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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)
+
+        Initializes the resource counter. As usual, should be the first
+        routine called for a new counter.
+
+ b. int res_counter_charge[_locked]
+                        (struct res_counter *rc, unsigned long val)
+
+        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).
+
+ c. 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.
+
+
+ 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 :)