 911 74053 688
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