Re: [PATCH v2 5/5] drm/i915: Start writeback from the shrinker

[Joonas Lahtinen <joonas.lahtinen@linux.intel.com>]

On pe, 2017-06-09 at 12:03 +0100, Chris Wilson wrote:
> When we are called to relieve mempressue via the shrinker, the only way
> we can make progress is either by discarding unwanted pages (those
> objects that userspace has marked MADV_DONTNEED) or by reclaiming the
> dirty objects via swap. As we know that is the only way to make further
> progress, we can initiate the writeback as we invalidate the objects.
> This means the objects we put onto the inactive anon lru list are
> already marked for reclaim+writeback and so will trigger a wait upon the
> writeback inside direct reclaim, greatly improving the success rate of
> direct reclaim on i915 objects.
> 
> The corollary is that we may start a slow swap on opportunistic
> mempressure from the likes of the compaction + migration kthreads. This
> is limited by those threads only being allowed to shrink idle pages, but
> also that if we reactivate the page before it is swapped out by gpu
> activity, we only page the cost of repinning the page. The cost is most
> felt when an object is reused after mempressure, which hopefully
> excludes the latency sensitive tasks (as we are just extending the
> impact of swap thrashing to them).
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Michal Hocko <mhocko@suse.com>

<SNIP>

> +static void __start_writeback(struct drm_i915_gem_object *obj)
> +{

<SNIP>

> +	/* Force any other users of this object to refault */
> +	mapping = obj->base.filp->f_mapping;
> +	unmap_mapping_range(mapping, 0, (loff_t)-1, 0);
> +
> +	/* Begin writeback on each dirty page */
> +	for (i = 0; i < obj->base.size >> PAGE_SHIFT; i++) {
> +		struct page *page;
> +
> +		page = find_lock_entry(mapping, i);
> +		if (!page || radix_tree_exceptional_entry(page))
> +			continue;
> +
> +		if (!page_mapped(page) && clear_page_dirty_for_io(page)) {
> +			int ret;
> +
> +			SetPageReclaim(page);
> +			ret = mapping->a_ops->writepage(page, &wbc);
> +			if (!PageWriteback(page))
> +				ClearPageReclaim(page);
> +			if (!ret)
> +				goto put;
> +		}
> +		unlock_page(page);
> +put:
> +		put_page(page);
> +	}

Apart from this part (which should probably be a helper function
outside of i915), the code is:

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

Regards, Joonas
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation
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