linux-kernel - Re: [PATCH V7 2/4] blk-mq: fix issue directly case when q is stopped or quiesced

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Date:   Wed, 14 Nov 2018 17:20:23 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jianchao Wang <jianchao.w.wang@...cle.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V7 2/4] blk-mq: fix issue directly case when q is stopped
 or quiesced

On Wed, Nov 14, 2018 at 04:45:29PM +0800, Jianchao Wang wrote:
> When try to issue request directly, if the queue is stopped or
> quiesced, 'bypass' will be ignored and return BLK_STS_OK to caller
> to avoid it dispatch request again. Then the request will be
> inserted with blk_mq_sched_insert_request. This is not correct
> for dm-rq case where we should avoid to pass through the underlying
> path's io scheduler.
> 
> To fix it, use blk_mq_request_bypass_insert to insert the request
> to hctx->dispatch when we cannot pass through io scheduler but have
> to insert.

Not sure if the current behaviour is wrong, or worth of a fix.

Bypassing io scheduler for dm-rq is only for sake of performance
because there has been io scheduler for dm device already, and we
just don't want to schedule these requests twice.

Given it can be thought as error handling, there shouldn't be big
difference to put request in scheduler queue or ->dispatch list
when queue is quiesced or stopped. What matters is that all these
requests can be dispatched again after queue switches back.

thanks,
Ming