__i915_spin_request() sucks

[Jens Axboe]

Hi,

So a few months ago I got an XPS13 laptop, the one with the high res 
screen. GUI performance was never really that great, I attributed it to 
coming from a more powerful laptop, and the i915 driving a lot of 
pixels. But yesterday I browsed from my wife's macbook, and was blown 
away. Wow, scrolling in chrome SUCKS on the xps13. Not just scrolling, 
basically anything in chrome. Molasses. So I got sick of it, fired up a 
quick perf record, did a bunch of stuff in chrome. No super smoking 
guns, but one thing did stick out - the path leading to 
__i915_spin_request().

So today, I figured I'd try just killing that spin. If it fails, we'll 
punt to normal completions, so easy change. And wow, MASSIVE difference. 
I can now scroll in chrome and not rage! It's like the laptop is 10x 
faster now.

Ran git blame, and found:

commit 2def4ad99befa25775dd2f714fdd4d92faec6e34
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date:   Tue Apr 7 16:20:41 2015 +0100

    drm/i915: Optimistically spin for the request completion

and read the commit message. Doesn't sound that impressive. Especially 
not for something that screws up interactive performance by a LOT.

What's the deal? Revert?

--
Jens Axboe

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