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Date: Thu, 9 Mar 2023 01:23:55 +0300
From: Kirill Tkhai <tkhai@...ru>
To: Qi Zheng <zhengqi.arch@...edance.com>, akpm@...ux-foundation.org,
hannes@...xchg.org, shakeelb@...gle.com, mhocko@...nel.org,
roman.gushchin@...ux.dev, muchun.song@...ux.dev, david@...hat.com,
shy828301@...il.com, rppt@...nel.org
Cc: sultan@...neltoast.com, dave@...olabs.net,
penguin-kernel@...ove.SAKURA.ne.jp, paulmck@...nel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/8] mm: vmscan: make memcg slab shrink lockless
On 07.03.2023 09:56, Qi Zheng wrote:
> 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 <zhengqi.arch@...edance.com>
Acked-by: Kirill Tkhai <tkhai@...ru>
> ---
> mm/vmscan.c | 46 +++++++++++++++++++++++++++-------------------
> 1 file changed, 27 insertions(+), 19 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 8515ac40bcaf..1de9bc3e5aa2 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -57,6 +57,7 @@
> #include <linux/khugepaged.h>
> #include <linux/rculist_nulls.h>
> #include <linux/random.h>
> +#include <linux/srcu.h>
>
> #include <asm/tlbflush.h>
> #include <asm/div64.h>
> @@ -221,8 +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 inline bool need_expand(int new_nr_max, int old_nr_max)
> @@ -269,7 +283,7 @@ 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;
> @@ -355,15 +369,16 @@ 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);
> }
> }
>
> @@ -377,7 +392,6 @@ 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;
> @@ -411,7 +425,7 @@ 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);
> }
>
> @@ -420,7 +434,7 @@ 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]);
> }
>
> @@ -898,15 +912,14 @@ 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;
>
> @@ -956,14 +969,9 @@ 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 */
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