mm, vmpressure: pass-through notification support

By default, vmpressure events are not pass-through, i.e.  they propagate
up through the memcg hierarchy until an event notifier is found for any
threshold level.

This presents a difficulty when a thread waiting on a read(2) for a
vmpressure event cannot distinguish between local memory pressure and
memory pressure in a descendant memcg, especially when that thread may
not control the memcg hierarchy.

Consider a user-controlled child memcg with a smaller limit than a
top-level memcg controlled by the "Activity Manager" specified in
Documentation/cgroup-v1/memory.txt.  It may register for memory pressure
notification for descendant memcgs to make a policy decision: oom kill a
low priority job, increase the limit, decrease other limits, etc.  If it
registers for memory pressure notification on the top-level memcg, it
currently cannot distinguish between memory pressure in its own memcg or
a descendant memcg, which is user-controlled.

Conversely, if a user registers for memory pressure notification on
their own descendant memcg, the Activity Manager does not receive any
pressure notification for that child memcg hierarchy.  Vmpressure events
are not received for ancestor memcgs if the memcg experiencing pressure
have notifiers registered, perhaps outside the knowledge of the thread
waiting on read(2) at the top level.

Both of these are consequences of vmpressure notification not being
pass-through.

This implements a pass-through behavior for vmpressure events.  When
writing to control.event_control, vmpressure event handlers may
optionally specify a mode.  There are two new modes:

 - "hierarchy": always propagate memory pressure events up the hierarchy
   regardless if descendant memcgs have their own notifiers registered,
   and

 - "local": only receive notifications when the memcg for which the
   event is registered experiences memory pressure.

Of course, processes may register for one notification of "low,local",
for example, and another for "low".

If no mode is specified, the current behavior is maintained for
backwards compatibility.

See the change to Documentation/cgroup-v1/memory.txt for full
specification.

[dan.carpenter@oracle.com: free the same pointer we allocated]
  Link: http://lkml.kernel.org/r/20170613191820.GA20003@elgon.mountain
Link: http://lkml.kernel.org/r/alpine.DEB.2.10.1705311421320.8946@chino.kir.corp.google.com
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Anton Vorontsov <anton@enomsg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroup-v1/memory.txt

@@ -789,23 +789,46 @@ way to trigger. Applications should do whatever they can to help the
 system. It might be too late to consult with vmstat or any other
 statistics, so it's advisable to take an immediate action.
 
-The events are propagated upward until the event is handled, i.e. the
-events are not pass-through. Here is what this means: for example you have
-three cgroups: A->B->C. Now you set up an event listener on cgroups A, B
-and C, and suppose group C experiences some pressure. In this situation,
-only group C will receive the notification, i.e. groups A and B will not
-receive it. This is done to avoid excessive "broadcasting" of messages,
-which disturbs the system and which is especially bad if we are low on
-memory or thrashing. So, organize the cgroups wisely, or propagate the
-events manually (or, ask us to implement the pass-through events,
-explaining why would you need them.)
+By default, events are propagated upward until the event is handled, i.e. the
+events are not pass-through. For example, you have three cgroups: A->B->C. Now
+you set up an event listener on cgroups A, B and C, and suppose group C
+experiences some pressure. In this situation, only group C will receive the
+notification, i.e. groups A and B will not receive it. This is done to avoid
+excessive "broadcasting" of messages, which disturbs the system and which is
+especially bad if we are low on memory or thrashing. Group B, will receive
+notification only if there are no event listers for group C.
+
+There are three optional modes that specify different propagation behavior:
+
+ - "default": this is the default behavior specified above. This mode is the
+   same as omitting the optional mode parameter, preserved by backwards
+   compatibility.
+
+ - "hierarchy": events always propagate up to the root, similar to the default
+   behavior, except that propagation continues regardless of whether there are
+   event listeners at each level, with the "hierarchy" mode. In the above
+   example, groups A, B, and C will receive notification of memory pressure.
+
+ - "local": events are pass-through, i.e. they only receive notifications when
+   memory pressure is experienced in the memcg for which the notification is
+   registered. In the above example, group C will receive notification if
+   registered for "local" notification and the group experiences memory
+   pressure. However, group B will never receive notification, regardless if
+   there is an event listener for group C or not, if group B is registered for
+   local notification.
+
+The level and event notification mode ("hierarchy" or "local", if necessary) are
+specified by a comma-delimited string, i.e. "low,hierarchy" specifies
+hierarchical, pass-through, notification for all ancestor memcgs. Notification
+that is the default, non pass-through behavior, does not specify a mode.
+"medium,local" specifies pass-through notification for the medium level.
 
 The file memory.pressure_level is only used to setup an eventfd. To
 register a notification, an application must:
 
 - create an eventfd using eventfd(2);
 - open memory.pressure_level;
-- write string like "<event_fd> <fd of memory.pressure_level> <level>"
+- write string as "<event_fd> <fd of memory.pressure_level> <level[,mode]>"
   to cgroup.event_control.
 
 Application will be notified through eventfd when memory pressure is at
@@ -821,7 +844,7 @@ Test:
    # cd /sys/fs/cgroup/memory/
    # mkdir foo
    # cd foo
-   # cgroup_event_listener memory.pressure_level low &
+   # cgroup_event_listener memory.pressure_level low,hierarchy &
    # echo 8000000 > memory.limit_in_bytes
    # echo 8000000 > memory.memsw.limit_in_bytes
    # echo $$ > tasks