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-
- The Resource Counter
-
-The resource counter, declared at include/linux/res_counter.h,
-is supposed to facilitate the resource management by controllers
-by providing common stuff for accounting.
-
-This "stuff" includes the res_counter structure and routines
-to work with it.
-
-
-
-1. Crucial parts of the res_counter structure
-
- a. unsigned long long usage
-
- The usage value shows the amount of a resource that is consumed
- by a group at a given time. The units of measurement should be
- determined by the controller that uses this counter. E.g. it can
- be bytes, items or any other unit the controller operates on.
-
- b. unsigned long long max_usage
-
- The maximal value of the usage over time.
-
- This value is useful when gathering statistical information about
- the particular group, as it shows the actual resource requirements
- for a particular group, not just some usage snapshot.
-
- c. unsigned long long limit
-
- The maximal allowed amount of resource to consume by the group. In
- case the group requests for more resources, so that the usage value
- would exceed the limit, the resource allocation is rejected (see
- the next section).
-
- d. unsigned long long failcnt
-
- The failcnt stands for "failures counter". This is the number of
- resource allocation attempts that failed.
-
- c. spinlock_t lock
-
- Protects changes of the above values.
-
-
-
-2. Basic accounting routines
-
- a. void res_counter_init(struct res_counter *rc,
- struct res_counter *rc_parent)
-
- Initializes the resource counter. As usual, should be the first
- routine called for a new counter.
-
- The struct res_counter *parent can be used to define a hierarchical
- child -> parent relationship directly in the res_counter structure,
- NULL can be used to define no relationship.
-
- c. int res_counter_charge(struct res_counter *rc, unsigned long val,
- struct res_counter **limit_fail_at)
-
- When a resource is about to be allocated it has to be accounted
- with the appropriate resource counter (controller should determine
- which one to use on its own). This operation is called "charging".
-
- This is not very important which operation - resource allocation
- or charging - is performed first, but
- * if the allocation is performed first, this may create a
- temporary resource over-usage by the time resource counter is
- charged;
- * if the charging is performed first, then it should be uncharged
- on error path (if the one is called).
-
- If the charging fails and a hierarchical dependency exists, the
- limit_fail_at parameter is set to the particular res_counter element
- where the charging failed.
-
- d. int res_counter_charge_locked
- (struct res_counter *rc, unsigned long val, bool force)
-
- The same as res_counter_charge(), but it must not acquire/release the
- res_counter->lock internally (it must be called with res_counter->lock
- held). The force parameter indicates whether we can bypass the limit.
-
- e. void res_counter_uncharge[_locked]
- (struct res_counter *rc, unsigned long val)
-
- When a resource is released (freed) it should be de-accounted
- from the resource counter it was accounted to. This is called
- "uncharging".
-
- The _locked routines imply that the res_counter->lock is taken.
-
- f. void res_counter_uncharge_until
- (struct res_counter *rc, struct res_counter *top,
- unsinged long val)
-
- Almost same as res_cunter_uncharge() but propagation of uncharge
- stops when rc == top. This is useful when kill a res_coutner in
- child cgroup.
-
- 2.1 Other accounting routines
-
- There are more routines that may help you with common needs, like
- checking whether the limit is reached or resetting the max_usage
- value. They are all declared in include/linux/res_counter.h.
-
-
-
-3. Analyzing the resource counter registrations
-
- a. If the failcnt value constantly grows, this means that the counter's
- limit is too tight. Either the group is misbehaving and consumes too
- many resources, or the configuration is not suitable for the group
- and the limit should be increased.
-
- b. The max_usage value can be used to quickly tune the group. One may
- set the limits to maximal values and either load the container with
- a common pattern or leave one for a while. After this the max_usage
- value shows the amount of memory the container would require during
- its common activity.
-
- Setting the limit a bit above this value gives a pretty good
- configuration that works in most of the cases.
-
- c. If the max_usage is much less than the limit, but the failcnt value
- is growing, then the group tries to allocate a big chunk of resource
- at once.
-
- d. If the max_usage is much less than the limit, but the failcnt value
- is 0, then this group is given too high limit, that it does not
- require. It is better to lower the limit a bit leaving more resource
- for other groups.
-
-
-
-4. Communication with the control groups subsystem (cgroups)
-
-All the resource controllers that are using cgroups and resource counters
-should provide files (in the cgroup filesystem) to work with the resource
-counter fields. They are recommended to adhere to the following rules:
-
- a. File names
-
- Field name File name
- ---------------------------------------------------
- usage usage_in_<unit_of_measurement>
- max_usage max_usage_in_<unit_of_measurement>
- limit limit_in_<unit_of_measurement>
- failcnt failcnt
- lock no file :)
-
- b. Reading from file should show the corresponding field value in the
- appropriate format.
-
- c. Writing to file
-
- Field Expected behavior
- ----------------------------------
- usage prohibited
- max_usage reset to usage
- limit set the limit
- failcnt reset to zero
-
-
-
-5. Usage example
-
- a. Declare a task group (take a look at cgroups subsystem for this) and
- fold a res_counter into it
-
- struct my_group {
- struct res_counter res;
-
- <other fields>
- }
-
- b. Put hooks in resource allocation/release paths
-
- int alloc_something(...)
- {
- if (res_counter_charge(res_counter_ptr, amount) < 0)
- return -ENOMEM;
-
- <allocate the resource and return to the caller>
- }
-
- void release_something(...)
- {
- res_counter_uncharge(res_counter_ptr, amount);
-
- <release the resource>
- }
-
- In order to keep the usage value self-consistent, both the
- "res_counter_ptr" and the "amount" in release_something() should be
- the same as they were in the alloc_something() when the releasing
- resource was allocated.
-
- c. Provide the way to read res_counter values and set them (the cgroups
- still can help with it).
-
- c. Compile and run :)