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Subject[PATCH v3 16/16] Add documentation about the kmem controller
Signed-off-by: Glauber Costa <>
CC: Randy Dunlap <>
CC: Christoph Lameter <>
CC: Pekka Enberg <>
CC: Michal Hocko <>
CC: Kamezawa Hiroyuki <>
CC: Johannes Weiner <>
CC: Suleiman Souhlal <>
CC: Mel Gorman <>

Documentation/cgroups/memory.txt | 73 +++++++++++++++++++++++++++++++++++++++-
1 file changed, 72 insertions(+), 1 deletion(-)

diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index 4372e6b..6356379 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -71,6 +71,11 @@ Brief summary of control files.
memory.oom_control # set/show oom controls.
memory.numa_stat # show the number of memory usage per numa node

+ memory.kmem.limit_in_bytes # set/show hard limit for kernel memory
+ memory.kmem.usage_in_bytes # show current kernel memory allocation
+ memory.kmem.failcnt # show the number of kernel memory usage hits limits
+ memory.kmem.max_usage_in_bytes # show max kernel memory usage recorded
memory.kmem.tcp.limit_in_bytes # set/show hard limit for tcp buf memory
memory.kmem.tcp.usage_in_bytes # show current tcp buf memory allocation
memory.kmem.tcp.failcnt # show the number of tcp buf memory usage hits limits
@@ -268,20 +273,80 @@ the amount of kernel memory used by the system. Kernel memory is fundamentally
different than user memory, since it can't be swapped out, which makes it
possible to DoS the system by consuming too much of this precious resource.

+Kernel memory won't be accounted at all until it is limited. This allows for
+existing setups to continue working without disruption. Note that it is
+possible to account it without an effective limit by setting the limits
+to a very high number (like RESOURCE_MAX -1page). After a controller is first
+limited, it will be kept being accounted until it is removed. The memory
+limitation itself, can of course be removed by writing -1 to
Kernel memory limits are not imposed for the root cgroup. Usage for the root
-cgroup may or may not be accounted.
+cgroup may or may not be accounted. The memory used is accumulated into
+memory.kmem.usage_in_bytes, or in a separate counter when it makes sense.
+The main "kmem" counter is fed into the main counter, so kmem charges will
+also be visible from the user counter.

Currently no soft limit is implemented for kernel memory. It is future work
to trigger slab reclaim when those limits are reached.

2.7.1 Current Kernel Memory resources accounted

+* stack pages: every process consumes some stack pages. By accounting into
+kernel memory, we prevent new processes from being created when the kernel
+memory usage is too high.
+* slab pages: pages allocated by the SLAB or SLUB allocator are tracked. A copy
+of each kmem_cache is created everytime the cache is touched by the first time
+from inside the memcg. The creation is done lazily, so some objects can still be
+skipped while the cache is being created. All objects in a slab page should
+belong to the same memcg. This only fails to hold when a task is migrated to a
+different memcg during the page allocation by the cache.
* sockets memory pressure: some sockets protocols have memory pressure
thresholds. The Memory Controller allows them to be controlled individually
per cgroup, instead of globally.

* tcp memory pressure: sockets memory pressure for the tcp protocol.

+2.7.2 The kmem_accounted field (internal, non user visible)
+The bitmap "kmem_accounted" is responsible for controlling the lifecycle of
+kmem accounting in a particular memcg. The bits have the following meaning:
+bit0, KMEM_ACCOUNTED_ACTIVE: will be set to indicate that the memcg is kmem
+ limited due to userspace action directly to this memcg.
+bit1, KMEM_ACCOUNTED_PARENT: only valid in hierarchical setups, will trigger
+ kernel memory accounting because any ancestor of this group was accounted.
+bit2, KMEM_ACCOUNTED_DEAD: will be set when a memcg is removed, and pending kmem
+ charges are still present. When the memcg can be removed without delays,
+ it will be, and this bit will never be set.
+2.7.3 Common use cases
+Because the "kmem" counter is fed to the main user counter, kernel memory can
+never be limited completely independently of user memory. Say "U" is the user
+limit, and "K" the kernel limit. There are three possible ways limits can be
+ U != 0, K = 0:
+ This is the standard memcg limitation mechanism already present before kmem
+ accounting. Kernel memory is completely ignored.
+ U,K != 0, K < U:
+ Kernel memory is effectively set as a percentage of the user memory. This
+ setup is useful in deployments where the total amount of memory per-cgroup
+ is overcommited. Overcommiting kernel memory limits is definitely not
+ recommended, since the box can still run out of non-reclaimable memory.
+ In this case, the admin could set up K so that the sum of all groups is
+ never greater than the total memory, and freely set U at the cost of his
+ QoS.
+ U,K != 0, K >= U:
+ Since kmem charges will also be fed to the user counter, this setup gives
+ the admin a unified view of memory. Reclaim will be triggered for the cgroup
+ for both kinds of memory.
3. User Interface

0. Configuration
@@ -290,6 +355,7 @@ a. Enable CONFIG_CGROUPS
d. Enable CONFIG_MEMCG_SWAP (to use swap extension)
+d. Enable CONFIG_MEMCG_KMEM (to use kmem extension)

1. Prepare the cgroups (see cgroups.txt, Why are cgroups needed?)
# mount -t tmpfs none /sys/fs/cgroup
@@ -402,6 +468,11 @@ About use_hierarchy, see Section 6.
moved to parent(if use_hierarchy==1) or root (if use_hierarchy==0) and this
cgroup will be empty.

+ Also, note that when memory.kmem.limit_in_bytes is set the charges due to
+ kernel pages will still be seen. This is not considered a failure and the
+ write will still return success. In this case, it is expected that
+ memory.kmem.usage_in_bytes == memory.usage_in_bytes.
Typical use case of this interface is that calling this before rmdir().
Because rmdir() moves all pages to parent, some out-of-use page caches can be
moved to the parent. If you want to avoid that, force_empty will be useful.

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