Hi all,
This patch series aims to make slab shrink lockless.
1. Background
=============
On our servers, we often find the following system cpu hotspots:
52.22% [kernel] [k] down_read_trylock
19.60% [kernel] [k] up_read
8.86% [kernel] [k] shrink_slab
2.44% [kernel] [k] idr_find
1.25% [kernel] [k] count_shadow_nodes
1.18% [kernel] [k] shrink lruvec
0.71% [kernel] [k] mem_cgroup_iter
0.71% [kernel] [k] shrink_node
0.55% [kernel] [k] find_next_bit
And we used bpftrace to capture its calltrace as follows:
@[
down_read_trylock+1
shrink_slab+128
shrink_node+371
do_try_to_free_pages+232
try_to_free_pages+243
_alloc_pages_slowpath+771
_alloc_pages_nodemask+702
pagecache_get_page+255
filemap_fault+1361
ext4_filemap_fault+44
__do_fault+76
handle_mm_fault+3543
do_user_addr_fault+442
do_page_fault+48
page_fault+62
]: 1161690
@[
down_read_trylock+1
shrink_slab+128
shrink_node+371
balance_pgdat+690
kswapd+389
kthread+246
ret_from_fork+31
]: 8424884
@[
down_read_trylock+1
shrink_slab+128
shrink_node+371
do_try_to_free_pages+232
try_to_free_pages+243
__alloc_pages_slowpath+771
__alloc_pages_nodemask+702
__do_page_cache_readahead+244
filemap_fault+1674
ext4_filemap_fault+44
__do_fault+76
handle_mm_fault+3543
do_user_addr_fault+442
do_page_fault+48
page_fault+62
]: 20917631
We can see that down_read_trylock() of shrinker_rwsem is being called with
high frequency at that time. Because of the poor multicore scalability of atomic
operations, this can lead to a significant drop in IPC (instructions per cycle).
And more, the shrinker_rwsem is a global read-write lock in shrinkers subsystem,
which protects most operations such as slab shrink, registration and
unregistration of shrinkers, etc. This can easily cause problems in the
following cases.
1) When the memory pressure is high and there are many filesystems mounted or
unmounted at the same time, slab shrink will be affected (down_read_trylock()
failed).
Such as the real workload mentioned by Kirill Tkhai:
```
One of the real workloads from my experience is start of an overcommitted
node containing many starting containers after node crash (or many resuming
containers after reboot for kernel update). In these cases memory pressure is
huge, and the node goes round in long reclaim.
```
2) If a shrinker is blocked (such as the case mentioned in [1]) and a writer
comes in (such as mount a fs), then this writer will be blocked and cause all
subsequent shrinker-related operations to be blocked.
[1]. https://lore.kernel.org/lkml/[email protected]/
All the above cases can be solved by replacing the shrinker_rwsem trylocks with
SRCU.
2. Survey
=========
Before doing the code implementation, I found that there were many similar
submissions in the community:
a. Davidlohr Bueso submitted a patch in 2015.
Subject: [PATCH -next v2] mm: srcu-ify shrinkers
Link: https://lore.kernel.org/all/[email protected]/
Result: It was finally merged into the linux-next branch, but failed on arm
allnoconfig (without CONFIG_SRCU)
b. Tetsuo Handa submitted a patchset in 2017.
Subject: [PATCH 1/2] mm,vmscan: Kill global shrinker lock.
Link: https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp/
Result: Finally chose to use the current simple way (break when
rwsem_is_contended()). And Christoph Hellwig suggested to using SRCU,
but SRCU was not unconditionally enabled at the time.
c. Kirill Tkhai submitted a patchset in 2018.
Subject: [PATCH RFC 00/10] Introduce lockless shrink_slab()
Link: https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/
Result: At that time, SRCU was not unconditionally enabled, and there were
some objections to enabling SRCU. Later, because Kirill's focus was
moved to other things, this patchset was not continued to be updated.
d. Sultan Alsawaf submitted a patch in 2021.
Subject: [PATCH] mm: vmscan: Replace shrinker_rwsem trylocks with SRCU protection
Link: https://lore.kernel.org/lkml/[email protected]/
Result: Rejected because SRCU was not unconditionally enabled.
We can find that almost all these historical commits were abandoned because SRCU
was not unconditionally enabled. But now SRCU has been unconditionally enable
by Paul E. McKenney in 2023 [2], so it's time to replace shrinker_rwsem trylocks
with SRCU.
[2] https://lore.kernel.org/lkml/20230105003759.GA1769545@paulmck-ThinkPad-P17-Gen-1/
3. Reproduction and testing
===========================
We can reproduce the down_read_trylock() hotspot through the following script:
```
#!/bin/bash
DIR="/root/shrinker/memcg/mnt"
do_create()
{
mkdir -p /sys/fs/cgroup/memory/test
mkdir -p /sys/fs/cgroup/perf_event/test
echo 4G > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
for i in `seq 0 $1`;
do
mkdir -p /sys/fs/cgroup/memory/test/$i;
echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs;
mkdir -p $DIR/$i;
done
}
do_mount()
{
for i in `seq $1 $2`;
do
mount -t tmpfs $i $DIR/$i;
done
}
do_touch()
{
for i in `seq $1 $2`;
do
echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs;
dd if=/dev/zero of=$DIR/$i/file$i bs=1M count=1 &
done
}
case "$1" in
touch)
do_touch $2 $3
;;
test)
do_create 4000
do_mount 0 4000
do_touch 0 3000
;;
*)
exit 1
;;
esac
```
Save the above script, then run test and touch commands. Then we can use the
following perf command to view hotspots:
perf top -U -F 999
1) Before applying this patchset:
32.31% [kernel] [k] down_read_trylock
19.40% [kernel] [k] pv_native_safe_halt
16.24% [kernel] [k] up_read
15.70% [kernel] [k] shrink_slab
4.69% [kernel] [k] _find_next_bit
2.62% [kernel] [k] shrink_node
1.78% [kernel] [k] shrink_lruvec
0.76% [kernel] [k] do_shrink_slab
2) After applying this patchset:
27.83% [kernel] [k] _find_next_bit
16.97% [kernel] [k] shrink_slab
15.82% [kernel] [k] pv_native_safe_halt
9.58% [kernel] [k] shrink_node
8.31% [kernel] [k] shrink_lruvec
5.64% [kernel] [k] do_shrink_slab
3.88% [kernel] [k] mem_cgroup_iter
At the same time, we use the following perf command to capture IPC information:
perf stat -e cycles,instructions -G test -a --repeat 5 -- sleep 10
1) Before applying this patchset:
Performance counter stats for 'system wide' (5 runs):
454187219766 cycles test ( +- 1.84% )
78896433101 instructions test # 0.17 insn per cycle ( +- 0.44% )
10.0020430 +- 0.0000366 seconds time elapsed ( +- 0.00% )
2) After applying this patchset:
Performance counter stats for 'system wide' (5 runs):
841954709443 cycles test ( +- 15.80% ) (98.69%)
527258677936 instructions test # 0.63 insn per cycle ( +- 15.11% ) (98.68%)
10.01064 +- 0.00831 seconds time elapsed ( +- 0.08% )
We can see that IPC drops very seriously when calling down_read_trylock() at
high frequency. After using SRCU, the IPC is at a normal level.
This series is based on next-20230306.
Comments and suggestions are welcome.
Thanks,
Qi.
Changelog in v4 -> v5:
- clean up [PATCH v4 1/8] (per Kirill)
- include linux/srcu.h in [PATCH v4 2/8] and [PATCH v4 5/8] (per Vlastimil)
- fix typo in the commit message of [PATCH v4 4/8] (per Vlastimil)
- add more explanation to the commit message of [PATCH v4 7/8]
- collect Acked-bys
Changelog in v3 -> v4:
- fix bug in [PATCH v3 1/7]
- revise commit messages
- rebase onto the next-20230306
Changelog in v2 -> v3:
- fix bug in [PATCH v2 1/7] (per Kirill)
- add Kirill's pacth which restore a check similar to the rwsem_is_contendent()
check by adding shrinker_srcu_generation
Changelog in v1 -> v2:
- add a map_nr_max field to shrinker_info (suggested by Kirill)
- use shrinker_mutex in reparent_shrinker_deferred() (pointed by Kirill)
Kirill Tkhai (1):
mm: vmscan: add shrinker_srcu_generation
Qi Zheng (7):
mm: vmscan: add a map_nr_max field to shrinker_info
mm: vmscan: make global slab shrink lockless
mm: vmscan: make memcg slab shrink lockless
mm: shrinkers: make count and scan in shrinker debugfs lockless
mm: vmscan: hold write lock to reparent shrinker nr_deferred
mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers()
mm: shrinkers: convert shrinker_rwsem to mutex
drivers/md/dm-cache-metadata.c | 2 +-
drivers/md/dm-thin-metadata.c | 2 +-
fs/super.c | 2 +-
include/linux/memcontrol.h | 1 +
mm/shrinker_debug.c | 39 ++++----
mm/vmscan.c | 160 ++++++++++++++++++---------------
6 files changed, 107 insertions(+), 99 deletions(-)
--
2.20.1
To prepare for the subsequent lockless memcg slab shrink,
add a map_nr_max field to struct shrinker_info to records
its own real shrinker_nr_max.
Signed-off-by: Qi Zheng <[email protected]>
Suggested-by: Kirill Tkhai <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
---
include/linux/memcontrol.h | 1 +
mm/vmscan.c | 35 ++++++++++++++++++-----------------
2 files changed, 19 insertions(+), 17 deletions(-)
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index b6eda2ab205d..aa69ea98e2d8 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -97,6 +97,7 @@ struct shrinker_info {
struct rcu_head rcu;
atomic_long_t *nr_deferred;
unsigned long *map;
+ int map_nr_max;
};
struct lruvec_stats_percpu {
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9414226218f0..9a2a6301052c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -226,7 +226,8 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
static int expand_one_shrinker_info(struct mem_cgroup *memcg,
int map_size, int defer_size,
- int old_map_size, int old_defer_size)
+ int old_map_size, int old_defer_size,
+ int new_nr_max)
{
struct shrinker_info *new, *old;
struct mem_cgroup_per_node *pn;
@@ -240,12 +241,17 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
if (!old)
return 0;
+ /* Already expanded this shrinker_info */
+ if (new_nr_max <= old->map_nr_max)
+ continue;
+
new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
if (!new)
return -ENOMEM;
new->nr_deferred = (atomic_long_t *)(new + 1);
new->map = (void *)new->nr_deferred + defer_size;
+ new->map_nr_max = new_nr_max;
/* map: set all old bits, clear all new bits */
memset(new->map, (int)0xff, old_map_size);
@@ -295,6 +301,7 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
}
info->nr_deferred = (atomic_long_t *)(info + 1);
info->map = (void *)info->nr_deferred + defer_size;
+ info->map_nr_max = shrinker_nr_max;
rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
}
up_write(&shrinker_rwsem);
@@ -302,23 +309,14 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
return ret;
}
-static inline bool need_expand(int nr_max)
-{
- return round_up(nr_max, BITS_PER_LONG) >
- round_up(shrinker_nr_max, BITS_PER_LONG);
-}
-
static int expand_shrinker_info(int new_id)
{
int ret = 0;
- int new_nr_max = new_id + 1;
+ int new_nr_max = round_up(new_id + 1, BITS_PER_LONG);
int map_size, defer_size = 0;
int old_map_size, old_defer_size = 0;
struct mem_cgroup *memcg;
- if (!need_expand(new_nr_max))
- goto out;
-
if (!root_mem_cgroup)
goto out;
@@ -332,7 +330,8 @@ static int expand_shrinker_info(int new_id)
memcg = mem_cgroup_iter(NULL, NULL, NULL);
do {
ret = expand_one_shrinker_info(memcg, map_size, defer_size,
- old_map_size, old_defer_size);
+ old_map_size, old_defer_size,
+ new_nr_max);
if (ret) {
mem_cgroup_iter_break(NULL, memcg);
goto out;
@@ -352,9 +351,11 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
rcu_read_lock();
info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
- /* Pairs with smp mb in shrink_slab() */
- smp_mb__before_atomic();
- set_bit(shrinker_id, info->map);
+ if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) {
+ /* Pairs with smp mb in shrink_slab() */
+ smp_mb__before_atomic();
+ set_bit(shrinker_id, info->map);
+ }
rcu_read_unlock();
}
}
@@ -432,7 +433,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
for_each_node(nid) {
child_info = shrinker_info_protected(memcg, nid);
parent_info = shrinker_info_protected(parent, nid);
- for (i = 0; i < shrinker_nr_max; i++) {
+ for (i = 0; i < child_info->map_nr_max; i++) {
nr = atomic_long_read(&child_info->nr_deferred[i]);
atomic_long_add(nr, &parent_info->nr_deferred[i]);
}
@@ -899,7 +900,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
if (unlikely(!info))
goto unlock;
- for_each_set_bit(i, info->map, shrinker_nr_max) {
+ for_each_set_bit(i, info->map, info->map_nr_max) {
struct shrink_control sc = {
.gfp_mask = gfp_mask,
.nid = nid,
--
2.20.1
Like global and memcg slab shrink, also use SRCU to
make count and scan operations in memory shrinker
debugfs lockless.
Signed-off-by: Qi Zheng <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Kirill Tkhai <[email protected]>
---
mm/shrinker_debug.c | 25 ++++++++-----------------
1 file changed, 8 insertions(+), 17 deletions(-)
diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index 39c3491e28a3..37d54d037495 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -5,10 +5,12 @@
#include <linux/seq_file.h>
#include <linux/shrinker.h>
#include <linux/memcontrol.h>
+#include <linux/srcu.h>
/* defined in vmscan.c */
extern struct rw_semaphore shrinker_rwsem;
extern struct list_head shrinker_list;
+extern struct srcu_struct shrinker_srcu;
static DEFINE_IDA(shrinker_debugfs_ida);
static struct dentry *shrinker_debugfs_root;
@@ -49,18 +51,13 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
struct mem_cgroup *memcg;
unsigned long total;
bool memcg_aware;
- int ret, nid;
+ int ret = 0, nid, srcu_idx;
count_per_node = kcalloc(nr_node_ids, sizeof(unsigned long), GFP_KERNEL);
if (!count_per_node)
return -ENOMEM;
- ret = down_read_killable(&shrinker_rwsem);
- if (ret) {
- kfree(count_per_node);
- return ret;
- }
- rcu_read_lock();
+ srcu_idx = srcu_read_lock(&shrinker_srcu);
memcg_aware = shrinker->flags & SHRINKER_MEMCG_AWARE;
@@ -91,8 +88,7 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
}
} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
- rcu_read_unlock();
- up_read(&shrinker_rwsem);
+ srcu_read_unlock(&shrinker_srcu, srcu_idx);
kfree(count_per_node);
return ret;
@@ -115,9 +111,8 @@ static ssize_t shrinker_debugfs_scan_write(struct file *file,
.gfp_mask = GFP_KERNEL,
};
struct mem_cgroup *memcg = NULL;
- int nid;
+ int nid, srcu_idx;
char kbuf[72];
- ssize_t ret;
read_len = size < (sizeof(kbuf) - 1) ? size : (sizeof(kbuf) - 1);
if (copy_from_user(kbuf, buf, read_len))
@@ -146,11 +141,7 @@ static ssize_t shrinker_debugfs_scan_write(struct file *file,
return -EINVAL;
}
- ret = down_read_killable(&shrinker_rwsem);
- if (ret) {
- mem_cgroup_put(memcg);
- return ret;
- }
+ srcu_idx = srcu_read_lock(&shrinker_srcu);
sc.nid = nid;
sc.memcg = memcg;
@@ -159,7 +150,7 @@ static ssize_t shrinker_debugfs_scan_write(struct file *file,
shrinker->scan_objects(shrinker, &sc);
- up_read(&shrinker_rwsem);
+ srcu_read_unlock(&shrinker_srcu, srcu_idx);
mem_cgroup_put(memcg);
return size;
--
2.20.1
Like global slab shrink, this commit also uses SRCU to make
memcg slab shrink lockless.
We can reproduce the down_read_trylock() hotspot through the
following script:
```
DIR="/root/shrinker/memcg/mnt"
do_create()
{
mkdir -p /sys/fs/cgroup/memory/test
mkdir -p /sys/fs/cgroup/perf_event/test
echo 4G > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
for i in `seq 0 $1`;
do
mkdir -p /sys/fs/cgroup/memory/test/$i;
echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs;
mkdir -p $DIR/$i;
done
}
do_mount()
{
for i in `seq $1 $2`;
do
mount -t tmpfs $i $DIR/$i;
done
}
do_touch()
{
for i in `seq $1 $2`;
do
echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs;
dd if=/dev/zero of=$DIR/$i/file$i bs=1M count=1 &
done
}
case "$1" in
touch)
do_touch $2 $3
;;
test)
do_create 4000
do_mount 0 4000
do_touch 0 3000
;;
*)
exit 1
;;
esac
```
Save the above script, then run test and touch commands.
Then we can use the following perf command to view hotspots:
perf top -U -F 999
1) Before applying this patchset:
32.31% [kernel] [k] down_read_trylock
19.40% [kernel] [k] pv_native_safe_halt
16.24% [kernel] [k] up_read
15.70% [kernel] [k] shrink_slab
4.69% [kernel] [k] _find_next_bit
2.62% [kernel] [k] shrink_node
1.78% [kernel] [k] shrink_lruvec
0.76% [kernel] [k] do_shrink_slab
2) After applying this patchset:
27.83% [kernel] [k] _find_next_bit
16.97% [kernel] [k] shrink_slab
15.82% [kernel] [k] pv_native_safe_halt
9.58% [kernel] [k] shrink_node
8.31% [kernel] [k] shrink_lruvec
5.64% [kernel] [k] do_shrink_slab
3.88% [kernel] [k] mem_cgroup_iter
At the same time, we use the following perf command to capture
IPC information:
perf stat -e cycles,instructions -G test -a --repeat 5 -- sleep 10
1) Before applying this patchset:
Performance counter stats for 'system wide' (5 runs):
454187219766 cycles test ( +- 1.84% )
78896433101 instructions test # 0.17 insn per cycle ( +- 0.44% )
10.0020430 +- 0.0000366 seconds time elapsed ( +- 0.00% )
2) After applying this patchset:
Performance counter stats for 'system wide' (5 runs):
841954709443 cycles test ( +- 15.80% ) (98.69%)
527258677936 instructions test # 0.63 insn per cycle ( +- 15.11% ) (98.68%)
10.01064 +- 0.00831 seconds time elapsed ( +- 0.08% )
We can see that IPC drops very seriously when calling
down_read_trylock() at high frequency. After using SRCU,
the IPC is at a normal level.
Signed-off-by: Qi Zheng <[email protected]>
Acked-by: Kirill Tkhai <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
---
mm/vmscan.c | 45 ++++++++++++++++++++++++++-------------------
1 file changed, 26 insertions(+), 19 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index db2ed6e08f67..ce7834030f75 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -222,8 +222,21 @@ static inline int shrinker_defer_size(int nr_items)
static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
int nid)
{
- return rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
- lockdep_is_held(&shrinker_rwsem));
+ return srcu_dereference_check(memcg->nodeinfo[nid]->shrinker_info,
+ &shrinker_srcu,
+ lockdep_is_held(&shrinker_rwsem));
+}
+
+static struct shrinker_info *shrinker_info_srcu(struct mem_cgroup *memcg,
+ int nid)
+{
+ return srcu_dereference(memcg->nodeinfo[nid]->shrinker_info,
+ &shrinker_srcu);
+}
+
+static void free_shrinker_info_rcu(struct rcu_head *head)
+{
+ kvfree(container_of(head, struct shrinker_info, rcu));
}
static int expand_one_shrinker_info(struct mem_cgroup *memcg,
@@ -264,7 +277,7 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
defer_size - old_defer_size);
rcu_assign_pointer(pn->shrinker_info, new);
- kvfree_rcu(old, rcu);
+ call_srcu(&shrinker_srcu, &old->rcu, free_shrinker_info_rcu);
}
return 0;
@@ -350,15 +363,16 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
{
if (shrinker_id >= 0 && memcg && !mem_cgroup_is_root(memcg)) {
struct shrinker_info *info;
+ int srcu_idx;
- rcu_read_lock();
- info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
+ srcu_idx = srcu_read_lock(&shrinker_srcu);
+ info = shrinker_info_srcu(memcg, nid);
if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) {
/* Pairs with smp mb in shrink_slab() */
smp_mb__before_atomic();
set_bit(shrinker_id, info->map);
}
- rcu_read_unlock();
+ srcu_read_unlock(&shrinker_srcu, srcu_idx);
}
}
@@ -372,7 +386,6 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
return -ENOSYS;
down_write(&shrinker_rwsem);
- /* This may call shrinker, so it must use down_read_trylock() */
id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
if (id < 0)
goto unlock;
@@ -406,7 +419,7 @@ static long xchg_nr_deferred_memcg(int nid, struct shrinker *shrinker,
{
struct shrinker_info *info;
- info = shrinker_info_protected(memcg, nid);
+ info = shrinker_info_srcu(memcg, nid);
return atomic_long_xchg(&info->nr_deferred[shrinker->id], 0);
}
@@ -415,7 +428,7 @@ static long add_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker,
{
struct shrinker_info *info;
- info = shrinker_info_protected(memcg, nid);
+ info = shrinker_info_srcu(memcg, nid);
return atomic_long_add_return(nr, &info->nr_deferred[shrinker->id]);
}
@@ -893,15 +906,14 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
{
struct shrinker_info *info;
unsigned long ret, freed = 0;
+ int srcu_idx;
int i;
if (!mem_cgroup_online(memcg))
return 0;
- if (!down_read_trylock(&shrinker_rwsem))
- return 0;
-
- info = shrinker_info_protected(memcg, nid);
+ srcu_idx = srcu_read_lock(&shrinker_srcu);
+ info = shrinker_info_srcu(memcg, nid);
if (unlikely(!info))
goto unlock;
@@ -951,14 +963,9 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
set_shrinker_bit(memcg, nid, i);
}
freed += ret;
-
- if (rwsem_is_contended(&shrinker_rwsem)) {
- freed = freed ? : 1;
- break;
- }
}
unlock:
- up_read(&shrinker_rwsem);
+ srcu_read_unlock(&shrinker_srcu, srcu_idx);
return freed;
}
#else /* CONFIG_MEMCG */
--
2.20.1
For now, reparent_shrinker_deferred() is the only holder
of read lock of shrinker_rwsem. And it already holds the
global cgroup_mutex, so it will not be called in parallel.
Therefore, in order to convert shrinker_rwsem to shrinker_mutex
later, here we change to hold the write lock of shrinker_rwsem
to reparent.
Signed-off-by: Qi Zheng <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Kirill Tkhai <[email protected]>
---
mm/vmscan.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5c2a22454320..8c1ae7ea8dea 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -445,7 +445,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
parent = root_mem_cgroup;
/* Prevent from concurrent shrinker_info expand */
- down_read(&shrinker_rwsem);
+ down_write(&shrinker_rwsem);
for_each_node(nid) {
child_info = shrinker_info_protected(memcg, nid);
parent_info = shrinker_info_protected(parent, nid);
@@ -454,7 +454,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
atomic_long_add(nr, &parent_info->nr_deferred[i]);
}
}
- up_read(&shrinker_rwsem);
+ up_write(&shrinker_rwsem);
}
static bool cgroup_reclaim(struct scan_control *sc)
--
2.20.1
From: Kirill Tkhai <[email protected]>
After we make slab shrink lockless with SRCU, the longest
sleep unregister_shrinker() will be a sleep waiting for
all do_shrink_slab() calls.
To avoid long unbreakable action in the unregister_shrinker(),
add shrinker_srcu_generation to restore a check similar to the
rwsem_is_contendent() check that we had before.
And for memcg slab shrink, we unlock SRCU and continue
iterations from the next shrinker id.
Signed-off-by: Kirill Tkhai <[email protected]>
Signed-off-by: Qi Zheng <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
---
mm/vmscan.c | 24 ++++++++++++++++++++----
1 file changed, 20 insertions(+), 4 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index ce7834030f75..5c2a22454320 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -204,6 +204,7 @@ static void set_task_reclaim_state(struct task_struct *task,
LIST_HEAD(shrinker_list);
DECLARE_RWSEM(shrinker_rwsem);
DEFINE_SRCU(shrinker_srcu);
+static atomic_t shrinker_srcu_generation = ATOMIC_INIT(0);
#ifdef CONFIG_MEMCG
static int shrinker_nr_max;
@@ -776,6 +777,7 @@ void unregister_shrinker(struct shrinker *shrinker)
debugfs_entry = shrinker_debugfs_remove(shrinker);
up_write(&shrinker_rwsem);
+ atomic_inc(&shrinker_srcu_generation);
synchronize_srcu(&shrinker_srcu);
debugfs_remove_recursive(debugfs_entry);
@@ -797,6 +799,7 @@ void synchronize_shrinkers(void)
{
down_write(&shrinker_rwsem);
up_write(&shrinker_rwsem);
+ atomic_inc(&shrinker_srcu_generation);
synchronize_srcu(&shrinker_srcu);
}
EXPORT_SYMBOL(synchronize_shrinkers);
@@ -906,18 +909,20 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
{
struct shrinker_info *info;
unsigned long ret, freed = 0;
- int srcu_idx;
- int i;
+ int srcu_idx, generation;
+ int i = 0;
if (!mem_cgroup_online(memcg))
return 0;
+again:
srcu_idx = srcu_read_lock(&shrinker_srcu);
info = shrinker_info_srcu(memcg, nid);
if (unlikely(!info))
goto unlock;
- for_each_set_bit(i, info->map, info->map_nr_max) {
+ generation = atomic_read(&shrinker_srcu_generation);
+ for_each_set_bit_from(i, info->map, info->map_nr_max) {
struct shrink_control sc = {
.gfp_mask = gfp_mask,
.nid = nid,
@@ -963,6 +968,11 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
set_shrinker_bit(memcg, nid, i);
}
freed += ret;
+ if (atomic_read(&shrinker_srcu_generation) != generation) {
+ srcu_read_unlock(&shrinker_srcu, srcu_idx);
+ i++;
+ goto again;
+ }
}
unlock:
srcu_read_unlock(&shrinker_srcu, srcu_idx);
@@ -1002,7 +1012,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
{
unsigned long ret, freed = 0;
struct shrinker *shrinker;
- int srcu_idx;
+ int srcu_idx, generation;
/*
* The root memcg might be allocated even though memcg is disabled
@@ -1016,6 +1026,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
srcu_idx = srcu_read_lock(&shrinker_srcu);
+ generation = atomic_read(&shrinker_srcu_generation);
list_for_each_entry_srcu(shrinker, &shrinker_list, list,
srcu_read_lock_held(&shrinker_srcu)) {
struct shrink_control sc = {
@@ -1028,6 +1039,11 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
if (ret == SHRINK_EMPTY)
ret = 0;
freed += ret;
+
+ if (atomic_read(&shrinker_srcu_generation) != generation) {
+ freed = freed ? : 1;
+ break;
+ }
}
srcu_read_unlock(&shrinker_srcu, srcu_idx);
--
2.20.1
Currently, the synchronize_shrinkers() is only used by TTM
pool. It only requires that no shrinkers run in parallel,
and doesn't care about registering and unregistering of
shrinkers.
Since slab shrink is protected by SRCU, synchronize_srcu()
is sufficient to ensure that no shrinker is running in
parallel. So the shrinker_rwsem in synchronize_shrinkers()
is no longer needed, just remove it.
Signed-off-by: Qi Zheng <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Kirill Tkhai <[email protected]>
---
mm/vmscan.c | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8c1ae7ea8dea..2b22a42d83c4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -790,15 +790,11 @@ EXPORT_SYMBOL(unregister_shrinker);
/**
* synchronize_shrinkers - Wait for all running shrinkers to complete.
*
- * This is equivalent to calling unregister_shrink() and register_shrinker(),
- * but atomically and with less overhead. This is useful to guarantee that all
- * shrinker invocations have seen an update, before freeing memory, similar to
- * rcu.
+ * This is useful to guarantee that all shrinker invocations have seen an
+ * update, before freeing memory.
*/
void synchronize_shrinkers(void)
{
- down_write(&shrinker_rwsem);
- up_write(&shrinker_rwsem);
atomic_inc(&shrinker_srcu_generation);
synchronize_srcu(&shrinker_srcu);
}
--
2.20.1
Now there are no readers of shrinker_rwsem, so we
can simply replace it with mutex lock.
Signed-off-by: Qi Zheng <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Kirill Tkhai <[email protected]>
---
drivers/md/dm-cache-metadata.c | 2 +-
drivers/md/dm-thin-metadata.c | 2 +-
fs/super.c | 2 +-
mm/shrinker_debug.c | 14 +++++++-------
mm/vmscan.c | 34 +++++++++++++++++-----------------
5 files changed, 27 insertions(+), 27 deletions(-)
diff --git a/drivers/md/dm-cache-metadata.c b/drivers/md/dm-cache-metadata.c
index acffed750e3e..9e0c69958587 100644
--- a/drivers/md/dm-cache-metadata.c
+++ b/drivers/md/dm-cache-metadata.c
@@ -1828,7 +1828,7 @@ int dm_cache_metadata_abort(struct dm_cache_metadata *cmd)
* Replacement block manager (new_bm) is created and old_bm destroyed outside of
* cmd root_lock to avoid ABBA deadlock that would result (due to life-cycle of
* shrinker associated with the block manager's bufio client vs cmd root_lock).
- * - must take shrinker_rwsem without holding cmd->root_lock
+ * - must take shrinker_mutex without holding cmd->root_lock
*/
new_bm = dm_block_manager_create(cmd->bdev, DM_CACHE_METADATA_BLOCK_SIZE << SECTOR_SHIFT,
CACHE_MAX_CONCURRENT_LOCKS);
diff --git a/drivers/md/dm-thin-metadata.c b/drivers/md/dm-thin-metadata.c
index fd464fb024c3..9f5cb52c5763 100644
--- a/drivers/md/dm-thin-metadata.c
+++ b/drivers/md/dm-thin-metadata.c
@@ -1887,7 +1887,7 @@ int dm_pool_abort_metadata(struct dm_pool_metadata *pmd)
* Replacement block manager (new_bm) is created and old_bm destroyed outside of
* pmd root_lock to avoid ABBA deadlock that would result (due to life-cycle of
* shrinker associated with the block manager's bufio client vs pmd root_lock).
- * - must take shrinker_rwsem without holding pmd->root_lock
+ * - must take shrinker_mutex without holding pmd->root_lock
*/
new_bm = dm_block_manager_create(pmd->bdev, THIN_METADATA_BLOCK_SIZE << SECTOR_SHIFT,
THIN_MAX_CONCURRENT_LOCKS);
diff --git a/fs/super.c b/fs/super.c
index 84332d5cb817..91a4037b1d95 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -54,7 +54,7 @@ static char *sb_writers_name[SB_FREEZE_LEVELS] = {
* One thing we have to be careful of with a per-sb shrinker is that we don't
* drop the last active reference to the superblock from within the shrinker.
* If that happens we could trigger unregistering the shrinker from within the
- * shrinker path and that leads to deadlock on the shrinker_rwsem. Hence we
+ * shrinker path and that leads to deadlock on the shrinker_mutex. Hence we
* take a passive reference to the superblock to avoid this from occurring.
*/
static unsigned long super_cache_scan(struct shrinker *shrink,
diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index 37d54d037495..fdd155fd35ed 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -8,7 +8,7 @@
#include <linux/srcu.h>
/* defined in vmscan.c */
-extern struct rw_semaphore shrinker_rwsem;
+extern struct mutex shrinker_mutex;
extern struct list_head shrinker_list;
extern struct srcu_struct shrinker_srcu;
@@ -168,7 +168,7 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
char buf[128];
int id;
- lockdep_assert_held(&shrinker_rwsem);
+ lockdep_assert_held(&shrinker_mutex);
/* debugfs isn't initialized yet, add debugfs entries later. */
if (!shrinker_debugfs_root)
@@ -211,7 +211,7 @@ int shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...)
if (!new)
return -ENOMEM;
- down_write(&shrinker_rwsem);
+ mutex_lock(&shrinker_mutex);
old = shrinker->name;
shrinker->name = new;
@@ -229,7 +229,7 @@ int shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...)
shrinker->debugfs_entry = entry;
}
- up_write(&shrinker_rwsem);
+ mutex_unlock(&shrinker_mutex);
kfree_const(old);
@@ -241,7 +241,7 @@ struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker)
{
struct dentry *entry = shrinker->debugfs_entry;
- lockdep_assert_held(&shrinker_rwsem);
+ lockdep_assert_held(&shrinker_mutex);
kfree_const(shrinker->name);
shrinker->name = NULL;
@@ -266,14 +266,14 @@ static int __init shrinker_debugfs_init(void)
shrinker_debugfs_root = dentry;
/* Create debugfs entries for shrinkers registered at boot */
- down_write(&shrinker_rwsem);
+ mutex_lock(&shrinker_mutex);
list_for_each_entry(shrinker, &shrinker_list, list)
if (!shrinker->debugfs_entry) {
ret = shrinker_debugfs_add(shrinker);
if (ret)
break;
}
- up_write(&shrinker_rwsem);
+ mutex_unlock(&shrinker_mutex);
return ret;
}
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2b22a42d83c4..8faac4310cb5 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -35,7 +35,7 @@
#include <linux/cpuset.h>
#include <linux/compaction.h>
#include <linux/notifier.h>
-#include <linux/rwsem.h>
+#include <linux/mutex.h>
#include <linux/delay.h>
#include <linux/kthread.h>
#include <linux/freezer.h>
@@ -202,7 +202,7 @@ static void set_task_reclaim_state(struct task_struct *task,
}
LIST_HEAD(shrinker_list);
-DECLARE_RWSEM(shrinker_rwsem);
+DEFINE_MUTEX(shrinker_mutex);
DEFINE_SRCU(shrinker_srcu);
static atomic_t shrinker_srcu_generation = ATOMIC_INIT(0);
@@ -225,7 +225,7 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
{
return srcu_dereference_check(memcg->nodeinfo[nid]->shrinker_info,
&shrinker_srcu,
- lockdep_is_held(&shrinker_rwsem));
+ lockdep_is_held(&shrinker_mutex));
}
static struct shrinker_info *shrinker_info_srcu(struct mem_cgroup *memcg,
@@ -304,7 +304,7 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
int nid, size, ret = 0;
int map_size, defer_size = 0;
- down_write(&shrinker_rwsem);
+ mutex_lock(&shrinker_mutex);
map_size = shrinker_map_size(shrinker_nr_max);
defer_size = shrinker_defer_size(shrinker_nr_max);
size = map_size + defer_size;
@@ -320,7 +320,7 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
info->map_nr_max = shrinker_nr_max;
rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
}
- up_write(&shrinker_rwsem);
+ mutex_unlock(&shrinker_mutex);
return ret;
}
@@ -336,7 +336,7 @@ static int expand_shrinker_info(int new_id)
if (!root_mem_cgroup)
goto out;
- lockdep_assert_held(&shrinker_rwsem);
+ lockdep_assert_held(&shrinker_mutex);
map_size = shrinker_map_size(new_nr_max);
defer_size = shrinker_defer_size(new_nr_max);
@@ -386,7 +386,7 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
if (mem_cgroup_disabled())
return -ENOSYS;
- down_write(&shrinker_rwsem);
+ mutex_lock(&shrinker_mutex);
id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
if (id < 0)
goto unlock;
@@ -400,7 +400,7 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
shrinker->id = id;
ret = 0;
unlock:
- up_write(&shrinker_rwsem);
+ mutex_unlock(&shrinker_mutex);
return ret;
}
@@ -410,7 +410,7 @@ static void unregister_memcg_shrinker(struct shrinker *shrinker)
BUG_ON(id < 0);
- lockdep_assert_held(&shrinker_rwsem);
+ lockdep_assert_held(&shrinker_mutex);
idr_remove(&shrinker_idr, id);
}
@@ -445,7 +445,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
parent = root_mem_cgroup;
/* Prevent from concurrent shrinker_info expand */
- down_write(&shrinker_rwsem);
+ mutex_lock(&shrinker_mutex);
for_each_node(nid) {
child_info = shrinker_info_protected(memcg, nid);
parent_info = shrinker_info_protected(parent, nid);
@@ -454,7 +454,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
atomic_long_add(nr, &parent_info->nr_deferred[i]);
}
}
- up_write(&shrinker_rwsem);
+ mutex_unlock(&shrinker_mutex);
}
static bool cgroup_reclaim(struct scan_control *sc)
@@ -703,9 +703,9 @@ void free_prealloced_shrinker(struct shrinker *shrinker)
shrinker->name = NULL;
#endif
if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
- down_write(&shrinker_rwsem);
+ mutex_lock(&shrinker_mutex);
unregister_memcg_shrinker(shrinker);
- up_write(&shrinker_rwsem);
+ mutex_unlock(&shrinker_mutex);
return;
}
@@ -715,11 +715,11 @@ void free_prealloced_shrinker(struct shrinker *shrinker)
void register_shrinker_prepared(struct shrinker *shrinker)
{
- down_write(&shrinker_rwsem);
+ mutex_lock(&shrinker_mutex);
list_add_tail_rcu(&shrinker->list, &shrinker_list);
shrinker->flags |= SHRINKER_REGISTERED;
shrinker_debugfs_add(shrinker);
- up_write(&shrinker_rwsem);
+ mutex_unlock(&shrinker_mutex);
}
static int __register_shrinker(struct shrinker *shrinker)
@@ -769,13 +769,13 @@ void unregister_shrinker(struct shrinker *shrinker)
if (!(shrinker->flags & SHRINKER_REGISTERED))
return;
- down_write(&shrinker_rwsem);
+ mutex_lock(&shrinker_mutex);
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);
+ mutex_unlock(&shrinker_mutex);
atomic_inc(&shrinker_srcu_generation);
synchronize_srcu(&shrinker_srcu);
--
2.20.1
On Mon, Mar 13, 2023 at 07:28:12PM +0800, Qi Zheng wrote:
> To prepare for the subsequent lockless memcg slab shrink,
> add a map_nr_max field to struct shrinker_info to records
> its own real shrinker_nr_max.
>
> Signed-off-by: Qi Zheng <[email protected]>
> Suggested-by: Kirill Tkhai <[email protected]>
> Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
, for the whole series.
Thank you!