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

[Jens Axboe <axboe@xxxxxxxxx>]

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);
}

static void blk_mq_update_dispatch_busy(struct blk_mq_hw_ctx *hctx, bool busy)
{
	if (hctx->queue->elevator)
		return;

	/*
	 * If we've never seen a busy condition, don't do anything.
	 */
	if (!test_bit(BLK_MQ_S_EWMA_ENABLED, &hctx->state)) {
		if (!busy)
			return;

		set_bit(BLK_MQ_S_EWMA, &hctx->state);
	}

	__blk_mq_update_dispatch_busy(hctx, busy);
}


-- 
Jens Axboe