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Subject: Re: [PATCH] scsi: storvsc: Cap scsi_driver.can_queue to fix a hang
 issue during boot
To:     Michael Kelley <mikelley@microsoft.com>,
        Dexuan Cui <decui@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "wei.liu@kernel.org" <wei.liu@kernel.org>,
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        Haiyang Zhang <haiyangz@microsoft.com>,
        "ming.lei@redhat.com" <ming.lei@redhat.com>,
        "bvanassche@acm.org" <bvanassche@acm.org>,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
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From:   John Garry <john.garry@huawei.com>
Message-ID: <fc4b3284-0218-8a03-a55c-94c10ac0b029@huawei.com>
Date:   Wed, 6 Oct 2021 17:24:00 +0100
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On 06/10/2021 17:08, Michael Kelley wrote:
>>> The calculated value of can_queue is not realistic.  The blk-mq layer
>>> caps the number of tags at 10240,
>> nit: 1024, I think
> I was thinking about BLK_MQ_MAX_DEPTH (#define'd as 10240), which
> is used to limit the tag set size in blk_mq_alloc_tag_set().   When running
> on large VMs on Hyper-V, we can see the "blk-mq: reduced tag depth
> to 10240" message.:-(

Ah, right. The other related capping is the sdev queue depth, which is 
now capped at max(1024, can_queue), see scsi_device_max_queue_depth().

Thanks,
John