Re: [PATCH v2] drm/i915: Disable shrinker for non-swapped backed objects

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Nov 25, 2015 at 06:36:56PM +0000, Chris Wilson wrote:
> If the system has no available swap pages, we cannot make forward
> progress in the shrinker by releasing active pages, only by releasing
> purgeable pages which are immediately reaped. Take total_swap_pages into
> account when counting up available objects to be shrunk and subsequently
> shrinking them. By doing so, we avoid unbinding objects that cannot be
> shrunk and so wasting CPU cycles flushing those objects from the GPU to
> the system and then immediately back again (as they will more than
> likely be reused shortly after).
> 
> Based on a patch by Akash Goel.
> 
> v2: Check for frontswap without physical swap (or dedicated swap space).
> If frontswap is available, we may be able to compress the GPU pages
> instead of swapping out to disk. In this case, we do want to shrink GPU
> objects and so make them available for compressing.

Frontswap always sits on top of an active swap device. It's enough to
check for available swap space.

> +static bool swap_available(void)
> +{
> +	return total_swap_pages || frontswap_enabled;
> +}

If you use get_nr_swap_pages() instead of total_swap_pages, this will
also stop scanning objects once the swap space is full. We do that in
the VM to stop scanning anonymous pages.

On a sidenote, frontswap_enabled is #defined to 1 when the feature is
compiled in, so this would be a no-op on most distro kernels.

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