Patch "drm/i915: Do not sync RCU during shrinking" has been added to the 4.11-stable tree

This is a note to let you know that I've just added the patch titled

drm/i915: Do not sync RCU during shrinking

to the 4.11-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
drm-i915-do-not-sync-rcu-during-shrinking.patch
and it can be found in the queue-4.11 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Joonas Lahtinen <[email protected]>
Date: Thu, 18 May 2017 11:49:39 +0300
Subject: drm/i915: Do not sync RCU during shrinking

From: Joonas Lahtinen <[email protected]>

commit 4681ee21d62cfed4364e09ec50ee8e88185dd628 upstream.

Due to the complex dependencies between workqueues and RCU, which are not easily detected by lockdep, do not synchronize RCU during shrinking.

On low-on-memory systems (mem=1G for example), the RCU sync leads to all system workqueus freezing and unrelated lockdep splats are displayed according to reports. GIT bisecting done by J. R. Okajima points to the commit where RCU syncing was extended.

RCU sync gains us very little benefit in real life scenarios where the amount of memory used by object backing storage is dominant over the metadata under RCU, so drop it altogether.

" Yeeeaah, if core could just, go ahead and reclaim RCU
queues, that'd be great. "

- Chris Wilson, 2016 (0eafec6d3244)

v2: More information to commit message.
v3: Remove "grep _rcu_" escapee from i915_gem_shrink_all (Andrea)

---
 drivers/gpu/drm/i915/i915_gem_shrinker.c |    5 -----
 1 file changed, 5 deletions(-)

@@ -59,9 +59,6 @@ static void i915_gem_shrinker_unlock(str
 		return;
 
 	mutex_unlock(&dev->struct_mutex);
-
-	/* expedite the RCU grace period to free some request slabs */
-	synchronize_rcu_expedited();
 }
 
 static bool any_vma_pinned(struct drm_i915_gem_object *obj)

@@ -271,8 +268,6 @@ unsigned long i915_gem_shrink_all(struct
 				I915_SHRINK_BOUND |
 				I915_SHRINK_UNBOUND |
 				I915_SHRINK_ACTIVE);
-	synchronize_rcu(); /* wait for our earlier RCU delayed slab frees */
-
 	return freed;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.11/drm-i915-fix-gvt-g-pvinfo-version-compatibility-check.patch queue-4.11/drm-i915-do-not-sync-rcu-during-shrinking.patch