drm/msm: Drop mm_lock in scan loop

lock_stat + mmm_donut[1] say that this reduces contention on mm_lock
significantly (~350x lower waittime-max, and ~100x lower waittime-avg)

[1] https://chromium.googlesource.com/chromiumos/platform/microbenchmarks/+/refs/heads/main/mmm_donut.py

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20210402211226.875726-1-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>

diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
index 2536340..6a42cdf 100644 (file)
--- a/drivers/gpu/drm/msm/msm_drv.h
+++ b/drivers/gpu/drm/msm/msm_drv.h
@@ -184,7 +184,8 @@ struct msm_drm_private {
        /**
         * Lists of inactive GEM objects.  Every bo is either in one of the
         * inactive lists (depending on whether or not it is shrinkable) or
-        * gpu->active_list (for the gpu it is active on[1])
+        * gpu->active_list (for the gpu it is active on[1]), or transiently
+        * on a temporary list as the shrinker is running.
         *
         * These lists are protected by mm_lock (which should be acquired
         * before per GEM object lock).  One should *not* hold mm_lock in

diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
index f146d9c..4e91b09 100644 (file)
--- a/drivers/gpu/drm/msm/msm_gem.c
+++ b/drivers/gpu/drm/msm/msm_gem.c
@@ -719,7 +719,7 @@ void msm_gem_purge(struct drm_gem_object *obj)
        put_iova_vmas(obj);
 
        msm_obj->madv = __MSM_MADV_PURGED;
-       mark_unpurgable(msm_obj);
+       update_inactive(msm_obj);
 
        drm_vma_node_unmap(&obj->vma_node, dev->anon_inode->i_mapping);
        drm_gem_free_mmap_offset(obj);

diff --git a/drivers/gpu/drm/msm/msm_gem_shrinker.c b/drivers/gpu/drm/msm/msm_gem_shrinker.c
index f3e948a..33a4964 100644 (file)
--- a/drivers/gpu/drm/msm/msm_gem_shrinker.c
+++ b/drivers/gpu/drm/msm/msm_gem_shrinker.c
@@ -22,26 +22,62 @@ msm_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
 {
        struct msm_drm_private *priv =
                container_of(shrinker, struct msm_drm_private, shrinker);
-       struct msm_gem_object *msm_obj;
+       struct list_head still_in_list;
        unsigned long freed = 0;
 
+       INIT_LIST_HEAD(&still_in_list);
+
        mutex_lock(&priv->mm_lock);
 
-       list_for_each_entry(msm_obj, &priv->inactive_dontneed, mm_list) {
-               if (freed >= sc->nr_to_scan)
+       while (freed < sc->nr_to_scan) {
+               struct msm_gem_object *msm_obj = list_first_entry_or_null(
+                               &priv->inactive_dontneed, typeof(*msm_obj), mm_list);
+
+               if (!msm_obj)
                        break;
-               /* Use trylock, because we cannot block on a obj that
-                * might be trying to acquire mm_lock
+
+               list_move_tail(&msm_obj->mm_list, &still_in_list);
+
+               /*
+                * If it is in the process of being freed, msm_gem_free_object
+                * can be blocked on mm_lock waiting to remove it.  So just
+                * skip it.
                 */
-               if (!msm_gem_trylock(&msm_obj->base))
+               if (!kref_get_unless_zero(&msm_obj->base.refcount))
                        continue;
+
+               /*
+                * Now that we own a reference, we can drop mm_lock for the
+                * rest of the loop body, to reduce contention with the
+                * retire_submit path (which could make more objects purgable)
+                */
+
+               mutex_unlock(&priv->mm_lock);
+
+               /*
+                * Note that this still needs to be trylock, since we can
+                * hit shrinker in response to trying to get backing pages
+                * for this obj (ie. while it's lock is already held)
+                */
+               if (!msm_gem_trylock(&msm_obj->base))
+                       goto tail;
+
                if (is_purgeable(msm_obj)) {
+                       /*
+                        * This will move the obj out of still_in_list to
+                        * the purged list
+                        */
                        msm_gem_purge(&msm_obj->base);
                        freed += msm_obj->base.size >> PAGE_SHIFT;
                }
                msm_gem_unlock(&msm_obj->base);
+
+tail:
+               drm_gem_object_put(&msm_obj->base);
+               mutex_lock(&priv->mm_lock);
        }
 
+       list_splice_tail(&still_in_list, &priv->inactive_dontneed);
        mutex_unlock(&priv->mm_lock);
 
        if (freed > 0) {