On 01/13/2017 05:41 PM, Omar Sandoval wrote:

On Fri, Jan 13, 2017 at 12:15:17PM +0100, Hannes Reinecke wrote:

quoted

On 01/11/2017 10:40 PM, Jens Axboe wrote:

quoted

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@fb.com>

[ .. ]

quoted

@@ -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

The entries in tags->rqs aren't slab objects, they're pointers into
pages allocated separately and tracked on tags->page_list. See
blk_mq_alloc_rqs(). In blk_mq_free_rqs(), we free all of the pages on
tags->page_list, so there shouldn't be a memory leak.

As for hctx->tags->rqs, entries are only overwritten when a scheduler is
enabled. In that case, the rqs array is storing pointers to requests
actually from hctx->sched_tags, so overwriting/leaking isn't an issue.

Ah. Thanks.
That explains it.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
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