When perf profiling a wide variety of different workloads, it was found
that vmacache_find() had higher than expected cost: up to 0.08% of cpu
utilization in some cases. This was found to rival other core VM
functions such as alloc_pages_vma() with thp enabled and default
mempolicy, and the conditionals in __get_vma_policy().
VMACACHE_HASH() determines which of the four per-task_struct slots a vma
is cached for a particular address. This currently depends on the pfn,
so pfn 5212 occupies a different vmacache slot than its neighboring
pfn 5213.
vmacache_find() iterates through all four of current's vmacache slots
when looking up an address. Hashing based on pfn, an address has
~1/VMACACHE_SIZE chance of being cached in the first vmacache slot, or
about 25%, *if* the vma is cached.
This patch hashes an address by its pmd instead of pte to optimize for
workloads with good spatial locality. This results in a higher
probability of vmas being cached in the first slot that is checked:
normally ~70% on the same workloads instead of 25%.
Signed-off-by: David Rientjes <rient...@google.com>
---
include/linux/vmacache.h | 6 ------
mm/vmacache.c | 32 ++++++++++++++++++++++----------
2 files changed, 22 insertions(+), 16 deletions(-)
diff --git a/include/linux/vmacache.h b/include/linux/vmacache.h
--- a/include/linux/vmacache.h
+++ b/include/linux/vmacache.h
@@ -5,12 +5,6 @@
#include <linux/sched.h>
#include <linux/mm.h>
-/*
- * Hash based on the page number. Provides a good hit rate for
- * workloads with good locality and those with random accesses as well.
- */
-#define VMACACHE_HASH(addr) ((addr >> PAGE_SHIFT) & VMACACHE_MASK)
-
static inline void vmacache_flush(struct task_struct *tsk)
{
memset(tsk->vmacache.vmas, 0, sizeof(tsk->vmacache.vmas));
diff --git a/mm/vmacache.c b/mm/vmacache.c
--- a/mm/vmacache.c
+++ b/mm/vmacache.c
@@ -7,6 +7,12 @@
#include <linux/mm.h>
#include <linux/vmacache.h>
+/*
+ * Hash based on the pmd of addr. Provides a good hit rate for workloads with
+ * spatial locality.
+ */
+#define VMACACHE_HASH(addr) ((addr >> PMD_SHIFT) & VMACACHE_MASK)
+
/*
* Flush vma caches for threads that share a given mm.
*
@@ -87,6 +93,7 @@ static bool vmacache_valid(struct mm_struct *mm)
struct vm_area_struct *vmacache_find(struct mm_struct *mm, unsigned long addr)
{
+ int idx = VMACACHE_HASH(addr);
int i;
count_vm_vmacache_event(VMACACHE_FIND_CALLS);
@@ -95,16 +102,18 @@ struct vm_area_struct *vmacache_find(struct mm_struct *mm,
unsigned long addr)
return NULL;
for (i = 0; i < VMACACHE_SIZE; i++) {
- struct vm_area_struct *vma = current->vmacache.vmas[i];
-
- if (!vma)
- continue;
- if (WARN_ON_ONCE(vma->vm_mm != mm))
- break;
- if (vma->vm_start <= addr && vma->vm_end > addr) {
- count_vm_vmacache_event(VMACACHE_FIND_HITS);
- return vma;
+ struct vm_area_struct *vma = current->vmacache.vmas[idx];
+
+ if (vma) {
+ if (WARN_ON_ONCE(vma->vm_mm != mm))
+ break;
+ if (vma->vm_start <= addr && vma->vm_end > addr) {
+ count_vm_vmacache_event(VMACACHE_FIND_HITS);
+ return vma;
+ }
}
+ if (++idx == VMACACHE_SIZE)
+ idx = 0;
}
return NULL;
@@ -115,6 +124,7 @@ struct vm_area_struct *vmacache_find_exact(struct mm_struct
*mm,
unsigned long start,
unsigned long end)
{
+ int idx = VMACACHE_HASH(addr);
int i;
count_vm_vmacache_event(VMACACHE_FIND_CALLS);
@@ -123,12 +133,14 @@ struct vm_area_struct *vmacache_find_exact(struct
mm_struct *mm,
return NULL;
for (i = 0; i < VMACACHE_SIZE; i++) {
- struct vm_area_struct *vma = current->vmacache.vmas[i];
+ struct vm_area_struct *vma = current->vmacache.vmas[idx];
if (vma && vma->vm_start == start && vma->vm_end == end) {
count_vm_vmacache_event(VMACACHE_FIND_HITS);
return vma;
}
+ if (++idx == VMACACHE_SIZE)
+ idx = 0;
}
return NULL;
