Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH RFC v2 01/24] scsi: add 'nr_reserved_cmds' field to the
 SCSI host template
To:     Ming Lei <ming.lei@redhat.com>, John Garry <john.garry@huawei.com>
Cc:     axboe@kernel.dk, jejb@linux.ibm.com, martin.petersen@oracle.com,
        bvanassche@acm.org, hch@infradead.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        esc.storagedev@microsemi.com, chenxiang66@hisilicon.com,
        Hannes Reinecke <hare@suse.com>
References: <1583857550-12049-1-git-send-email-john.garry@huawei.com>
 <1583857550-12049-2-git-send-email-john.garry@huawei.com>
 <20200310230835.GA16056@ming.t460p>
From:   Hannes Reinecke <hare@suse.de>
Openpgp: preference=signencrypt
Autocrypt: addr=hare@suse.de; prefer-encrypt=mutual; keydata=
 mQINBE6KyREBEACwRN6XKClPtxPiABx5GW+Yr1snfhjzExxkTYaINHsWHlsLg13kiemsS6o7
 qrc+XP8FmhcnCOts9e2jxZxtmpB652lxRB9jZE40mcSLvYLM7S6aH0WXKn8bOqpqOGJiY2bc
 6qz6rJuqkOx3YNuUgiAxjuoYauEl8dg4bzex3KGkGRuxzRlC8APjHlwmsr+ETxOLBfUoRNuE
 b4nUtaseMPkNDwM4L9+n9cxpGbdwX0XwKFhlQMbG3rWA3YqQYWj1erKIPpgpfM64hwsdk9zZ
 QO1krgfULH4poPQFpl2+yVeEMXtsSou915jn/51rBelXeLq+cjuK5+B/JZUXPnNDoxOG3j3V
 VSZxkxLJ8RO1YamqZZbVP6jhDQ/bLcAI3EfjVbxhw9KWrh8MxTcmyJPn3QMMEp3wpVX9nSOQ
 tzG72Up/Py67VQe0x8fqmu7R4MmddSbyqgHrab/Nu+ak6g2RRn3QHXAQ7PQUq55BDtj85hd9
 W2iBiROhkZ/R+Q14cJkWhzaThN1sZ1zsfBNW0Im8OVn/J8bQUaS0a/NhpXJWv6J1ttkX3S0c
 QUratRfX4D1viAwNgoS0Joq7xIQD+CfJTax7pPn9rT////hSqJYUoMXkEz5IcO+hptCH1HF3
 qz77aA5njEBQrDRlslUBkCZ5P+QvZgJDy0C3xRGdg6ZVXEXJOQARAQABtCpIYW5uZXMgUmVp
 bmVja2UgKFN1U0UgTGFicykgPGhhcmVAc3VzZS5kZT6JAkEEEwECACsCGwMFCRLMAwAGCwkI
 BwMCBhUIAgkKCwQWAgMBAh4BAheABQJOisquAhkBAAoJEGz4yi9OyKjPOHoQAJLeLvr6JNHx
 GPcHXaJLHQiinz2QP0/wtsT8+hE26dLzxb7hgxLafj9XlAXOG3FhGd+ySlQ5wSbbjdxNjgsq
 FIjqQ88/Lk1NfnqG5aUTPmhEF+PzkPogEV7Pm5Q17ap22VK623MPaltEba+ly6/pGOODbKBH
 ak3gqa7Gro5YCQzNU0QVtMpWyeGF7xQK76DY/atvAtuVPBJHER+RPIF7iv5J3/GFIfdrM+wS
 BubFVDOibgM7UBnpa7aohZ9RgPkzJpzECsbmbttxYaiv8+EOwark4VjvOne8dRaj50qeyJH6
 HLpBXZDJH5ZcYJPMgunghSqghgfuUsd5fHmjFr3hDb5EoqAfgiRMSDom7wLZ9TGtT6viDldv
 hfWaIOD5UhpNYxfNgH6Y102gtMmN4o2P6g3UbZK1diH13s9DA5vI2mO2krGz2c5BOBmcctE5
 iS+JWiCizOqia5Op+B/tUNye/YIXSC4oMR++Fgt30OEafB8twxydMAE3HmY+foawCpGq06yM
 vAguLzvm7f6wAPesDAO9vxRNC5y7JeN4Kytl561ciTICmBR80Pdgs/Obj2DwM6dvHquQbQrU
 Op4XtD3eGUW4qgD99DrMXqCcSXX/uay9kOG+fQBfK39jkPKZEuEV2QdpE4Pry36SUGfohSNq
 xXW+bMc6P+irTT39VWFUJMcSuQINBE6KyREBEACvEJggkGC42huFAqJcOcLqnjK83t4TVwEn
 JRisbY/VdeZIHTGtcGLqsALDzk+bEAcZapguzfp7cySzvuR6Hyq7hKEjEHAZmI/3IDc9nbdh
 EgdCiFatah0XZ/p4vp7KAelYqbv8YF/ORLylAdLh9rzLR6yHFqVaR4WL4pl4kEWwFhNSHLxe
 55G56/dxBuoj4RrFoX3ynerXfbp4dH2KArPc0NfoamqebuGNfEQmDbtnCGE5zKcR0zvmXsRp
 qU7+caufueZyLwjTU+y5p34U4PlOO2Q7/bdaPEdXfpgvSpWk1o3H36LvkPV/PGGDCLzaNn04
 BdiiiPEHwoIjCXOAcR+4+eqM4TSwVpTn6SNgbHLjAhCwCDyggK+3qEGJph+WNtNU7uFfscSP
 k4jqlxc8P+hn9IqaMWaeX9nBEaiKffR7OKjMdtFFnBRSXiW/kOKuuRdeDjL5gWJjY+IpdafP
 KhjvUFtfSwGdrDUh3SvB5knSixE3qbxbhbNxmqDVzyzMwunFANujyyVizS31DnWC6tKzANkC
 k15CyeFC6sFFu+WpRxvC6fzQTLI5CRGAB6FAxz8Hu5rpNNZHsbYs9Vfr/BJuSUfRI/12eOCL
 IvxRPpmMOlcI4WDW3EDkzqNAXn5Onx/b0rFGFpM4GmSPriEJdBb4M4pSD6fN6Y/Jrng/Bdwk
 SQARAQABiQIlBBgBAgAPBQJOiskRAhsMBQkSzAMAAAoJEGz4yi9OyKjPgEwQAIP/gy/Xqc1q
 OpzfFScswk3CEoZWSqHxn/fZasa4IzkwhTUmukuIvRew+BzwvrTxhHcz9qQ8hX7iDPTZBcUt
 ovWPxz+3XfbGqE+q0JunlIsP4N+K/I10nyoGdoFpMFMfDnAiMUiUatHRf9Wsif/nT6oRiPNJ
 T0EbbeSyIYe+ZOMFfZBVGPqBCbe8YMI+JiZeez8L9JtegxQ6O3EMQ//1eoPJ5mv5lWXLFQfx
 f4rAcKseM8DE6xs1+1AIsSIG6H+EE3tVm+GdCkBaVAZo2VMVapx9k8RMSlW7vlGEQsHtI0FT
 c1XNOCGjaP4ITYUiOpfkh+N0nUZVRTxWnJqVPGZ2Nt7xCk7eoJWTSMWmodFlsKSgfblXVfdM
 9qoNScM3u0b9iYYuw/ijZ7VtYXFuQdh0XMM/V6zFrLnnhNmg0pnK6hO1LUgZlrxHwLZk5X8F
 uD/0MCbPmsYUMHPuJd5dSLUFTlejVXIbKTSAMd0tDSP5Ms8Ds84z5eHreiy1ijatqRFWFJRp
 ZtWlhGRERnDH17PUXDglsOA08HCls0PHx8itYsjYCAyETlxlLApXWdVl9YVwbQpQ+i693t/Y
 PGu8jotn0++P19d3JwXW8t6TVvBIQ1dRZHx1IxGLMn+CkDJMOmHAUMWTAXX2rf5tUjas8/v2
 azzYF4VRJsdl+d0MCaSy8mUh
Message-ID: <fecc400c-fe6b-144a-51f9-1b3b2704c1a2@suse.de>
Date:   Wed, 11 Mar 2020 07:55:46 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.7.2
MIME-Version: 1.0
In-Reply-To: <20200310230835.GA16056@ming.t460p>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 3/11/20 12:08 AM, Ming Lei wrote:
> On Wed, Mar 11, 2020 at 12:25:27AM +0800, John Garry wrote:
>> From: Hannes Reinecke <hare@suse.com>
>>
>> Add a new field 'nr_reserved_cmds' to the SCSI host template to
>> instruct the block layer to set aside a tag space for reserved
>> commands.
>>
>> Signed-off-by: Hannes Reinecke <hare@suse.com>
>> ---
>>  drivers/scsi/scsi_lib.c  | 1 +
>>  include/scsi/scsi_host.h | 6 ++++++
>>  2 files changed, 7 insertions(+)
>>
>> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
>> index 610ee41fa54c..2967325df7a0 100644
>> --- a/drivers/scsi/scsi_lib.c
>> +++ b/drivers/scsi/scsi_lib.c
>> @@ -1896,6 +1896,7 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
>>  		shost->tag_set.ops = &scsi_mq_ops_no_commit;
>>  	shost->tag_set.nr_hw_queues = shost->nr_hw_queues ? : 1;
>>  	shost->tag_set.queue_depth = shost->can_queue;
>> +	shost->tag_set.reserved_tags = shost->nr_reserved_cmds;
> 
> You reserve tags for special usage, meantime the whole queue depth
> isn't increased, that means the tags for IO request is decreased given
> reserved tags can't be used for IO, so IO performance may be effected.
> 
> If not the case, please explain a bit in commit log.
> 
The overall idea of this patchset is to fold _existing_ management
command handling into using the blk-mq bitmap.
Currently, quite a lot of drivers are using management commands
internally, which typically use the same hardware tag pool (ie they are
being allocated from the same hardware resources) than the 'normal' I/O
commands.
But as they are using the same tagpool these drivers already decrement
the available number of commands; check eg. hpsa:

static int hpsa_scsi_host_alloc(struct ctlr_info *h)
{
	struct Scsi_Host *sh;

	sh = scsi_host_alloc(&hpsa_driver_template, sizeof(h));
	if (sh == NULL) {
		dev_err(&h->pdev->dev, "scsi_host_alloc failed\n");
		return -ENOMEM;
	}

	sh->io_port = 0;
	sh->n_io_port = 0;
	sh->this_id = -1;
	sh->max_channel = 3;
	sh->max_cmd_len = MAX_COMMAND_SIZE;
	sh->max_lun = HPSA_MAX_LUN;
	sh->max_id = HPSA_MAX_LUN;
	sh->can_queue = h->nr_cmds - HPSA_NRESERVED_CMDS;
	sh->cmd_per_lun = sh->can_queue;

So the idea of this patchset is to convert existing use-cases; seeing
that they already reserve commands internally performance will not be
affected.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), GF: Felix Imendörffer