On 11/14/18 11:22 PM, Jens Axboe wrote:

On 11/14/18 2:43 AM, Ming Lei wrote:

quoted

On Wed, Nov 14, 2018 at 05:23:48PM +0800, jianchao.wang wrote:

quoted

Hi Ming

On 11/14/18 5:11 PM, Ming Lei wrote:

quoted

quoted

 
-	if (!blk_mq_get_dispatch_budget(hctx))
-		goto insert;
+	if (unlikely(!blk_mq_get_dispatch_budget(hctx)))
+		goto out_unlock;

The unlikely annotation is a bit misleading, since out-of-budget can
happen frequently in case of low queue depth, and there are lots of
such examples.

This could be good for the case for no .get_budget and getting budget success.
In case of out-of-budget, we insert the request which is slow path.

In case of low queue depth, it is hard to say that 'insert request' is
done in slow path, cause it happens quite frequently.

I suggest to remove these two unlikely() since modern CPU's branch prediction
should work well enough.

Especially the annotation of unlikely() often means that this branch is
missed in most of times for all settings, and it is obviously not true
in this case.

Agree, unlikely() should only be used for the error handling case or
similar that does indeed almost never trigger. It should not be used
for cases that don't trigger a lot in "most" circumstances.

That's really appreciated for all of your kindly response.
Fair enough with 'unlikely'.
I will remove these two wrong 'unlikely' in next version.

Thanks
Jianchao