+ mm-vmscan-make-memcg-slab-shrink-lockless.patch added to mm-unstable branch

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The patch titled
     Subject: mm: vmscan: make memcg slab shrink lockless
has been added to the -mm mm-unstable branch.  Its filename is
     mm-vmscan-make-memcg-slab-shrink-lockless.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-vmscan-make-memcg-slab-shrink-lockless.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days

------------------------------------------------------
From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Subject: mm: vmscan: make memcg slab shrink lockless
Date: Mon, 13 Mar 2023 19:28:14 +0800

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.

Link: https://lkml.kernel.org/r/20230313112819.38938-4-zhengqi.arch@xxxxxxxxxxxxx
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Acked-by: Kirill Tkhai <tkhai@xxxxx>
Acked-by: Vlastimil Babka <Vbabka@xxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---


--- a/mm/vmscan.c~mm-vmscan-make-memcg-slab-shrink-lockless
+++ a/mm/vmscan.c
@@ -222,8 +222,21 @@ static inline int shrinker_defer_size(in
 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(stru
 		       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
 {
 	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(struc
 		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 n
 {
 	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 n
 {
 	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(g
 {
 	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(g
 				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 */
_

Patches currently in -mm which might be from zhengqi.arch@xxxxxxxxxxxxx are

mm-vmscan-add-a-map_nr_max-field-to-shrinker_info.patch
mm-vmscan-make-global-slab-shrink-lockless.patch
mm-vmscan-make-memcg-slab-shrink-lockless.patch
mm-shrinkers-make-count-and-scan-in-shrinker-debugfs-lockless.patch
mm-vmscan-hold-write-lock-to-reparent-shrinker-nr_deferred.patch
mm-vmscan-remove-shrinker_rwsem-from-synchronize_shrinkers.patch
mm-shrinkers-convert-shrinker_rwsem-to-mutex.patch




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