linux-kernel - Re: [PATCH] SCSI: run queue if SCSI device queue isn't ready and queue is idle

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Date:   Tue, 5 Dec 2017 16:41:46 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "ming.lei@...hat.com" <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>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "axboe@...com" <axboe@...com>, "hare@...e.com" <hare@...e.com>,
        "holger@...lied-asynchrony.com" <holger@...lied-asynchrony.com>,
        "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>
Subject: Re: [PATCH] SCSI: run queue if SCSI device queue isn't ready and
 queue is idle

On Wed, 2017-12-06 at 00:28 +0800, Ming Lei wrote:
> This is still a workaround for RESTART, see my comment before:
> 
> 	https://marc.info/?l=linux-block&m=151217500929341&w=2

A quote from that e-mail: "The theory about using BLK_MQ_S_SCHED_RESTART in
current way is that we mark it after requests are added to hctx->dispatch".
Reading that makes me wonder whether you understand the purpose of the
BLK_MQ_S_SCHED_RESTART flag? That flag is not set after requests are added
to the dispatch list but after requests have been *removed*. The purpose of
that flag is to detect whether another thread has run the queue after
requests were removed from the dispatch list and before these were readded.
If so, the queue needs to be rerun.

Bart.