Re: [PATCH 3/3] mm: Count list_lru_one::nr_items lockless
From: Kirill Tkhai
Date: Wed Aug 23 2017 - 08:26:17 EST
On 23.08.2017 11:27, Vladimir Davydov wrote:
> On Wed, Aug 23, 2017 at 11:00:56AM +0300, Kirill Tkhai wrote:
>> On 22.08.2017 22:47, Vladimir Davydov wrote:
>>> On Tue, Aug 22, 2017 at 03:29:35PM +0300, Kirill Tkhai wrote:
>>>> During the reclaiming slab of a memcg, shrink_slab iterates
>>>> over all registered shrinkers in the system, and tries to count
>>>> and consume objects related to the cgroup. In case of memory
>>>> pressure, this behaves bad: I observe high system time and
>>>> time spent in list_lru_count_one() for many processes on RHEL7
>>>> kernel (collected via $perf record --call-graph fp -j k -a):
>>>>
>>>> 0,50% nixstatsagent [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
>>>> 0,26% nixstatsagent [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
>>>> 0,23% nixstatsagent [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
>>>> 0,15% nixstatsagent [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
>>>> 0,15% nixstatsagent [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2
>>>>
>>>> 0,94% mysqld [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
>>>> 0,57% mysqld [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
>>>> 0,51% mysqld [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
>>>> 0,32% mysqld [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
>>>> 0,32% mysqld [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2
>>>>
>>>> 0,73% sshd [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
>>>> 0,35% sshd [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
>>>> 0,32% sshd [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
>>>> 0,21% sshd [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
>>>> 0,21% sshd [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2
>>>
>>> It would be nice to see how this is improved by this patch.
>>> Can you try to record the traces on the vanilla kernel with
>>> and without this patch?
>>
>> Sadly, the talk is about a production node, and it's impossible to use vanila kernel there.
>
> I see :-( Then maybe you could try to come up with a contrived test?
I've tried and I'm not sure I'm able to reproduce on my test 8-cpu node the situation like I saw on production node
via a test. Maybe you have an idea how to measure that?
I've changed the places, you commented, and the merged patch is below.
How are you about it?
[PATCH]mm: Make count list_lru_one::nr_items lockless
During the reclaiming slab of a memcg, shrink_slab iterates
over all registered shrinkers in the system, and tries to count
and consume objects related to the cgroup. In case of memory
pressure, this behaves bad: I observe high system time and
time spent in list_lru_count_one() for many processes on RHEL7
kernel (collected via $perf record --call-graph fp -j k -a):
0,50% nixstatsagent [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
0,26% nixstatsagent [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
0,23% nixstatsagent [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
0,15% nixstatsagent [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
0,15% nixstatsagent [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2
0,94% mysqld [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
0,57% mysqld [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
0,51% mysqld [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
0,32% mysqld [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
0,32% mysqld [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2
0,73% sshd [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
0,35% sshd [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
0,32% sshd [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
0,21% sshd [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
0,21% sshd [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2
This patch aims to make super_cache_count() (and other functions,
which count LRU nr_items) more effective.
It allows list_lru_node::memcg_lrus to be RCU-accessed, and makes
__list_lru_count_one() count nr_items lockless to minimize
overhead introduced by locking operation, and to make parallel
reclaims more scalable.
Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
---
include/linux/list_lru.h | 3 ++-
mm/list_lru.c | 59 ++++++++++++++++++++++++++++++------------------
2 files changed, 39 insertions(+), 23 deletions(-)
diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
index fa7fd03cb5f9..a55258100e40 100644
--- a/include/linux/list_lru.h
+++ b/include/linux/list_lru.h
@@ -31,6 +31,7 @@ struct list_lru_one {
};
struct list_lru_memcg {
+ struct rcu_head rcu;
/* array of per cgroup lists, indexed by memcg_cache_id */
struct list_lru_one *lru[0];
};
@@ -42,7 +43,7 @@ struct list_lru_node {
struct list_lru_one lru;
#if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
/* for cgroup aware lrus points to per cgroup lists, otherwise NULL */
- struct list_lru_memcg *memcg_lrus;
+ struct list_lru_memcg __rcu *memcg_lrus;
#endif
long nr_items;
} ____cacheline_aligned_in_smp;
diff --git a/mm/list_lru.c b/mm/list_lru.c
index 7a40fa2be858..9fdb24818dae 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -52,14 +52,15 @@ static inline bool list_lru_memcg_aware(struct list_lru *lru)
static inline struct list_lru_one *
list_lru_from_memcg_idx(struct list_lru_node *nlru, int idx)
{
+ struct list_lru_memcg *memcg_lrus;
/*
- * The lock protects the array of per cgroup lists from relocation
- * (see memcg_update_list_lru_node).
+ * Either lock or RCU protects the array of per cgroup lists
+ * from relocation (see memcg_update_list_lru_node).
*/
- lockdep_assert_held(&nlru->lock);
- if (nlru->memcg_lrus && idx >= 0)
- return nlru->memcg_lrus->lru[idx];
-
+ memcg_lrus = rcu_dereference_check(nlru->memcg_lrus,
+ lockdep_is_held(&nlru->lock));
+ if (memcg_lrus && idx >= 0)
+ return memcg_lrus->lru[idx];
return &nlru->lru;
}
@@ -168,10 +169,10 @@ static unsigned long __list_lru_count_one(struct list_lru *lru,
struct list_lru_one *l;
unsigned long count;
- spin_lock(&nlru->lock);
+ rcu_read_lock();
l = list_lru_from_memcg_idx(nlru, memcg_idx);
count = l->nr_items;
- spin_unlock(&nlru->lock);
+ rcu_read_unlock();
return count;
}
@@ -323,24 +324,33 @@ static int __memcg_init_list_lru_node(struct list_lru_memcg *memcg_lrus,
static int memcg_init_list_lru_node(struct list_lru_node *nlru)
{
+ struct list_lru_memcg *memcg_lrus;
int size = memcg_nr_cache_ids;
- nlru->memcg_lrus = kmalloc(size * sizeof(void *), GFP_KERNEL);
- if (!nlru->memcg_lrus)
+ memcg_lrus = kmalloc(sizeof(*memcg_lrus) +
+ size * sizeof(void *), GFP_KERNEL);
+ if (!memcg_lrus)
return -ENOMEM;
- if (__memcg_init_list_lru_node(nlru->memcg_lrus, 0, size)) {
- kfree(nlru->memcg_lrus);
+ if (__memcg_init_list_lru_node(memcg_lrus, 0, size)) {
+ kfree(memcg_lrus);
return -ENOMEM;
}
+ RCU_INIT_POINTER(nlru->memcg_lrus, memcg_lrus);
return 0;
}
static void memcg_destroy_list_lru_node(struct list_lru_node *nlru)
{
- __memcg_destroy_list_lru_node(nlru->memcg_lrus, 0, memcg_nr_cache_ids);
- kfree(nlru->memcg_lrus);
+ struct list_lru_memcg *memcg_lrus;
+ /*
+ * This is called when shrinker has already been unregistered,
+ * and nobody can use it. So, there is no need to use kfree_rcu().
+ */
+ memcg_lrus = rcu_dereference_protected(nlru->memcg_lrus, true);
+ __memcg_destroy_list_lru_node(memcg_lrus, 0, memcg_nr_cache_ids);
+ kfree(memcg_lrus);
}
static int memcg_update_list_lru_node(struct list_lru_node *nlru,
@@ -350,8 +360,9 @@ static int memcg_update_list_lru_node(struct list_lru_node *nlru,
BUG_ON(old_size > new_size);
- old = nlru->memcg_lrus;
- new = kmalloc(new_size * sizeof(void *), GFP_KERNEL);
+ old = rcu_dereference_protected(nlru->memcg_lrus,
+ lockdep_is_held(&list_lrus_mutex));
+ new = kmalloc(sizeof(*new) + new_size * sizeof(void *), GFP_KERNEL);
if (!new)
return -ENOMEM;
@@ -360,29 +371,33 @@ static int memcg_update_list_lru_node(struct list_lru_node *nlru,
return -ENOMEM;
}
- memcpy(new, old, old_size * sizeof(void *));
+ memcpy(&new->lru, &old->lru, old_size * sizeof(void *));
/*
- * The lock guarantees that we won't race with a reader
- * (see list_lru_from_memcg_idx).
+ * The locking below allows readers that hold nlru->lock avoid taking
+ * rcu_read_lock (see list_lru_from_memcg_idx).
*
* Since list_lru_{add,del} may be called under an IRQ-safe lock,
* we have to use IRQ-safe primitives here to avoid deadlock.
*/
spin_lock_irq(&nlru->lock);
- nlru->memcg_lrus = new;
+ rcu_assign_pointer(nlru->memcg_lrus, new);
spin_unlock_irq(&nlru->lock);
- kfree(old);
+ kfree_rcu(old, rcu);
return 0;
}
static void memcg_cancel_update_list_lru_node(struct list_lru_node *nlru,
int old_size, int new_size)
{
+ struct list_lru_memcg *memcg_lrus;
+
+ memcg_lrus = rcu_dereference_protected(nlru->memcg_lrus,
+ lockdep_is_held(&list_lrus_mutex));
/* do not bother shrinking the array back to the old size, because we
* cannot handle allocation failures here */
- __memcg_destroy_list_lru_node(nlru->memcg_lrus, old_size, new_size);
+ __memcg_destroy_list_lru_node(memcg_lrus, old_size, new_size);
}
static int memcg_init_list_lru(struct list_lru *lru, bool memcg_aware)