Commit ddbf369c authored by David Rientjes's avatar David Rientjes Committed by Linus Torvalds
Browse files

mm, vmacache: hash addresses based on pmd 

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%.

[rientjes@google.com: various updates]
  Link: http://lkml.kernel.org/r/alpine.DEB.2.21.1807231532290.109445@chino.kir.corp.google.com
Link: http://lkml.kernel.org/r/alpine.DEB.2.21.1807091749150.114630@chino.kir.corp.google.com


Signed-off-by: default avatarDavid Rientjes <rientjes@google.com>
Reviewed-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 6b51e881

include/linux/vmacache.h

+0 −6
Original line number Diff line number Diff line
@@ -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));

mm/vmacache.c

+29 −9
Original line number Diff line number Diff line
@@ -6,6 +6,18 @@ 
#include <linux/sched/task.h>

#include <linux/mm.h>

#include <linux/vmacache.h>

#include <asm/pgtable.h>



/*

 * Hash based on the pmd of addr if configured with MMU, which provides a good

 * hit rate for workloads with spatial locality.  Otherwise, use pages.

 */

#ifdef CONFIG_MMU

#define VMACACHE_SHIFT	PMD_SHIFT

#else

#define VMACACHE_SHIFT	PAGE_SHIFT

#endif

#define VMACACHE_HASH(addr) ((addr >> VMACACHE_SHIFT) & VMACACHE_MASK)



/*

 * Flush vma caches for threads that share a given mm.
@@ -87,6 +99,7 @@ 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);
@@ -95,17 +108,21 @@ 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];

		struct vm_area_struct *vma = current->vmacache.vmas[idx];



		if (!vma)

			continue;

		if (vma) {

#ifdef CONFIG_DEBUG_VM_VMACACHE

			if (WARN_ON_ONCE(vma->vm_mm != mm))

				break;

#endif

			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 +132,7 @@ struct vm_area_struct *vmacache_find_exact(struct mm_struct *mm, 
					   unsigned long start,

					   unsigned long end)

{

	int idx = VMACACHE_HASH(start);

	int i;



	count_vm_vmacache_event(VMACACHE_FIND_CALLS);
@@ -123,12 +141,14 @@ 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;

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