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Message-ID: <2940c150-577a-30a8-fac3-cf59a49b84b4@I-love.SAKURA.ne.jp>
Date: Thu, 9 Nov 2017 19:26:50 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Shakeel Butt <shakeelb@...gle.com>,
Minchan Kim <minchan@...nel.org>,
Huang Ying <ying.huang@...el.com>,
Mel Gorman <mgorman@...hsingularity.net>,
Vladimir Davydov <vdavydov.dev@...il.com>,
Michal Hocko <mhocko@...nel.org>,
Greg Thelen <gthelen@...gle.com>,
Johannes Weiner <hannes@...xchg.org>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm, shrinker: make shrinker_list lockless
On 2017/11/09 2:37, Shakeel Butt wrote:
> In our production, we have observed that the job loader gets stuck for
> 10s of seconds while doing mount operation. It turns out that it was
> stuck in register_shrinker() and some unrelated job was under memory
> pressure and spending time in shrink_slab(). Our machines have a lot
> of shrinkers registered and jobs under memory pressure has to traverse
> all of those memcg-aware shrinkers and do affect unrelated jobs which
> want to register their own shrinkers.
>
> This patch has made the shrinker_list traversal lockless and shrinker
> register remain fast. For the shrinker unregister, atomic counter
> has been introduced to avoid synchronize_rcu() call. The fields of
> struct shrinker has been rearraged to make sure that the size does
> not increase for x86_64.
>
> The shrinker functions are allowed to reschedule() and thus can not
> be called with rcu read lock. One way to resolve that is to use
> srcu read lock but then ifdefs has to be used as SRCU is behind
> CONFIG_SRCU. Another way is to just release the rcu read lock before
> calling the shrinker and reacquire on the return. The atomic counter
> will make sure that the shrinker entry will not be freed under us.
>
> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> ---
> Changelog since v1:
> - release and reacquire rcu lock across shrinker call.
>
> include/linux/shrinker.h | 4 +++-
> mm/vmscan.c | 54 ++++++++++++++++++++++++++++++------------------
> 2 files changed, 37 insertions(+), 21 deletions(-)
>
If you can accept serialized register_shrinker()/unregister_shrinker(),
I think that something like shown below can do it.
----------
diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 388ff29..e2272dd 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; /* Counted only if !SHRINKER_PERMANENT */
struct list_head list;
/* objs pending delete, per node */
atomic_long_t *nr_deferred;
@@ -74,6 +75,7 @@ struct shrinker {
/* Flags */
#define SHRINKER_NUMA_AWARE (1 << 0)
#define SHRINKER_MEMCG_AWARE (1 << 1)
+#define SHRINKER_PERMANENT (1 << 2)
extern int register_shrinker(struct shrinker *);
extern void unregister_shrinker(struct shrinker *);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1c1bc95..e963359 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 +298,14 @@ int register_shrinker(struct shrinker *shrinker)
*/
void unregister_shrinker(struct shrinker *shrinker)
{
- down_write(&shrinker_rwsem);
- list_del(&shrinker->list);
- up_write(&shrinker_rwsem);
+ BUG_ON(shrinker->flags & SHRINKER_PERMANENT);
+ mutex_lock(&shrinker_lock);
+ list_del_rcu(&shrinker->list);
+ synchronize_rcu();
+ while (atomic_read(&shrinker->nr_active))
+ msleep(1);
+ synchronize_rcu();
+ mutex_unlock(&shrinker_lock);
kfree(shrinker->nr_deferred);
}
EXPORT_SYMBOL(unregister_shrinker);
@@ -468,18 +474,9 @@ 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) {
+ bool permanent;
struct shrink_control sc = {
.gfp_mask = gfp_mask,
.nid = nid,
@@ -498,11 +495,16 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
sc.nid = 0;
+ permanent = (shrinker->flags & SHRINKER_PERMANENT);
+ if (!permanent)
+ atomic_inc(&shrinker->nr_active);
+ rcu_read_unlock();
freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
+ rcu_read_lock();
+ if (!permanent)
+ atomic_dec(&shrinker->nr_active);
}
-
- up_read(&shrinker_rwsem);
-out:
+ rcu_read_unlock();
cond_resched();
return freed;
}
----------
If you want parallel register_shrinker()/unregister_shrinker(), something like
shown below on top of shown above will do it.
----------
diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index e2272dd..471b2f6 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -67,6 +67,7 @@ struct shrinker {
/* These are for internal use */
atomic_t nr_active; /* Counted only if !SHRINKER_PERMANENT */
struct list_head list;
+ struct list_head gc_list;
/* objs pending delete, per node */
atomic_long_t *nr_deferred;
};
diff --git a/mm/vmscan.c b/mm/vmscan.c
index e963359..a216dc5 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 DEFINE_MUTEX(shrinker_lock);
+static DEFINE_SPINLOCK(shrinker_lock);
#ifdef CONFIG_MEMCG
static bool global_reclaim(struct scan_control *sc)
@@ -286,9 +286,9 @@ int register_shrinker(struct shrinker *shrinker)
return -ENOMEM;
atomic_set(&shrinker->nr_active, 0);
- mutex_lock(&shrinker_lock);
+ spin_lock(&shrinker_lock);
list_add_tail_rcu(&shrinker->list, &shrinker_list);
- mutex_unlock(&shrinker_lock);
+ spin_unlock(&shrinker_lock);
return 0;
}
EXPORT_SYMBOL(register_shrinker);
@@ -298,15 +298,30 @@ int register_shrinker(struct shrinker *shrinker)
*/
void unregister_shrinker(struct shrinker *shrinker)
{
+ static LIST_HEAD(shrinker_gc_list);
+ struct shrinker *gc;
+
BUG_ON(shrinker->flags & SHRINKER_PERMANENT);
- mutex_lock(&shrinker_lock);
+ spin_lock(&shrinker_lock);
list_del_rcu(&shrinker->list);
+ /*
+ * Need to update ->list.next if concurrently unregistering shrinkers
+ * can find this shrinker, for this shrinker's unregistration might
+ * complete before their unregistrations complete.
+ */
+ list_for_each_entry(gc, &shrinker_gc_list, gc_list) {
+ if (gc->list.next == &shrinker->list)
+ rcu_assign_pointer(gc->list.next, shrinker->list.next);
+ }
+ list_add_tail(&shrinker->gc_list, &shrinker_gc_list);
+ spin_unlock(&shrinker_lock);
synchronize_rcu();
while (atomic_read(&shrinker->nr_active))
msleep(1);
synchronize_rcu();
- mutex_unlock(&shrinker_lock);
+ spin_lock(&shrinker_lock);
+ list_del(&shrinker->gc_list);
+ spin_unlock(&shrinker_lock);
kfree(shrinker->nr_deferred);
}
EXPORT_SYMBOL(unregister_shrinker);
----------
F.Y.I. When I posted above change at
http://lkml.kernel.org/r/201411231350.DHI12456.OLOFFJSFtQVMHO@I-love.SAKURA.ne.jp ,
Michal Hocko commented like below.
I thought that {un}register_shrinker are really unlikely
paths called during initialization and tear down which usually do not
happen during OOM conditions.
I cannot judge the patch itself as this is out of my area but is the
complexity worth it?
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