linux-kernel - Re: [PATCH V7 4/6] blk-mq: introduce .get_budget and .put_budget in blk_mq

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Message-ID: <782e3b9b-2aae-e013-bedc-3c35cb0f7edd@wdc.com>
Date:   Tue, 17 Oct 2017 11:09:15 -0700
From:   Bart Van Assche <bart.vanassche@....com>
To:     Hannes Reinecke <hare@...e.de>, Ming Lei <ming.lei@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "tom81094@...il.com" <tom81094@...il.com>,
        "himanshu.madhani@...ium.com" <himanshu.madhani@...ium.com>,
        "paolo.valente@...aro.org" <paolo.valente@...aro.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "oleksandr@...alenko.name" <oleksandr@...alenko.name>,
        "john.garry@...wei.com" <john.garry@...wei.com>,
        "osandov@...com" <osandov@...com>,
        "loberman@...hat.com" <loberman@...hat.com>,
        "james.smart@...adcom.com" <james.smart@...adcom.com>
Subject: Re: [PATCH V7 4/6] blk-mq: introduce .get_budget and .put_budget in
 blk_mq_ops

On 10/16/17 23:38, Hannes Reinecke wrote:
> Again, these are just some pre-defined values to avoid I/O starvation
> when having several LUNs. _if_ we can guarantee I/O fairness between
> several (hundreds!) devices all sharing the same tagspace we wouldn't
> need these variables.

I think we already have two mechanisms for guaranteeing I/O fairness:
* Scsi_Host.starved_list for both the legacy SCSI core and scsi-mq.
* For scsi-mq, the active_queues counter in the blk-mq core. See also
   hctx_may_queue().

Bart.