[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, 21 insertions, 2 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index bccee7cf9ccd..7c89b4549049 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -14,22 +14,41 @@
 char *task_mem(struct mm_struct *mm, char *buffer)
 {
         unsigned long data, text, lib;
+        unsigned long hiwater_vm, total_vm, hiwater_rss, total_rss;
+
+        /*
+         * Note: to minimize their overhead, mm maintains hiwater_vm and
+         * hiwater_rss only when about to *lower* total_vm or rss.  Any
+         * collector of these hiwater stats must therefore get total_vm
+         * and rss too, which will usually be the higher.  Barriers? not
+         * worth the effort, such snapshots can always be inconsistent.
+         */
+        hiwater_vm = total_vm = mm->total_vm;
+        if (hiwater_vm < mm->hiwater_vm)
+                hiwater_vm = mm->hiwater_vm;
+        hiwater_rss = total_rss = get_mm_rss(mm);
+        if (hiwater_rss < mm->hiwater_rss)
+                hiwater_rss = mm->hiwater_rss;
 
         data = mm->total_vm - mm->shared_vm - mm->stack_vm;
         text = (PAGE_ALIGN(mm->end_code) - (mm->start_code & PAGE_MASK)) >> 10;
         lib = (mm->exec_vm << (PAGE_SHIFT-10)) - text;
         buffer += sprintf(buffer,
+                "VmPeak:\t%8lu kB\n"
                 "VmSize:\t%8lu kB\n"
                 "VmLck:\t%8lu kB\n"
+                "VmHWM:\t%8lu kB\n"
                 "VmRSS:\t%8lu kB\n"
                 "VmData:\t%8lu kB\n"
                 "VmStk:\t%8lu kB\n"
                 "VmExe:\t%8lu kB\n"
                 "VmLib:\t%8lu kB\n"
                 "VmPTE:\t%8lu kB\n",
-                (mm->total_vm - mm->reserved_vm) << (PAGE_SHIFT-10),
+                hiwater_vm << (PAGE_SHIFT-10),
+                (total_vm - mm->reserved_vm) << (PAGE_SHIFT-10),
                 mm->locked_vm << (PAGE_SHIFT-10),
-                get_mm_rss(mm) << (PAGE_SHIFT-10),
+                hiwater_rss << (PAGE_SHIFT-10),
+                total_rss << (PAGE_SHIFT-10),
                 data << (PAGE_SHIFT-10),
                 mm->stack_vm << (PAGE_SHIFT-10), text, lib,
                 (PTRS_PER_PTE*sizeof(pte_t)*mm->nr_ptes) >> 10);