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Date: Wed, 27 Apr 2011 15:36:05 +0100
From: Matt Fleming <matt@...sole-pimps.org>
To: Dave Hansen <dave@...ux.vnet.ibm.com>
Cc: Hugh Dickins <hughd@...gle.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-mm@...r.kernel.org, linux-kernel@...r.kernel.org,
Matt Fleming <matt.fleming@...ux.intel.com>
Subject: [PATCH] mm: Delete non-atomic mm counter implementation
From: Matt Fleming <matt.fleming@...ux.intel.com>
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@...ux.intel.com>
---
Dave, you might want to take this into your pagetable counters series
so that you only need to worry about atomic mm counters.
include/linux/mm.h | 44 +++++++-------------------------------------
include/linux/mm_types.h | 9 +++------
2 files changed, 10 insertions(+), 43 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd87a78..ee64af2 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1034,65 +1034,35 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
/*
* 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 --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index ca01ab2..b8ca318 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -205,19 +205,16 @@ 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 */
--
1.7.4.4
--
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