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
