linux-kernel - Re: [PATCH 08/10] blk-mq-sched: add framework for MQ capable IO schedulers

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Date:   Fri, 13 Jan 2017 12:15:17 +0100
From:   Hannes Reinecke <hare@...e.de>
To:     Jens Axboe <axboe@...com>, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org
Cc:     osandov@...ndov.com, bart.vanassche@...disk.com
Subject: Re: [PATCH 08/10] blk-mq-sched: add framework for MQ capable IO
 schedulers

On 01/11/2017 10:40 PM, Jens Axboe wrote:
> This adds a set of hooks that intercepts the blk-mq path of
> allocating/inserting/issuing/completing requests, allowing
> us to develop a scheduler within that framework.
> 
> We reuse the existing elevator scheduler API on the registration
> side, but augment that with the scheduler flagging support for
> the blk-mq interfce, and with a separate set of ops hooks for MQ
> devices.
> 
> We split driver and scheduler tags, so we can run the scheduling
> independent of device queue depth.
> 
> Signed-off-by: Jens Axboe <axboe@...com>
[ .. ]
> @@ -823,6 +847,35 @@ static inline unsigned int queued_to_index(unsigned int queued)
>  	return min(BLK_MQ_MAX_DISPATCH_ORDER - 1, ilog2(queued) + 1);
>  }
>  
> +static bool blk_mq_get_driver_tag(struct request *rq,
> +				  struct blk_mq_hw_ctx **hctx, bool wait)
> +{
> +	struct blk_mq_alloc_data data = {
> +		.q = rq->q,
> +		.ctx = rq->mq_ctx,
> +		.hctx = blk_mq_map_queue(rq->q, rq->mq_ctx->cpu),
> +		.flags = wait ? 0 : BLK_MQ_REQ_NOWAIT,
> +	};
> +
> +	if (blk_mq_hctx_stopped(data.hctx))
> +		return false;
> +
> +	if (rq->tag != -1) {
> +done:
> +		if (hctx)
> +			*hctx = data.hctx;
> +		return true;
> +	}
> +
> +	rq->tag = blk_mq_get_tag(&data);
> +	if (rq->tag >= 0) {
> +		data.hctx->tags->rqs[rq->tag] = rq;
> +		goto done;
> +	}
> +
> +	return false;
> +}
> +
What happens with the existing request at 'rqs[rq->tag]' ?
Surely there is one already, right?
Things like '->init_request' assume a fully populated array, so moving
one entry to another location is ... interesting.

I would have thought we need to do a request cloning here,
otherwise this would introduce a memory leak, right?
(Not to mention a potential double completion, as the request is now at
two positions in the array)

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@...e.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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