[PATCH 4/4] mm, swap_cgroup: remove global swap cgroup lock

Kairui Song
2024-12-02T18:42:29Z

From: Kairui Song <kasong@tencent.com>

commit e9e58a4ec3b1 ("memcg: avoid use cmpxchg in swap cgroup maintainance")
replaced the cmpxchg/xchg with a global irq spinlock because some archs
doesn't support 2 bytes cmpxchg/xchg. Clearly this won't scale well.

And as commented in swap_cgroup.c, this lock is not needed for map
synchronization.

Emulation of 2 bytes cmpxchg/xchg with atomic isn't hard, so implement
it to get rid of this lock.

Testing using 64G brd and build with build kernel with make -j96 in 1.5G
memory cgroup using 4k folios showed below improvement (10 test run):

Before this series:
Sys time: 10730.08 (stdev 49.030728)
Real time: 171.03 (stdev 0.850355)

After this commit:
Sys time: 9612.24 (stdev 66.310789), -10.42%
Real time: 159.78 (stdev 0.577193), -6.57%

With 64k folios and 2G memcg:
Before this series:
Sys time: 7626.77 (stdev 43.545517)
Real time: 136.22 (stdev 1.265544)

After this commit:
Sys time: 6936.03 (stdev 39.996280), -9.06%
Real time: 129.65 (stdev 0.880039), -4.82%

Sequential swapout of 8G 4k zero folios (24 test run):
Before this series:
5461409.12 us (stdev 183957.827084)

After this commit:
5420447.26 us (stdev 196419.240317)

Sequential swapin of 8G 4k zero folios (24 test run):
Before this series:
19736958.916667 us (stdev 189027.246676)

After this commit:
19662182.629630 us (stdev 172717.640614)

Performance is better or at least not worse for all tests above.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
mm/swap_cgroup.c | 56 +++++++++++++++++++++++++++++++++++-------------
1 file changed, 41 insertions(+), 15 deletions(-)

diff --git a/mm/swap_cgroup.c b/mm/swap_cgroup.c
index a76afdc3666a..028f5e6be3f0 100644
--- a/mm/swap_cgroup.c
+++ b/mm/swap_cgroup.c
@@ -5,6 +5,15 @@

#include <linux/swapops.h> /* depends on mm.h include */

+#define ID_PER_UNIT (sizeof(atomic_t) / sizeof(unsigned short))
+struct swap_cgroup_unit {
+ union {
+ int raw;
+ atomic_t val;
+ unsigned short __id[ID_PER_UNIT];
+ };
+};
+
static DEFINE_MUTEX(swap_cgroup_mutex);

struct swap_cgroup {
@@ -12,8 +21,10 @@ struct swap_cgroup {
};

struct swap_cgroup_ctrl {
- unsigned short *map;
- spinlock_t lock;
+ union {
+ struct swap_cgroup_unit *units;
+ unsigned short *map;
+ };
};

static struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
@@ -31,6 +42,24 @@ static struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
*
* TODO: we can push these buffers out to HIGHMEM.
*/
+static unsigned short __swap_cgroup_xchg(void *map,
+ pgoff_t offset,
+ unsigned int new_id)
+{
+ unsigned int old_id;
+ struct swap_cgroup_unit *units = map;
+ struct swap_cgroup_unit *unit = &units[offset / ID_PER_UNIT];
+ struct swap_cgroup_unit new, old = { .raw = atomic_read(&unit->val) };
+
+ do {
+ new.raw = old.raw;
+ old_id = old.__id[offset % ID_PER_UNIT];
+ new.__id[offset % ID_PER_UNIT] = new_id;
+ } while (!atomic_try_cmpxchg(&unit->val, &old.raw, new.raw));
+
+ return old_id;
+}
+
/**
* swap_cgroup_record - record mem_cgroup for a set of swap entries
* @ent: the first swap entry to be recorded into
@@ -44,22 +73,19 @@ unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id,
unsigned int nr_ents)
{
struct swap_cgroup_ctrl *ctrl;
- unsigned short *map;
- unsigned short old;
- unsigned long flags;
pgoff_t offset = swp_offset(ent);
pgoff_t end = offset + nr_ents;
+ unsigned short old, iter;
+ unsigned short *map;

ctrl = &swap_cgroup_ctrl[swp_type(ent)];
map = ctrl->map;

- spin_lock_irqsave(&ctrl->lock, flags);
- old = map[offset];
+ old = READ_ONCE(map[offset]);
do {
- VM_BUG_ON(map[offset] != old);
- map[offset] = id;
+ iter = __swap_cgroup_xchg(map, offset, id);
+ VM_BUG_ON(iter != old);
} while (++offset != end);
- spin_unlock_irqrestore(&ctrl->lock, flags);

return old;
}
@@ -85,20 +111,20 @@ unsigned short lookup_swap_cgroup_id(swp_entry_t ent)

int swap_cgroup_swapon(int type, unsigned long max_pages)
{
- void *map;
+ struct swap_cgroup_unit *units;
struct swap_cgroup_ctrl *ctrl;

if (mem_cgroup_disabled())
return 0;

- map = vzalloc(max_pages * sizeof(unsigned short));
- if (!map)
+ units = vzalloc(DIV_ROUND_UP(max_pages, ID_PER_UNIT) *
+ sizeof(struct swap_cgroup_unit));
+ if (!units)
goto nomem;

ctrl = &swap_cgroup_ctrl[type];
mutex_lock(&swap_cgroup_mutex);
- ctrl->map = map;
- spin_lock_init(&ctrl->lock);
+ ctrl->units = units;
mutex_unlock(&swap_cgroup_mutex);

return 0;
--
2.47.0