To: Daniel Vetter <daniel.vetter@ffwll.ch>, chris@chris-wilson.co.uk
Cc: DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>
From: Jens Axboe <axboe@kernel.dk>
Subject: __i915_spin_request() sucks
Message-ID: <5644F850.2060803@kernel.dk>
Date: Thu, 12 Nov 2015 13:36:32 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.3.0
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1462
Lines: 38

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@chris-wilson.co.uk>
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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/