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Date:   Wed, 3 Jun 2020 20:12:03 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org,
        Dongli Zhang <dongli.zhang@oracle.com>,
        John Garry <john.garry@huawei.com>,
        Hannes Reinecke <hare@suse.de>, Daniel Wagner <dwagner@suse.de>
Subject: Re: [PATCH] blk-mq: don't fail driver tag allocation because of
 inactive hctx
Message-ID: <20200603181203.GA2032@lst.de>
References: <20200603105128.2147139-1-ming.lei@redhat.com> <20200603115347.GA8653@lst.de> <20200603133608.GA2149752@T590>
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On Wed, Jun 03, 2020 at 09:36:08PM +0800, Ming Lei wrote:
> > +bool __blk_mq_get_driver_tag(struct request *rq)
> > +{
> > +	struct sbitmap_queue *bt = &rq->mq_hctx->tags->bitmap_tags;
> > +	unsigned int tag_offset = rq->mq_hctx->tags->nr_reserved_tags;
> > +	bool shared = blk_mq_tag_busy(rq->mq_hctx);
> 
> Not necessary to add 'shared' which is just used once.

blk_mq_tag_busy also increments active_queues, and the existing code
does that before various early returns.  To keep the behavior as-is
the call to blk_mq_tag_busy can't be moved around.

> > +	int tag;
> > +
> > +	if (blk_mq_tag_is_reserved(rq->mq_hctx->sched_tags, rq->internal_tag)) {
> > +		bt = &rq->mq_hctx->tags->breserved_tags;
> 
> Too many rq->mq_hctx->tags, you can add one local variable to store it.

Really just three of them.  And with a local variable confusing it with
rq->mq_hctx->sched_tags becomes much easier.