linux-kernel - Re: [PATCH V5 7/7] blk-mq-sched: don't dequeue request until all in ->dispatch are flushed

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Message-ID: <20171009104019.GC22168@ming.t460p>
Date:   Mon, 9 Oct 2017 18:40:20 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...com>, linux-block@...r.kernel.org,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Laurence Oberman <loberman@...hat.com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Tom Nguyen <tom81094@...il.com>, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, Omar Sandoval <osandov@...com>
Subject: Re: [PATCH V5 7/7] blk-mq-sched: don't dequeue request until all in
 ->dispatch are flushed

On Tue, Oct 03, 2017 at 02:11:28AM -0700, Christoph Hellwig wrote:
> This looks good in general:
> 
> Reviewed-by: Christoph Hellwig <hch@....de>
> 
> Minor nitpicks below:
> 
> >  	const bool has_sched_dispatch = e && e->type->ops.mq.dispatch_request;
> 
> This is now only tested once, so you can remove the local variable
> for it.

There are still two users of the local variable, so I suggest to
keep it.

> 
> > +		/*
> > +		 * We may clear DISPATCH_BUSY just after it
> > +		 * is set from another context, the only cost
> > +		 * is that one request is dequeued a bit early,
> > +		 * we can survive that. Given the window is
> > +		 * small enough, no need to worry about performance
> > +		 * effect.
> > +		 */
> 
> Use your 80 line real estate for comments please.

OK.

> 
> >  	if (!has_sched_dispatch)
> > +		if (!q->queue_depth) {
> > +			blk_mq_flush_busy_ctxs(hctx, &rq_list);
> > +			blk_mq_dispatch_rq_list(q, &rq_list);
> > +		} else {
> > +			blk_mq_do_dispatch_ctx(q, hctx);
> > +		}
> > +	} else {
> >  		blk_mq_do_dispatch_sched(q, e, hctx);
> > +	}
> 
> Maybe flatten this out to:
> 
> 	if (e && e->type->ops.mq.dispatch_request) {
> 		blk_mq_do_dispatch_sched(q, e, hctx);
> 	} else if (q->queue_depth) {
> 		blk_mq_do_dispatch_ctx(q, hctx);
> 	} else {
> 		blk_mq_flush_busy_ctxs(hctx, &rq_list);
> 		blk_mq_dispatch_rq_list(q, &rq_list);
> 	}
> 

OK.


-- 
Ming