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Diffstat (limited to 'Documentation/cgroup-v2.txt')
| -rw-r--r-- | Documentation/cgroup-v2.txt | 180 |
1 files changed, 179 insertions, 1 deletions
diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt index ff49cf901148..4cc07ce3b8dd 100644 --- a/Documentation/cgroup-v2.txt +++ b/Documentation/cgroup-v2.txt @@ -47,6 +47,11 @@ CONTENTS 5-3. IO 5-3-1. IO Interface Files 5-3-2. Writeback +6. Namespace + 6-1. Basics + 6-2. The Root and Views + 6-3. Migration and setns(2) + 6-4. Interaction with Other Namespaces P. Information on Kernel Programming P-1. Filesystem Support for Writeback D. Deprecated v1 Core Features @@ -132,6 +137,12 @@ strongly discouraged for production use. It is recommended to decide the hierarchies and controller associations before starting using the controllers after system boot. +During transition to v2, system management software might still +automount the v1 cgroup filesystem and so hijack all controllers +during boot, before manual intervention is possible. To make testing +and experimenting easier, the kernel parameter cgroup_no_v1= allows +disabling controllers in v1 and make them always available in v2. + 2-2. Organizing Processes @@ -843,6 +854,15 @@ PAGE_SIZE multiple when read back. Amount of memory used to cache filesystem data, including tmpfs and shared memory. + kernel_stack + + Amount of memory allocated to kernel stacks. + + slab + + Amount of memory used for storing in-kernel data + structures. + sock Amount of memory used in network transmission buffers @@ -871,6 +891,16 @@ PAGE_SIZE multiple when read back. on the internal memory management lists used by the page reclaim algorithm + slab_reclaimable + + Part of "slab" that might be reclaimed, such as + dentries and inodes. + + slab_unreclaimable + + Part of "slab" that cannot be reclaimed on memory + pressure. + pgfault Total number of page faults incurred @@ -896,7 +926,7 @@ PAGE_SIZE multiple when read back. limit, anonymous meomry of the cgroup will not be swapped out. -5-2-2. General Usage +5-2-2. Usage Guidelines "memory.high" is the main mechanism to control memory usage. Over-committing on high limit (sum of high limits > available memory) @@ -1089,6 +1119,148 @@ writeback as follows. vm.dirty[_background]_ratio. +6. Namespace + +6-1. Basics + +cgroup namespace provides a mechanism to virtualize the view of the +"/proc/$PID/cgroup" file and cgroup mounts. The CLONE_NEWCGROUP clone +flag can be used with clone(2) and unshare(2) to create a new cgroup +namespace. The process running inside the cgroup namespace will have +its "/proc/$PID/cgroup" output restricted to cgroupns root. The +cgroupns root is the cgroup of the process at the time of creation of +the cgroup namespace. + +Without cgroup namespace, the "/proc/$PID/cgroup" file shows the +complete path of the cgroup of a process. In a container setup where +a set of cgroups and namespaces are intended to isolate processes the +"/proc/$PID/cgroup" file may leak potential system level information +to the isolated processes. For Example: + + # cat /proc/self/cgroup + 0::/batchjobs/container_id1 + +The path '/batchjobs/container_id1' can be considered as system-data +and undesirable to expose to the isolated processes. cgroup namespace +can be used to restrict visibility of this path. For example, before +creating a cgroup namespace, one would see: + + # ls -l /proc/self/ns/cgroup + lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835] + # cat /proc/self/cgroup + 0::/batchjobs/container_id1 + +After unsharing a new namespace, the view changes. + + # ls -l /proc/self/ns/cgroup + lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183] + # cat /proc/self/cgroup + 0::/ + +When some thread from a multi-threaded process unshares its cgroup +namespace, the new cgroupns gets applied to the entire process (all +the threads). This is natural for the v2 hierarchy; however, for the +legacy hierarchies, this may be unexpected. + +A cgroup namespace is alive as long as there are processes inside or +mounts pinning it. When the last usage goes away, the cgroup +namespace is destroyed. The cgroupns root and the actual cgroups +remain. + + +6-2. The Root and Views + +The 'cgroupns root' for a cgroup namespace is the cgroup in which the +process calling unshare(2) is running. For example, if a process in +/batchjobs/container_id1 cgroup calls unshare, cgroup +/batchjobs/container_id1 becomes the cgroupns root. For the +init_cgroup_ns, this is the real root ('/') cgroup. + +The cgroupns root cgroup does not change even if the namespace creator +process later moves to a different cgroup. + + # ~/unshare -c # unshare cgroupns in some cgroup + # cat /proc/self/cgroup + 0::/ + # mkdir sub_cgrp_1 + # echo 0 > sub_cgrp_1/cgroup.procs + # cat /proc/self/cgroup + 0::/sub_cgrp_1 + +Each process gets its namespace-specific view of "/proc/$PID/cgroup" + +Processes running inside the cgroup namespace will be able to see +cgroup paths (in /proc/self/cgroup) only inside their root cgroup. +From within an unshared cgroupns: + + # sleep 100000 & + [1] 7353 + # echo 7353 > sub_cgrp_1/cgroup.procs + # cat /proc/7353/cgroup + 0::/sub_cgrp_1 + +From the initial cgroup namespace, the real cgroup path will be +visible: + + $ cat /proc/7353/cgroup + 0::/batchjobs/container_id1/sub_cgrp_1 + +From a sibling cgroup namespace (that is, a namespace rooted at a +different cgroup), the cgroup path relative to its own cgroup +namespace root will be shown. For instance, if PID 7353's cgroup +namespace root is at '/batchjobs/container_id2', then it will see + + # cat /proc/7353/cgroup + 0::/../container_id2/sub_cgrp_1 + +Note that the relative path always starts with '/' to indicate that +its relative to the cgroup namespace root of the caller. + + +6-3. Migration and setns(2) + +Processes inside a cgroup namespace can move into and out of the +namespace root if they have proper access to external cgroups. For +example, from inside a namespace with cgroupns root at +/batchjobs/container_id1, and assuming that the global hierarchy is +still accessible inside cgroupns: + + # cat /proc/7353/cgroup + 0::/sub_cgrp_1 + # echo 7353 > batchjobs/container_id2/cgroup.procs + # cat /proc/7353/cgroup + 0::/../container_id2 + +Note that this kind of setup is not encouraged. A task inside cgroup +namespace should only be exposed to its own cgroupns hierarchy. + +setns(2) to another cgroup namespace is allowed when: + +(a) the process has CAP_SYS_ADMIN against its current user namespace +(b) the process has CAP_SYS_ADMIN against the target cgroup + namespace's userns + +No implicit cgroup changes happen with attaching to another cgroup +namespace. It is expected that the someone moves the attaching +process under the target cgroup namespace root. + + +6-4. Interaction with Other Namespaces + +Namespace specific cgroup hierarchy can be mounted by a process +running inside a non-init cgroup namespace. + + # mount -t cgroup2 none $MOUNT_POINT + +This will mount the unified cgroup hierarchy with cgroupns root as the +filesystem root. The process needs CAP_SYS_ADMIN against its user and +mount namespaces. + +The virtualization of /proc/self/cgroup file combined with restricting +the view of cgroup hierarchy by namespace-private cgroupfs mount +provides a properly isolated cgroup view inside the container. + + P. Information on Kernel Programming This section contains kernel programming information in the areas @@ -1368,6 +1540,12 @@ system than killing the group. Otherwise, memory.max is there to limit this type of spillover and ultimately contain buggy or even malicious applications. +Setting the original memory.limit_in_bytes below the current usage was +subject to a race condition, where concurrent charges could cause the +limit setting to fail. memory.max on the other hand will first set the +limit to prevent new charges, and then reclaim and OOM kill until the +new limit is met - or the task writing to memory.max is killed. + The combined memory+swap accounting and limiting is replaced by real control over swap space. |