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Subject[PATCH 1/2] mm,vmscan: Kill global shrinker lock.
Date
When shrinker_rwsem was introduced, it was assumed that
register_shrinker()/unregister_shrinker() are really unlikely paths
which are called during initialization and tear down. But nowadays,
register_shrinker()/unregister_shrinker() might be called regularly.
This patch prepares for allowing parallel registration/unregistration
of shrinkers.

Since do_shrink_slab() can reschedule, we cannot protect shrinker_list
using one RCU section. But using atomic_inc()/atomic_dec() for each
do_shrink_slab() call will not impact so much.

This patch uses polling loop with short sleep for unregister_shrinker()
rather than wait_on_atomic_t(), for we can save reader's cost (plain
atomic_dec() compared to atomic_dec_and_test()), we can expect that
do_shrink_slab() of unregistering shrinker likely returns shortly, and
we can avoid khungtaskd warnings when do_shrink_slab() of unregistering
shrinker unexpectedly took so long.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
include/linux/shrinker.h | 3 ++-
mm/vmscan.c | 41 +++++++++++++++++++----------------------
2 files changed, 21 insertions(+), 23 deletions(-)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 388ff29..333a1d0 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -62,9 +62,10 @@ struct shrinker {

int seeks; /* seeks to recreate an obj */
long batch; /* reclaim batch size, 0 = default */
- unsigned long flags;
+ unsigned int flags;

/* These are for internal use */
+ atomic_t nr_active;
struct list_head list;
/* objs pending delete, per node */
atomic_long_t *nr_deferred;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1c1bc95..c8996e8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -157,7 +157,7 @@ struct scan_control {
unsigned long vm_total_pages;

static LIST_HEAD(shrinker_list);
-static DECLARE_RWSEM(shrinker_rwsem);
+static DEFINE_MUTEX(shrinker_lock);

#ifdef CONFIG_MEMCG
static bool global_reclaim(struct scan_control *sc)
@@ -285,9 +285,10 @@ int register_shrinker(struct shrinker *shrinker)
if (!shrinker->nr_deferred)
return -ENOMEM;

- down_write(&shrinker_rwsem);
- list_add_tail(&shrinker->list, &shrinker_list);
- up_write(&shrinker_rwsem);
+ atomic_set(&shrinker->nr_active, 0);
+ mutex_lock(&shrinker_lock);
+ list_add_tail_rcu(&shrinker->list, &shrinker_list);
+ mutex_unlock(&shrinker_lock);
return 0;
}
EXPORT_SYMBOL(register_shrinker);
@@ -297,9 +298,13 @@ int register_shrinker(struct shrinker *shrinker)
*/
void unregister_shrinker(struct shrinker *shrinker)
{
- down_write(&shrinker_rwsem);
- list_del(&shrinker->list);
- up_write(&shrinker_rwsem);
+ mutex_lock(&shrinker_lock);
+ list_del_rcu(&shrinker->list);
+ synchronize_rcu();
+ while (atomic_read(&shrinker->nr_active))
+ schedule_timeout_uninterruptible(1);
+ synchronize_rcu();
+ mutex_unlock(&shrinker_lock);
kfree(shrinker->nr_deferred);
}
EXPORT_SYMBOL(unregister_shrinker);
@@ -468,18 +473,8 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
if (nr_scanned == 0)
nr_scanned = SWAP_CLUSTER_MAX;

- if (!down_read_trylock(&shrinker_rwsem)) {
- /*
- * If we would return 0, our callers would understand that we
- * have nothing else to shrink and give up trying. By returning
- * 1 we keep it going and assume we'll be able to shrink next
- * time.
- */
- freed = 1;
- goto out;
- }
-
- list_for_each_entry(shrinker, &shrinker_list, list) {
+ rcu_read_lock();
+ list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
struct shrink_control sc = {
.gfp_mask = gfp_mask,
.nid = nid,
@@ -498,11 +493,13 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
sc.nid = 0;

+ atomic_inc(&shrinker->nr_active);
+ rcu_read_unlock();
freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
+ rcu_read_lock();
+ atomic_dec(&shrinker->nr_active);
}
-
- up_read(&shrinker_rwsem);
-out:
+ rcu_read_unlock();
cond_resched();
return freed;
}
--
1.8.3.1
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