Re: [PATCH V4] blk-mq: dequeue request one by one from sw queue iff hctx is busy

[Ming Lei <ming.lei@redhat.com>]

On Tue, Jul 03, 2018 at 08:03:23AM -0600, Jens Axboe wrote:
> On 7/3/18 2:34 AM, Ming Lei wrote:
> > It won't be efficient to dequeue request one by one from sw queue,
> > but we have to do that when queue is busy for better merge performance.
> > 
> > This patch takes the Exponential Weighted Moving Average(EWMA) to figure
> > out if queue is busy, then only dequeue request one by one from sw queue
> > when queue is busy.
> 
> I've started to come around to the approach, but can we add something
> that only triggers this busy tracking if we've even seen a BUSY
> condition? Basically, this:
> 
> blk_mq_update_dispatch_busy(hctx, false);
> 
> should be a no-op, if we've never called:
> 
> blk_mq_update_dispatch_busy(hctx, true);
> 
> Something ala the below, with the BLK_MQ_S_EWMA bit added, of course.
> 
> static void __blk_mq_update_dispatch_busy(struct blk_mq_hw_ctx *hctx, bool busy)
> {
> 	unsigned int ewma = READ_ONCE(hctx->dispatch_busy);
> 
> 	ewma *= BLK_MQ_DISPATCH_BUSY_EWMA_WEIGHT - 1;
> 	if (busy)
> 		ewma += 1 << BLK_MQ_DISPATCH_BUSY_EWMA_FACTOR;
> 	ewma /= BLK_MQ_DISPATCH_BUSY_EWMA_WEIGHT;
> 
> 	WRITE_ONCE(hctx->dispatch_busy, ewma);
> }

How about doing it in the following(simpler) way? By adding the check
at the entry of __blk_mq_update_dispatch_busy().

	if (!ewma && !busy)
		return;

Thanks,
Ming