linux/mm/vmacache.c
David Rientjes ddbf369c0a 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: David Rientjes <rientjes@google.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2018-08-17 16:20:32 -07:00

157 lines
3.7 KiB
C

// SPDX-License-Identifier: GPL-2.0
/*
* Copyright (C) 2014 Davidlohr Bueso.
*/
#include <linux/sched/signal.h>
#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.
*
* The operation is safe because the caller holds the mmap_sem
* exclusively and other threads accessing the vma cache will
* have mmap_sem held at least for read, so no extra locking
* is required to maintain the vma cache.
*/
void vmacache_flush_all(struct mm_struct *mm)
{
struct task_struct *g, *p;
count_vm_vmacache_event(VMACACHE_FULL_FLUSHES);
/*
* Single threaded tasks need not iterate the entire
* list of process. We can avoid the flushing as well
* since the mm's seqnum was increased and don't have
* to worry about other threads' seqnum. Current's
* flush will occur upon the next lookup.
*/
if (atomic_read(&mm->mm_users) == 1)
return;
rcu_read_lock();
for_each_process_thread(g, p) {
/*
* Only flush the vmacache pointers as the
* mm seqnum is already set and curr's will
* be set upon invalidation when the next
* lookup is done.
*/
if (mm == p->mm)
vmacache_flush(p);
}
rcu_read_unlock();
}
/*
* This task may be accessing a foreign mm via (for example)
* get_user_pages()->find_vma(). The vmacache is task-local and this
* task's vmacache pertains to a different mm (ie, its own). There is
* nothing we can do here.
*
* Also handle the case where a kernel thread has adopted this mm via use_mm().
* That kernel thread's vmacache is not applicable to this mm.
*/
static inline bool vmacache_valid_mm(struct mm_struct *mm)
{
return current->mm == mm && !(current->flags & PF_KTHREAD);
}
void vmacache_update(unsigned long addr, struct vm_area_struct *newvma)
{
if (vmacache_valid_mm(newvma->vm_mm))
current->vmacache.vmas[VMACACHE_HASH(addr)] = newvma;
}
static bool vmacache_valid(struct mm_struct *mm)
{
struct task_struct *curr;
if (!vmacache_valid_mm(mm))
return false;
curr = current;
if (mm->vmacache_seqnum != curr->vmacache.seqnum) {
/*
* First attempt will always be invalid, initialize
* the new cache for this task here.
*/
curr->vmacache.seqnum = mm->vmacache_seqnum;
vmacache_flush(curr);
return false;
}
return true;
}
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);
if (!vmacache_valid(mm))
return NULL;
for (i = 0; i < VMACACHE_SIZE; i++) {
struct vm_area_struct *vma = current->vmacache.vmas[idx];
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;
}
#ifndef CONFIG_MMU
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);
if (!vmacache_valid(mm))
return NULL;
for (i = 0; i < VMACACHE_SIZE; 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;
}
#endif