The patch titled
mm: delete non-atomic mm counter implementation
has been added to the -mm tree. Its filename is
mm-delete-non-atomic-mm-counter-implementation.patch
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------------------------------------------------------
Subject: mm: delete non-atomic mm counter implementation
From: Matt Fleming <matt.fleming@xxxxxxxxxxxxxxx>
The problem with having two different types of counters is that developers
adding new code need to keep in mind whether it's safe to use both the
atomic and non-atomic implementations. For example, when adding new
callers of the *_mm_counter() functions a developer needs to ensure that
those paths are always executed with page_table_lock held, in case we're
using the non-atomic implementation of mm counters.
Hugh Dickins introduced the atomic mm counters in commit f412ac08c986
("[PATCH] mm: fix rss and mmlist locking"). When asked why he left the
non-atomic counters around he said,
| The only reason was to avoid adding costly atomic operations into a
| configuration that had no need for them there: the page_table_lock
| sufficed.
|
| Certainly it would be simpler just to delete the non-atomic variant.
|
| And I think it's fair to say that any configuration on which we're
| measuring performance to that degree (rather than "does it boot fast?"
| type measurements), would already be going the split ptlocks route.
Removing the non-atomic counters eases the maintenance burden because
developers no longer have to mindful of the two implementations when using
*_mm_counter().
Note that all architectures provide a means of atomically updating
atomic_long_t variables, even if they have to revert to the generic
spinlock implementation because they don't support 64-bit atomic
instructions (see lib/atomic64.c).
Signed-off-by: Matt Fleming <matt.fleming@xxxxxxxxxxxxxxx>
Acked-by: Dave Hansen <dave@xxxxxxxxxxxxxxxxxx>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm.h | 44 +++++--------------------------------
include/linux/mm_types.h | 9 ++-----
2 files changed, 10 insertions(+), 43 deletions(-)
diff -puN include/linux/mm.h~mm-delete-non-atomic-mm-counter-implementation include/linux/mm.h
--- a/include/linux/mm.h~mm-delete-non-atomic-mm-counter-implementation
+++ a/include/linux/mm.h
@@ -1031,65 +1031,35 @@ int __get_user_pages_fast(unsigned long
/*
* per-process(per-mm_struct) statistics.
*/
-#if defined(SPLIT_RSS_COUNTING)
-/*
- * The mm counters are not protected by its page_table_lock,
- * so must be incremented atomically.
- */
static inline void set_mm_counter(struct mm_struct *mm, int member, long value)
{
atomic_long_set(&mm->rss_stat.count[member], value);
}
+#if defined(SPLIT_RSS_COUNTING)
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)
-{
- atomic_long_add(value, &mm->rss_stat.count[member]);
-}
-
-static inline void inc_mm_counter(struct mm_struct *mm, int member)
-{
- atomic_long_inc(&mm->rss_stat.count[member]);
-}
-
-static inline void dec_mm_counter(struct mm_struct *mm, int member)
-{
- atomic_long_dec(&mm->rss_stat.count[member]);
-}
-
-#else /* !USE_SPLIT_PTLOCKS */
-/*
- * The mm counters are protected by its page_table_lock,
- * so can be incremented directly.
- */
-static inline void set_mm_counter(struct mm_struct *mm, int member, long value)
-{
- mm->rss_stat.count[member] = value;
-}
-
+#else
static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
{
- return mm->rss_stat.count[member];
+ return atomic_long_read(&mm->rss_stat.count[member]);
}
+#endif
static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
{
- mm->rss_stat.count[member] += value;
+ atomic_long_add(value, &mm->rss_stat.count[member]);
}
static inline void inc_mm_counter(struct mm_struct *mm, int member)
{
- mm->rss_stat.count[member]++;
+ atomic_long_inc(&mm->rss_stat.count[member]);
}
static inline void dec_mm_counter(struct mm_struct *mm, int member)
{
- mm->rss_stat.count[member]--;
+ atomic_long_dec(&mm->rss_stat.count[member]);
}
-#endif /* !USE_SPLIT_PTLOCKS */
-
static inline unsigned long get_mm_rss(struct mm_struct *mm)
{
return get_mm_counter(mm, MM_FILEPAGES) +
diff -puN include/linux/mm_types.h~mm-delete-non-atomic-mm-counter-implementation include/linux/mm_types.h
--- a/include/linux/mm_types.h~mm-delete-non-atomic-mm-counter-implementation
+++ a/include/linux/mm_types.h
@@ -204,19 +204,16 @@ enum {
#if USE_SPLIT_PTLOCKS && defined(CONFIG_MMU)
#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 */
+#endif /* USE_SPLIT_PTLOCKS */
+
struct mm_rss_stat {
- unsigned long count[NR_MM_COUNTERS];
+ atomic_long_t count[NR_MM_COUNTERS];
};
-#endif /* !USE_SPLIT_PTLOCKS */
struct mm_struct {
struct vm_area_struct * mmap; /* list of VMAs */
_
Patches currently in -mm which might be from matt.fleming@xxxxxxxxxxxxxxx are
linux-next.patch
mm-delete-non-atomic-mm-counter-implementation.patch
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