linux-kernel - Re: [PATCH] i915: slab shrinker have to return -1 if it cant shrink any objects

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Message-ID: <4E1C15B2.9020800@jp.fujitsu.com>
Date:	Tue, 12 Jul 2011 18:36:50 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	chris@...is-wilson.co.uk
CC:	keithp@...thp.com, linux-kernel@...r.kernel.org, airlied@...ux.ie,
	dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] i915: slab shrinker have to return -1 if it cant shrink
 any objects

Hi,

sorry for the delay.

> On Wed, 29 Jun 2011 20:53:54 -0700, Keith Packard <keithp@...thp.com> wrote:
>> On Fri, 24 Jun 2011 17:03:22 +0900, KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
>>
>>> Now, i915_gem_inactive_shrink() should return -1 instead of 0 if it
>>> can't take a lock. Otherwise, vmscan is getting a lot of confusing
>>> because vmscan can't distinguish "can't take a lock temporary" and
>>> "we've shrank all of i915 objects".
>>
>> This doesn't look like the cleanest change possible. I think it would be
>> better if the shrink function could uniformly return an error
>> indication so that we wouldn't need the weird looking conditional return.

shrink_icache_memory() is good sample code.
It doesn't take a lock if sc->nr_to_scan==0. i915_gem_inactive_shrink() should do
it too, ideally.

My patch only take a first-aid.

Plus, if I understand correctly, i915_gem_inactive_shrink() have more fundamental
issue. actually, shrinker code shouldn't use mutex. Instead, use spinlock.
IOW, Don't call kmalloc(GFP_KERNEL) while taking dev->struct_mutex. Otherwise,
vmscan in its call path completely fail to shrink i915 cache and it makes big
memory reclaim confusing if i915 have a lot of shrinkable pages.

> Unless I am mistaken, and there are more patches in flight, the return
> code from i915_gem_inactive_shrink() is promoted to unsigned long and then
> used in the calculation of how may objects to evict...

shrinker->shrink has int type value. you can't change i915_gem_inactive_shrink()
unless generic shrinker code.
Do you really want to change it?

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