[PATCH mm-unstable v2 10/10] mm: multi-gen LRU: use mmu_notifier_test_clear_young()

Yu Zhao
2023-05-26T23:46:00Z

Use mmu_notifier_test_clear_young() to handle KVM PTEs in batches
when the fast path is supported. This reduces the contention on
kvm->mmu_lock when the host is under heavy memory pressure.

An existing selftest can quickly demonstrate the effectiveness of
this patch. On a generic workstation equipped with 128 CPUs and 256GB
DRAM:

$ sudo max_guest_memory_test -c 64 -m 250 -s 250

MGLRU run2
------------------
Before [1] ~64s
After ~51s

kswapd (MGLRU before)
100.00% balance_pgdat
100.00% shrink_node
100.00% shrink_one
99.99% try_to_shrink_lruvec
99.71% evict_folios
97.29% shrink_folio_list
==>> 13.05% folio_referenced
12.83% rmap_walk_file
12.31% folio_referenced_one
7.90% __mmu_notifier_clear_young
7.72% kvm_mmu_notifier_clear_young
7.34% _raw_write_lock

kswapd (MGLRU after)
100.00% balance_pgdat
100.00% shrink_node
100.00% shrink_one
99.99% try_to_shrink_lruvec
99.59% evict_folios
80.37% shrink_folio_list
==>> 3.74% folio_referenced
3.59% rmap_walk_file
3.19% folio_referenced_one
2.53% lru_gen_look_around
1.06% __mmu_notifier_test_clear_young

[1] "mm: rmap: Don't flush TLB after checking PTE young for page
reference" was included so that the comparison is apples to
apples.
https://lore.kernel.org/r/20220706112041.3831-1-21cnbao@gmail.com/

Signed-off-by: Yu Zhao <yuzhao@google.com>
---
Documentation/admin-guide/mm/multigen_lru.rst | 6 +-
include/linux/mmzone.h | 6 +-
mm/rmap.c | 8 +-
mm/vmscan.c | 139 ++++++++++++++++--
4 files changed, 138 insertions(+), 21 deletions(-)

diff --git a/Documentation/admin-guide/mm/multigen_lru.rst b/Documentation/admin-guide/mm/multigen_lru.rst
index 33e068830497..0ae2a6d4d94c 100644
--- a/Documentation/admin-guide/mm/multigen_lru.rst
+++ b/Documentation/admin-guide/mm/multigen_lru.rst
@@ -48,6 +48,10 @@ Values Components
verified on x86 varieties other than Intel and AMD. If it is
disabled, the multi-gen LRU will suffer a negligible
performance degradation.
+0x0008 Clearing the accessed bit in KVM page table entries in large
+ batches, when KVM MMU sets it (e.g., on x86_64). This can
+ improve the performance of guests when the host is under memory
+ pressure.
[yYnN] Apply to all the components above.
====== ===============================================================

@@ -56,7 +60,7 @@ E.g.,

echo y >/sys/kernel/mm/lru_gen/enabled
cat /sys/kernel/mm/lru_gen/enabled
- 0x0007
+ 0x000f
echo 5 >/sys/kernel/mm/lru_gen/enabled
cat /sys/kernel/mm/lru_gen/enabled
0x0005
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 5a7ada0413da..1b148f39fabc 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -369,6 +369,7 @@ enum {
LRU_GEN_CORE,
LRU_GEN_MM_WALK,
LRU_GEN_NONLEAF_YOUNG,
+ LRU_GEN_KVM_MMU_WALK,
NR_LRU_GEN_CAPS
};

@@ -471,7 +472,7 @@ struct lru_gen_mm_walk {
};

void lru_gen_init_lruvec(struct lruvec *lruvec);
-void lru_gen_look_around(struct page_vma_mapped_walk *pvmw);
+bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw);

#ifdef CONFIG_MEMCG

@@ -559,8 +560,9 @@ static inline void lru_gen_init_lruvec(struct lruvec *lruvec)
{
}

-static inline void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
+static inline bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
{
+ return false;
}

#ifdef CONFIG_MEMCG
diff --git a/mm/rmap.c b/mm/rmap.c
index ae127f60a4fb..3a2c4e6a0887 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -825,12 +825,10 @@ static bool folio_referenced_one(struct folio *folio,
return false; /* To break the loop */
}

- if (pvmw.pte) {
- if (lru_gen_enabled() && pte_young(*pvmw.pte)) {
- lru_gen_look_around(&pvmw);
+ if (lru_gen_enabled() && pvmw.pte) {
+ if (lru_gen_look_around(&pvmw))
referenced++;
- }
-
+ } else if (pvmw.pte) {
if (ptep_clear_flush_young_notify(vma, address,
pvmw.pte))
referenced++;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index ef687f9be13c..3f734588b600 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -58,6 +58,7 @@
#include <linux/rculist_nulls.h>
#include <linux/random.h>
#include <linux/srcu.h>
+#include <linux/mmu_notifier.h>

#include <asm/tlbflush.h>
#include <asm/div64.h>
@@ -3244,6 +3245,20 @@ static bool should_clear_pmd_young(void)
return arch_has_hw_nonleaf_pmd_young() && get_cap(LRU_GEN_NONLEAF_YOUNG);
}

+#if IS_ENABLED(CONFIG_KVM)
+#include <linux/kvm_host.h>
+
+static bool should_walk_kvm_mmu(void)
+{
+ return kvm_arch_has_test_clear_young() && get_cap(LRU_GEN_KVM_MMU_WALK);
+}
+#else
+static bool should_walk_kvm_mmu(void)
+{
+ return false;
+}
+#endif
+
/******************************************************************************
* shorthand helpers
******************************************************************************/
@@ -3982,6 +3997,55 @@ static struct folio *get_pfn_folio(unsigned long pfn, struct mem_cgroup *memcg,
return folio;
}

+static bool test_spte_young(struct mm_struct *mm, unsigned long addr, unsigned long end,
+ unsigned long *bitmap, unsigned long *last)
+{
+ if (!should_walk_kvm_mmu())
+ return false;
+
+ if (*last > addr)
+ goto done;
+
+ *last = end - addr > MIN_LRU_BATCH * PAGE_SIZE ?
+ addr + MIN_LRU_BATCH * PAGE_SIZE - 1 : end - 1;
+ bitmap_zero(bitmap, MIN_LRU_BATCH);
+
+ mmu_notifier_test_clear_young(mm, addr, *last + 1, false, bitmap);
+done:
+ return test_bit((*last - addr) / PAGE_SIZE, bitmap);
+}
+
+static void clear_spte_young(struct mm_struct *mm, unsigned long addr,
+ unsigned long *bitmap, unsigned long *last)
+{
+ int i;
+ unsigned long start, end = *last + 1;
+
+ if (addr + PAGE_SIZE != end)
+ return;
+
+ i = find_last_bit(bitmap, MIN_LRU_BATCH);
+ if (i == MIN_LRU_BATCH)
+ return;
+
+ start = end - (i + 1) * PAGE_SIZE;
+
+ i = find_first_bit(bitmap, MIN_LRU_BATCH);
+
+ end -= i * PAGE_SIZE;
+
+ mmu_notifier_test_clear_young(mm, start, end, true, bitmap);
+}
+
+static void skip_spte_young(struct mm_struct *mm, unsigned long addr,
+ unsigned long *bitmap, unsigned long *last)
+{
+ if (*last > addr)
+ __clear_bit((*last - addr) / PAGE_SIZE, bitmap);
+
+ clear_spte_young(mm, addr, bitmap, last);
+}
+
static bool suitable_to_scan(int total, int young)
{
int n = clamp_t(int, cache_line_size() / sizeof(pte_t), 2, 8);
@@ -3997,6 +4061,8 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
pte_t *pte;
spinlock_t *ptl;
unsigned long addr;
+ DECLARE_BITMAP(bitmap, MIN_LRU_BATCH);
+ unsigned long last = 0;
int total = 0;
int young = 0;
struct lru_gen_mm_walk *walk = args->private;
@@ -4015,6 +4081,7 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
pte = pte_offset_map(pmd, start & PMD_MASK);
restart:
for (i = pte_index(start), addr = start; addr != end; i++, addr += PAGE_SIZE) {
+ bool ret;
unsigned long pfn;
struct folio *folio;

@@ -4022,20 +4089,27 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
walk->mm_stats[MM_LEAF_TOTAL]++;

pfn = get_pte_pfn(pte[i], args->vma, addr);
- if (pfn == -1)
+ if (pfn == -1) {
+ skip_spte_young(args->vma->vm_mm, addr, bitmap, &last);
continue;
+ }

- if (!pte_young(pte[i])) {
+ ret = test_spte_young(args->vma->vm_mm, addr, end, bitmap, &last);
+ if (!ret && !pte_young(pte[i])) {
+ skip_spte_young(args->vma->vm_mm, addr, bitmap, &last);
walk->mm_stats[MM_LEAF_OLD]++;
continue;
}

folio = get_pfn_folio(pfn, memcg, pgdat, walk->can_swap);
- if (!folio)
+ if (!folio) {
+ skip_spte_young(args->vma->vm_mm, addr, bitmap, &last);
continue;
+ }

- if (!ptep_test_and_clear_young(args->vma, addr, pte + i))
- VM_WARN_ON_ONCE(true);
+ clear_spte_young(args->vma->vm_mm, addr, bitmap, &last);
+ if (pte_young(pte[i]))
+ ptep_test_and_clear_young(args->vma, addr, pte + i);

young++;
walk->mm_stats[MM_LEAF_YOUNG]++;
@@ -4629,6 +4703,23 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
* rmap/PT walk feedback
******************************************************************************/

+static bool should_look_around(struct vm_area_struct *vma, unsigned long addr,
+ pte_t *pte, int *young)
+{
+ int ret = mmu_notifier_clear_young(vma->vm_mm, addr, addr + PAGE_SIZE);
+
+ if (pte_young(*pte)) {
+ ptep_test_and_clear_young(vma, addr, pte);
+ *young = true;
+ return true;
+ }
+
+ if (ret)
+ *young = true;
+
+ return ret & MMU_NOTIFIER_RANGE_LOCKLESS;
+}
+
/*
* This function exploits spatial locality when shrink_folio_list() walks the
* rmap. It scans the adjacent PTEs of a young PTE and promotes hot pages. If
@@ -4636,12 +4727,14 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
* the PTE table to the Bloom filter. This forms a feedback loop between the
* eviction and the aging.
*/
-void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
+bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
{
int i;
unsigned long start;
unsigned long end;
struct lru_gen_mm_walk *walk;
+ DECLARE_BITMAP(bitmap, MIN_LRU_BATCH);
+ unsigned long last = 0;
int young = 0;
pte_t *pte = pvmw->pte;
unsigned long addr = pvmw->address;
@@ -4655,8 +4748,11 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
lockdep_assert_held(pvmw->ptl);
VM_WARN_ON_ONCE_FOLIO(folio_test_lru(folio), folio);

+ if (!should_look_around(pvmw->vma, addr, pte, &young))
+ return young;
+
if (spin_is_contended(pvmw->ptl))
- return;
+ return young;

/* avoid taking the LRU lock under the PTL when possible */
walk = current->reclaim_state ? current->reclaim_state->mm_walk : NULL;
@@ -4664,6 +4760,9 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
start = max(addr & PMD_MASK, pvmw->vma->vm_start);
end = min(addr | ~PMD_MASK, pvmw->vma->vm_end - 1) + 1;

+ if (end - start == PAGE_SIZE)
+ return young;
+
if (end - start > MIN_LRU_BATCH * PAGE_SIZE) {
if (addr - start < MIN_LRU_BATCH * PAGE_SIZE / 2)
end = start + MIN_LRU_BATCH * PAGE_SIZE;
@@ -4677,28 +4776,37 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)

/* folio_update_gen() requires stable folio_memcg() */
if (!mem_cgroup_trylock_pages(memcg))
- return;
+ return young;

arch_enter_lazy_mmu_mode();

pte -= (addr - start) / PAGE_SIZE;

for (i = 0, addr = start; addr != end; i++, addr += PAGE_SIZE) {
+ bool ret;
unsigned long pfn;

pfn = get_pte_pfn(pte[i], pvmw->vma, addr);
- if (pfn == -1)
+ if (pfn == -1) {
+ skip_spte_young(pvmw->vma->vm_mm, addr, bitmap, &last);
continue;
+ }

- if (!pte_young(pte[i]))
+ ret = test_spte_young(pvmw->vma->vm_mm, addr, end, bitmap, &last);
+ if (!ret && !pte_young(pte[i])) {
+ skip_spte_young(pvmw->vma->vm_mm, addr, bitmap, &last);
continue;
+ }

folio = get_pfn_folio(pfn, memcg, pgdat, !walk || walk->can_swap);
- if (!folio)
+ if (!folio) {
+ skip_spte_young(pvmw->vma->vm_mm, addr, bitmap, &last);
continue;
+ }

- if (!ptep_test_and_clear_young(pvmw->vma, addr, pte + i))
- VM_WARN_ON_ONCE(true);
+ clear_spte_young(pvmw->vma->vm_mm, addr, bitmap, &last);
+ if (pte_young(pte[i]))
+ ptep_test_and_clear_young(pvmw->vma, addr, pte + i);

young++;

@@ -4728,6 +4836,8 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
/* feedback from rmap walkers to page table walkers */
if (suitable_to_scan(i, young))
update_bloom_filter(lruvec, max_seq, pvmw->pmd);
+
+ return young;
}

/******************************************************************************
@@ -5745,6 +5855,9 @@ static ssize_t enabled_show(struct kobject *kobj, struct kobj_attribute *attr, c
if (should_clear_pmd_young())
caps |= BIT(LRU_GEN_NONLEAF_YOUNG);

+ if (should_walk_kvm_mmu())
+ caps |= BIT(LRU_GEN_KVM_MMU_WALK);
+
return sysfs_emit(buf, "0x%04x\n", caps);
}

--
2.41.0.rc0.172.g3f132b7071-goog