[PATCH] mm: update_hiwaters just in time

update_mem_hiwater has attracted various criticisms, in particular from those
concerned with mm scalability.  Originally it was called whenever rss or
total_vm got raised.  Then many of those callsites were replaced by a timer
tick call from account_system_time.  Now Frank van Maarseveen reports that to
be found inadequate.  How about this?  Works for Frank.

Replace update_mem_hiwater, a poor combination of two unrelated ops, by macros
update_hiwater_rss and update_hiwater_vm.  Don't attempt to keep
mm->hiwater_rss up to date at timer tick, nor every time we raise rss (usually
by 1): those are hot paths.  Do the opposite, update only when about to lower
rss (usually by many), or just before final accounting in do_exit.  Handle
mm->hiwater_vm in the same way, though it's much less of an issue.  Demand
that whoever collects these hiwater statistics do the work of taking the
maximum with rss or total_vm.

And there has been no collector of these hiwater statistics in the tree.  The
new convention needs an example, so match Frank's usage by adding a VmPeak
line above VmSize to /proc/<pid>/status, and also a VmHWM line above VmRSS
(High-Water-Mark or High-Water-Memory).

There was a particular anomaly during mremap move, that hiwater_vm might be
captured too high.  A fleeting such anomaly remains, but it's quickly
corrected now, whereas before it would stick.

What locking?  None: if the app is racy then these statistics will be racy,
it's not worth any overhead to make them exact.  But whenever it suits,
hiwater_vm is updated under exclusive mmap_sem, and hiwater_rss under
page_table_lock (for now) or with preemption disabled (later on): without
going to any trouble, minimize the time between reading current values and
updating, to minimize those occasions when a racing thread bumps a count up
and back down in between.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index afcaac66cbd..a9c0b7d2630 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -256,6 +256,16 @@ extern void arch_unmap_area_topdown(struct mm_struct *, unsigned long);
 #define dec_mm_counter(mm, member) (mm)->_##member--
 #define get_mm_rss(mm) ((mm)->_file_rss + (mm)->_anon_rss)
 
+#define update_hiwater_rss(mm)	do {			\
+	unsigned long _rss = get_mm_rss(mm);		\
+	if ((mm)->hiwater_rss < _rss)			\
+		(mm)->hiwater_rss = _rss;		\
+} while (0)
+#define update_hiwater_vm(mm)	do {			\
+	if ((mm)->hiwater_vm < (mm)->total_vm)		\
+		(mm)->hiwater_vm = (mm)->total_vm;	\
+} while (0)
+
 typedef unsigned long mm_counter_t;
 
 struct mm_struct {