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mm: delete non-atomic mm counter implementation

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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 f412ac08c9
("[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@linux.intel.com>
Acked-by: Dave Hansen <dave@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Matt Fleming 2011-05-24 17:12:36 -07:00 committed by Linus Torvalds
parent a197b59ae6
commit 172703b08c
2 changed files with 10 additions and 43 deletions
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44
include/linux/mm.h
View File

@ -1053,17 +1053,19 @@ 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);
#else
static inline unsigned long get_mm_counter(struct mm_struct *mm, int 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)
{
@ -1080,38 +1082,6 @@ 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;
}
static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
{
return mm->rss_stat.count[member];
}
static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
{
mm->rss_stat.count[member] += value;
}
static inline void inc_mm_counter(struct mm_struct *mm, int member)
{
mm->rss_stat.count[member]++;
}
static inline void dec_mm_counter(struct mm_struct *mm, int member)
{
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) +

9
include/linux/mm_types.h
View File

@ -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 */