[RFC PATCH v23 05/10] mm: report the approximate RSS in the rss_stat tracepoint

[Mathieu Desnoyers]

The rss_stat tracepoint is emitted on every RSS update, including each
page fault.  When it is enabled, it reports the precise sum of the RSS
counter, which iterates over all possible CPUs for each event: 384 per
page fault on a machine with 384 logical CPUs.

Report the approximate sum of the hierarchical per-cpu counter instead,
through get_mm_counter(), which reads the root of the counter tree.  The
size field of the event, the RSS counter after the update, is therefore
within the accuracy bound of the counter from the precise value: within
158.5MB on a machine with 384 logical CPUs and 4kB pages.

The tracepoint reports the precise sum since commit f1a7941243c1 ("mm:
convert mm's rss stats into percpu_counter").  Before that, it reported
an atomic counter which only received the per-task cached updates in
batches.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Dennis Zhou <dennis@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxxx>
Cc: Martin Liu <liumartin@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: christian.koenig@xxxxxxx
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: SeongJae Park <sj@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Sweet Tea Dorminy <sweettea-kernel@xxxxxxxxxx>
Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
Cc: Liam R. Howlett <liam@xxxxxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Wei Yang <richard.weiyang@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Mateusz Guzik <mjguzik@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Aboorva Devarajan <aboorvad@xxxxxxxxxxxxx>
Cc: David Carlier <devnexen@xxxxxxxxx>
Cc: Josh Law <objecting@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
---
Changes since v22:
- Split from "mm: improve RSS counter approximation accuracy for proc
  interfaces", which now keeps reporting the precise sum.
---
 include/trace/events/kmem.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
index ac4c96db7638..8ee6d5df5961 100644
--- a/include/trace/events/kmem.h
+++ b/include/trace/events/kmem.h
@@ -448,7 +448,7 @@ TRACE_EVENT(rss_stat,
 		 */
 		__entry->curr = current->mm == mm && !(current->flags & PF_KTHREAD);
 		__entry->member = member;
-		__entry->size = get_mm_counter_sum(mm, member) << PAGE_SHIFT;
+		__entry->size = get_mm_counter(mm, member) << PAGE_SHIFT;
 	),
 
 	TP_printk("mm_id=%u curr=%d type=%s size=%ldB",
-- 
2.47.3