[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>

2 files changed, 4 insertions, 3 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 3b25b182d2b..79f52b85d6e 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -839,7 +839,10 @@ fastcall NORET_TYPE void do_exit(long code)
 				preempt_count());
 
 	acct_update_integrals(tsk);
-	update_mem_hiwater(tsk);
+	if (tsk->mm) {
+		update_hiwater_rss(tsk->mm);
+		update_hiwater_vm(tsk->mm);
+	}
 	group_dead = atomic_dec_and_test(&tsk->signal->live);
 	if (group_dead) {
  		del_timer_sync(&tsk->signal->real_timer);
diff --git a/kernel/sched.c b/kernel/sched.c
index 1e5cafdf4e2..4f26c544d02 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2511,8 +2511,6 @@ void account_system_time(struct task_struct *p, int hardirq_offset,
 		cpustat->idle = cputime64_add(cpustat->idle, tmp);
 	/* Account for system time used */
 	acct_update_integrals(p);
-	/* Update rss highwater mark */
-	update_mem_hiwater(p);
 }
 
 /*