[mmotm][PATCH 2/5] mm : avoid false sharing on mm_counter
From: KAMEZAWA Hiroyuki
Date: Tue Dec 15 2009 - 04:16:48 EST
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Considering the nature of per mm stats, it's the shared object among
threads and can be a cache-miss point in the page fault path.
This patch adds per-thread cache for mm_counter. RSS value will be counted
into a struct in task_struct and synchronized with mm's one at events.
Now, in this patch, the event is the number of calls to handle_mm_fault.
Per-thread value is added to mm at each 64 calls.
rough estimation with small benchmark on parallel thread (2threads) shows
[before]
4.5 cache-miss/faults
[after]
4.0 cache-miss/faults
Anyway, the most contended object is mmap_sem if the number of threads grows.
Changlog: 2009/12/15
- added Documentation
- removed all hooks from scheduler and ticks.
- added event counter instead of them.
- make counter per thread rather than per cpu. This removes many
complicated codes.
- added SPLIT_RSS_COUNTING instead of reusing USE_SPLIT_PTLOCKS.
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
Documentation/filesystems/proc.txt | 6 ++
fs/exec.c | 1
include/linux/mm.h | 8 +--
include/linux/mm_types.h | 6 ++
include/linux/sched.h | 4 +
kernel/exit.c | 3 -
mm/memory.c | 94 +++++++++++++++++++++++++++++++++----
7 files changed, 107 insertions(+), 15 deletions(-)
Index: mmotm-2.6.32-Dec8-pth/include/linux/mm_types.h
===================================================================
--- mmotm-2.6.32-Dec8-pth.orig/include/linux/mm_types.h
+++ mmotm-2.6.32-Dec8-pth/include/linux/mm_types.h
@@ -200,9 +200,15 @@ enum {
};
#if USE_SPLIT_PTLOCKS
+#define SPLIT_RSS_COUNTING
struct mm_rss_stat {
atomic_long_t count[NR_MM_COUNTERS];
};
+/* per-thread cached information, */
+struct task_rss_stat {
+ int events; /* for synchronization threshold */
+ int count[NR_MM_COUNTERS];
+};
#else /* !USE_SPLIT_PTLOCKS */
struct mm_rss_stat {
unsigned long count[NR_MM_COUNTERS];
Index: mmotm-2.6.32-Dec8-pth/include/linux/sched.h
===================================================================
--- mmotm-2.6.32-Dec8-pth.orig/include/linux/sched.h
+++ mmotm-2.6.32-Dec8-pth/include/linux/sched.h
@@ -1222,7 +1222,9 @@ struct task_struct {
struct plist_node pushable_tasks;
struct mm_struct *mm, *active_mm;
-
+#if defined(SPLIT_RSS_COUNTING)
+ struct task_rss_stat rss_stat;
+#endif
/* task state */
int exit_state;
int exit_code, exit_signal;
Index: mmotm-2.6.32-Dec8-pth/mm/memory.c
===================================================================
--- mmotm-2.6.32-Dec8-pth.orig/mm/memory.c
+++ mmotm-2.6.32-Dec8-pth/mm/memory.c
@@ -122,6 +122,79 @@ static int __init init_zero_pfn(void)
core_initcall(init_zero_pfn);
+#if defined(SPLIT_RSS_COUNTING)
+
+void __sync_task_rss_stat(struct task_struct *task, struct mm_struct *mm)
+{
+ int i;
+
+ for (i = 0; i < NR_MM_COUNTERS; i++) {
+ if (task->rss_stat.count[i]) {
+ add_mm_counter(mm, i, task->rss_stat.count[i]);
+ task->rss_stat.count[i] = 0;
+ }
+ }
+ task->rss_stat.events = 0;
+}
+
+static void add_mm_counter_fast(struct mm_struct *mm, int member, int val)
+{
+ struct task_struct *task = current;
+
+ if (likely(task->mm == mm))
+ task->rss_stat.count[member] += val;
+ else
+ add_mm_counter(mm, member, val);
+}
+#define inc_mm_counter_fast(mm, member) add_mm_counter_fast(mm, member,1)
+#define dec_mm_counter_fast(mm, member) add_mm_counter_fast(mm, member,-1)
+
+/* sync counter once per 64 page faults */
+#define TASK_RSS_EVENTS_THRESH (64)
+static void check_sync_rss_stat(struct task_struct *task)
+{
+ if (unlikely(task != current))
+ return;
+ if (unlikely(task->rss_stat.events++ > TASK_RSS_EVENTS_THRESH))
+ __sync_task_rss_stat(task, task->mm);
+}
+
+unsigned long get_mm_counter(struct mm_struct *mm, int member)
+{
+ long val = 0;
+
+ /*
+ * Don't use task->mm here...for avoiding to use task_get_mm()..
+ * The caller must guarantee task->mm is not invalid.
+ */
+ val = atomic_long_read(&mm->rss_stat.count[member]);
+ /*
+ * counter is updated in asynchronous manner and may go to minus.
+ * But it's never be expected number for users.
+ */
+ if (val < 0)
+ return 0;
+ return (unsigned long)val;
+}
+
+void sync_mm_rss(struct task_struct *task, struct mm_struct *mm)
+{
+ __sync_task_rss_stat(task, mm);
+}
+#else
+
+#define inc_mm_counter_fast(mm, member) inc_mm_counter(mm, member)
+#define dec_mm_counter_fast(mm, member) dec_mm_counter(mm, member)
+
+static void check_sync_rss_stat(struct task_struct *task)
+{
+}
+
+void sync_mm_rss(struct task_struct *task, struct mm_struct *mm)
+{
+}
+#endif
+
/*
* If a p?d_bad entry is found while walking page tables, report
* the error, before resetting entry to p?d_none. Usually (but
@@ -386,6 +459,8 @@ static inline void add_mm_rss_vec(struct
{
int i;
+ if (current->mm == mm)
+ sync_mm_rss(current, mm);
for (i = 0; i < NR_MM_COUNTERS; i++)
if (rss[i])
add_mm_counter(mm, i, rss[i]);
@@ -1539,7 +1614,7 @@ static int insert_page(struct vm_area_st
/* Ok, finally just insert the thing.. */
get_page(page);
- inc_mm_counter(mm, MM_FILEPAGES);
+ inc_mm_counter_fast(mm, MM_FILEPAGES);
page_add_file_rmap(page);
set_pte_at(mm, addr, pte, mk_pte(page, prot));
@@ -2175,11 +2250,11 @@ gotten:
if (likely(pte_same(*page_table, orig_pte))) {
if (old_page) {
if (!PageAnon(old_page)) {
- dec_mm_counter(mm, MM_FILEPAGES);
- inc_mm_counter(mm, MM_ANONPAGES);
+ dec_mm_counter_fast(mm, MM_FILEPAGES);
+ inc_mm_counter_fast(mm, MM_ANONPAGES);
}
} else
- inc_mm_counter(mm, MM_ANONPAGES);
+ inc_mm_counter_fast(mm, MM_ANONPAGES);
flush_cache_page(vma, address, pte_pfn(orig_pte));
entry = mk_pte(new_page, vma->vm_page_prot);
entry = maybe_mkwrite(pte_mkdirty(entry), vma);
@@ -2612,7 +2687,7 @@ static int do_swap_page(struct mm_struct
* discarded at swap_free().
*/
- inc_mm_counter(mm, MM_ANONPAGES);
+ inc_mm_counter_fast(mm, MM_ANONPAGES);
pte = mk_pte(page, vma->vm_page_prot);
if ((flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
pte = maybe_mkwrite(pte_mkdirty(pte), vma);
@@ -2696,7 +2771,7 @@ static int do_anonymous_page(struct mm_s
if (!pte_none(*page_table))
goto release;
- inc_mm_counter(mm, MM_ANONPAGES);
+ inc_mm_counter_fast(mm, MM_ANONPAGES);
page_add_new_anon_rmap(page, vma, address);
setpte:
set_pte_at(mm, address, page_table, entry);
@@ -2850,10 +2925,10 @@ static int __do_fault(struct mm_struct *
if (flags & FAULT_FLAG_WRITE)
entry = maybe_mkwrite(pte_mkdirty(entry), vma);
if (anon) {
- inc_mm_counter(mm, MM_ANONPAGES);
+ inc_mm_counter_fast(mm, MM_ANONPAGES);
page_add_new_anon_rmap(page, vma, address);
} else {
- inc_mm_counter(mm, MM_FILEPAGES);
+ inc_mm_counter_fast(mm, MM_FILEPAGES);
page_add_file_rmap(page);
if (flags & FAULT_FLAG_WRITE) {
dirty_page = page;
@@ -3031,6 +3106,9 @@ int handle_mm_fault(struct mm_struct *mm
count_vm_event(PGFAULT);
+ /* do counter updates before entering really critical section. */
+ check_sync_rss_stat(current);
+
if (unlikely(is_vm_hugetlb_page(vma)))
return hugetlb_fault(mm, vma, address, flags);
Index: mmotm-2.6.32-Dec8-pth/include/linux/mm.h
===================================================================
--- mmotm-2.6.32-Dec8-pth.orig/include/linux/mm.h
+++ mmotm-2.6.32-Dec8-pth/include/linux/mm.h
@@ -871,7 +871,7 @@ int __get_user_pages_fast(unsigned long
/*
* per-process(per-mm_struct) statistics.
*/
-#if USE_SPLIT_PTLOCKS
+#if defined(SPLIT_RSS_COUNTING)
/*
* The mm counters are not protected by its page_table_lock,
* so must be incremented atomically.
@@ -881,10 +881,7 @@ static inline void set_mm_counter(struct
atomic_long_set(&mm->rss_stat.count[member], value);
}
-static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
-{
- return (unsigned long)atomic_long_read(&mm->rss_stat.count[member]);
-}
+unsigned long get_mm_counter(struct mm_struct *mm, int member);
static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
{
@@ -972,6 +969,7 @@ static inline void setmax_mm_hiwater_rss
*maxrss = hiwater_rss;
}
+void sync_mm_rss(struct task_struct *task, struct mm_struct *mm);
/*
* A callback you can register to apply pressure to ageable caches.
Index: mmotm-2.6.32-Dec8-pth/fs/exec.c
===================================================================
--- mmotm-2.6.32-Dec8-pth.orig/fs/exec.c
+++ mmotm-2.6.32-Dec8-pth/fs/exec.c
@@ -702,6 +702,7 @@ static int exec_mmap(struct mm_struct *m
/* Notify parent that we're no longer interested in the old VM */
tsk = current;
old_mm = current->mm;
+ sync_mm_rss(tsk, old_mm);
mm_release(tsk, old_mm);
if (old_mm) {
Index: mmotm-2.6.32-Dec8-pth/kernel/exit.c
===================================================================
--- mmotm-2.6.32-Dec8-pth.orig/kernel/exit.c
+++ mmotm-2.6.32-Dec8-pth/kernel/exit.c
@@ -944,7 +944,8 @@ NORET_TYPE void do_exit(long code)
preempt_count());
acct_update_integrals(tsk);
-
+ /* sync mm's RSS info before statistics gathering */
+ sync_mm_rss(tsk, tsk->mm);
group_dead = atomic_dec_and_test(&tsk->signal->live);
if (group_dead) {
hrtimer_cancel(&tsk->signal->real_timer);
Index: mmotm-2.6.32-Dec8-pth/Documentation/filesystems/proc.txt
===================================================================
--- mmotm-2.6.32-Dec8-pth.orig/Documentation/filesystems/proc.txt
+++ mmotm-2.6.32-Dec8-pth/Documentation/filesystems/proc.txt
@@ -189,6 +189,12 @@ memory usage. Its seven fields are expla
contains details information about the process itself. Its fields are
explained in Table 1-4.
+(for SMP CONFIG users)
+For making accounting scalable, RSS related information are handled in
+asynchronous manner and the vaule may not be very precise. To see a precise
+snapshot of a moment, you can see /proc/<pid>/smaps file and scan page table.
+It's slow but very precise.
+
Table 1-2: Contents of the statm files (as of 2.6.30-rc7)
..............................................................................
Field Content
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