[PATCH v2 2/7] mm: vmscan: make global slab shrink lockless

[Qi Zheng]

The shrinker_rwsem is a global lock in shrinkers subsystem,
it is easy to cause blocking in the following cases:

a. the write lock of shrinker_rwsem was held for too long.
   For example, there are many memcgs in the system, which
   causes some paths to hold locks and traverse it for too
   long. (e.g. expand_shrinker_info())
b. the read lock of shrinker_rwsem was held for too long,
   and a writer came at this time. Then this writer will be
   forced to wait and block all subsequent readers.
   For example:
   - be scheduled when the read lock of shrinker_rwsem is
     held in do_shrink_slab()
   - some shrinker are blocked for too long. Like the case
     mentioned in the patchset[1].

Therefore, many times in history ([2],[3],[4],[5]), some
people wanted to replace shrinker_rwsem reader with SRCU,
but they all gave up because SRCU was not unconditionally
enabled.

But now, since commit 1cd0bd06093c ("rcu: Remove CONFIG_SRCU"),
the SRCU is unconditionally enabled. So it's time to use
SRCU to protect readers who previously held shrinker_rwsem.

[1]. https://lore.kernel.org/lkml/20191129214541.3110-1-ptikhomirov@xxxxxxxxxxxxx/
[2]. https://lore.kernel.org/all/1437080113.3596.2.camel@xxxxxxxxxxxx/
[3]. https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx/
[4]. https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/
[5]. https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@xxxxxxxxxxxxxxx/

Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
 mm/vmscan.c | 27 +++++++++++----------------
 1 file changed, 11 insertions(+), 16 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9f895ca6216c..02987a6f95d1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -202,6 +202,7 @@ static void set_task_reclaim_state(struct task_struct *task,
 
 LIST_HEAD(shrinker_list);
 DECLARE_RWSEM(shrinker_rwsem);
+DEFINE_SRCU(shrinker_srcu);
 
 #ifdef CONFIG_MEMCG
 static int shrinker_nr_max;
@@ -706,7 +707,7 @@ void free_prealloced_shrinker(struct shrinker *shrinker)
 void register_shrinker_prepared(struct shrinker *shrinker)
 {
 	down_write(&shrinker_rwsem);
-	list_add_tail(&shrinker->list, &shrinker_list);
+	list_add_tail_rcu(&shrinker->list, &shrinker_list);
 	shrinker->flags |= SHRINKER_REGISTERED;
 	shrinker_debugfs_add(shrinker);
 	up_write(&shrinker_rwsem);
@@ -760,13 +761,15 @@ void unregister_shrinker(struct shrinker *shrinker)
 		return;
 
 	down_write(&shrinker_rwsem);
-	list_del(&shrinker->list);
+	list_del_rcu(&shrinker->list);
 	shrinker->flags &= ~SHRINKER_REGISTERED;
 	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
 		unregister_memcg_shrinker(shrinker);
 	debugfs_entry = shrinker_debugfs_remove(shrinker);
 	up_write(&shrinker_rwsem);
 
+	synchronize_srcu(&shrinker_srcu);
+
 	debugfs_remove_recursive(debugfs_entry);
 
 	kfree(shrinker->nr_deferred);
@@ -786,6 +789,7 @@ void synchronize_shrinkers(void)
 {
 	down_write(&shrinker_rwsem);
 	up_write(&shrinker_rwsem);
+	synchronize_srcu(&shrinker_srcu);
 }
 EXPORT_SYMBOL(synchronize_shrinkers);
 
@@ -996,6 +1000,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 {
 	unsigned long ret, freed = 0;
 	struct shrinker *shrinker;
+	int srcu_idx;
 
 	/*
 	 * The root memcg might be allocated even though memcg is disabled
@@ -1007,10 +1012,10 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 	if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
 		return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
 
-	if (!down_read_trylock(&shrinker_rwsem))
-		goto out;
+	srcu_idx = srcu_read_lock(&shrinker_srcu);
 
-	list_for_each_entry(shrinker, &shrinker_list, list) {
+	list_for_each_entry_srcu(shrinker, &shrinker_list, list,
+				 srcu_read_lock_held(&shrinker_srcu)) {
 		struct shrink_control sc = {
 			.gfp_mask = gfp_mask,
 			.nid = nid,
@@ -1021,19 +1026,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 		if (ret == SHRINK_EMPTY)
 			ret = 0;
 		freed += ret;
-		/*
-		 * Bail out if someone want to register a new shrinker to
-		 * prevent the registration from being stalled for long periods
-		 * by parallel ongoing shrinking.
-		 */
-		if (rwsem_is_contended(&shrinker_rwsem)) {
-			freed = freed ? : 1;
-			break;
-		}
 	}
 
-	up_read(&shrinker_rwsem);
-out:
+	srcu_read_unlock(&shrinker_srcu, srcu_idx);
 	cond_resched();
 	return freed;
 }
-- 
2.20.1