[PATCH] mm/shrinker: add SHRINKER_NO_DIRECT_RECLAIM

[lipeifeng]

From: Peifeng Li <lipeifeng@xxxxxxxx>

In the case of insufficient memory, threads will be in direct_reclaim to
reclaim memory, direct_reclaim will call shrink_slab to run sequentially
each shrinker callback. If there is a lock-contention in the shrinker
callback,such as spinlock,mutex_lock and so on, threads may be likely to
be stuck in direct_reclaim for a long time, even if the memfree has reached
the high watermarks of the zone, resulting in poor performance of threads.

Example 1: shrinker callback may wait for spinlock
static unsigned long mb_cache_shrink(struct mb_cache *cache,
                                     unsigned long nr_to_scan)
{
        struct mb_cache_entry *entry;
        unsigned long shrunk = 0;

        spin_lock(&cache->c_list_lock);
        while (nr_to_scan-- && !list_empty(&cache->c_list)) {
                entry = list_first_entry(&cache->c_list,
                                         struct mb_cache_entry, e_list);
                if (test_bit(MBE_REFERENCED_B, &entry->e_flags) ||
                    atomic_cmpxchg(&entry->e_refcnt, 1, 0) != 1) {
                        clear_bit(MBE_REFERENCED_B, &entry->e_flags);
                        list_move_tail(&entry->e_list, &cache->c_list);
                        continue;
                }
                list_del_init(&entry->e_list);
                cache->c_entry_count--;
                spin_unlock(&cache->c_list_lock);
                __mb_cache_entry_free(cache, entry);
                shrunk++;
                cond_resched();
                spin_lock(&cache->c_list_lock);
        }
        spin_unlock(&cache->c_list_lock);

        return shrunk;
}
Example 2: shrinker callback may wait for mutex lock
static
unsigned long kbase_mem_evictable_reclaim_scan_objects(struct shrinker *s,
		struct shrink_control *sc)
{
	struct kbase_context *kctx;
	struct kbase_mem_phy_alloc *alloc;
	struct kbase_mem_phy_alloc *tmp;
	unsigned long freed = 0;

	kctx = container_of(s, struct kbase_context, reclaim);

	// MTK add to prevent false alarm
	lockdep_off();

	mutex_lock(&kctx->jit_evict_lock);

	list_for_each_entry_safe(alloc, tmp, &kctx->evict_list, evict_node) {
		int err;

		err = kbase_mem_shrink_gpu_mapping(kctx, alloc->reg,
				0, alloc->nents);
		if (err != 0) {
			freed = -1;
			goto out_unlock;
		}

		alloc->evicted = alloc->nents;

		kbase_free_phy_pages_helper(alloc, alloc->evicted);
		freed += alloc->evicted;
		list_del_init(&alloc->evict_node);

		kbase_jit_backing_lost(alloc->reg);

		if (freed > sc->nr_to_scan)
			break;
	}
out_unlock:
	mutex_unlock(&kctx->jit_evict_lock);

	// MTK add to prevent false alarm
	lockdep_on();

	return freed;
}

In mobile-phone,threads are likely to be stuck in shrinker callback during
direct_reclaim, with example like the following:
<...>-2806    [004] ..... 866458.339840: mm_shrink_slab_start:
			dynamic_mem_shrink_scan+0x0/0xb8 ... priority 2
<...>-2806    [004] ..... 866459.339933: mm_shrink_slab_end:
			dynamic_mem_shrink_scan+0x0/0xb8 ...

For the above reason, the patch introduces SHRINKER_NO_DIRECT_RECLAIM that
allows driver to set shrinker callback not to be called in direct_reclaim
unless sc->priority is 0.

The reason why sc->priority=0 allows shrinker callback to be called in
direct_reclaim is for two reasons:
1.Always call all shrinker callback in drop_slab that priority is 0.
2.sc->priority is 0 during direct_reclaim, allow direct_reclaim to call
shrinker callback, to reclaim memory timely.

Note:
1.Register_shrinker_prepared() default not to set
SHRINKER_NO_DIRECT_RECLAIM, to maintain the current behavior of the code.
2.Logic of kswapd and drop_slab to call shrinker callback isn't affected.

Signed-off-by: Peifeng Li <lipeifeng@xxxxxxxx>
---
 include/linux/shrinker.h |  5 +++++
 mm/shrinker.c            | 36 ++++++++++++++++++++++++++++++++++--
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 1a00be90d93a..2d5a8b3a720b 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -130,6 +130,11 @@ struct shrinker {
  * non-MEMCG_AWARE shrinker should not have this flag set.
  */
 #define SHRINKER_NONSLAB	BIT(4)
+/*
+ * Can shrinker callback be called in direct_relcaim unless
+ * sc->priority is 0?
+ */
+#define SHRINKER_NO_DIRECT_RECLAIM	BIT(5)
 
 __printf(2, 3)
 struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...);
diff --git a/mm/shrinker.c b/mm/shrinker.c
index dc5d2a6fcfc4..3ac50da72494 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -544,7 +544,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 			if (!memcg_kmem_online() &&
 			    !(shrinker->flags & SHRINKER_NONSLAB))
 				continue;
-
+			/*
+			 * SHRINKER_NO_DIRECT_RECLAIM, mean that shrinker callback
+			 * should not be called in direct_reclaim unless priority
+			 * is 0.
+			 */
+			if ((shrinker->flags & SHRINKER_NO_DIRECT_RECLAIM) &&
+					!current_is_kswapd()) {
+				/*
+				 * 1.Always call shrinker callback in drop_slab that
+				 * priority is 0.
+				 * 2.sc->priority is 0 during direct_reclaim, allow
+				 * direct_reclaim to call shrinker callback, to reclaim
+				 * memory timely.
+				 */
+				if (priority)
+					continue;
+			}
 			ret = do_shrink_slab(&sc, shrinker, priority);
 			if (ret == SHRINK_EMPTY) {
 				clear_bit(offset, unit->map);
@@ -658,7 +674,23 @@ unsigned long shrink_slab(gfp_t gfp_mask, int nid, struct mem_cgroup *memcg,
 			continue;
 
 		rcu_read_unlock();
-
+		/*
+		 * SHRINKER_NO_DIRECT_RECLAIM, mean that shrinker callback
+		 * should not be called in direct_reclaim unless priority
+		 * is 0.
+		 */
+		if ((shrinker->flags & SHRINKER_NO_DIRECT_RECLAIM) &&
+				!current_is_kswapd()) {
+			/*
+			 * 1.Always call shrinker callback in drop_slab that
+			 * priority is 0.
+			 * 2.sc->priority is 0 during direct_reclaim, allow
+			 * direct_reclaim to call shrinker callback, to reclaim
+			 * memory timely.
+			 */
+			if (priority)
+				continue;
+		}
 		ret = do_shrink_slab(&sc, shrinker, priority);
 		if (ret == SHRINK_EMPTY)
 			ret = 0;
-- 
2.34.1