On 12/20/2016 04:55 AM, Paolo Valente wrote:

quoted

+struct request *blk_mq_sched_get_request(struct request_queue *q,
+					 struct bio *bio,
+					 unsigned int op,
+					 struct blk_mq_alloc_data *data)
+{
+	struct elevator_queue *e = q->elevator;
+	struct blk_mq_hw_ctx *hctx;
+	struct blk_mq_ctx *ctx;
+	struct request *rq;
+
+	blk_queue_enter_live(q);
+	ctx = blk_mq_get_ctx(q);
+	hctx = blk_mq_map_queue(q, ctx->cpu);
+
+	blk_mq_set_alloc_data(data, q, 0, ctx, hctx);
+
+	if (e && e->type->ops.mq.get_request)
+		rq = e->type->ops.mq.get_request(q, op, data);

bio is not passed to the scheduler here.  Yet bfq uses bio to get the
blkcg (invoking bio_blkcg).  I'm not finding any workaround.

One important note here - what I'm posting is a work in progress, it's
by no means set in stone. So when you find missing items like this, feel
free to fix them up and send a patch. I will then fold in that patch. Or
if you don't feel comfortable fixing it up, let me know, and I'll fix it
up next time I touch it.

quoted

+	else
+		rq = __blk_mq_alloc_request(data, op);
+
+	if (rq) {
+		rq->elv.icq = NULL;
+		if (e && e->type->icq_cache)
+			blk_mq_sched_assign_ioc(q, rq, bio);

bfq needs rq->elv.icq to be consistent in bfq_get_request, but the
needed initialization seems to occur only after mq.get_request is
invoked.

Note: to minimize latency, I'm reporting immediately each problem that
apparently cannot be solved by just modifying bfq.  But, if the
resulting higher number of micro-emails is annoying for you, I can
buffer my questions, and send you cumulative emails less frequently.

That's perfectly fine, I prefer knowing earlier rather than later. But
do also remember that it's fine to send a patch to fix those things up,
you don't have to wait for me.

-- 
Jens Axboe