linux/Documentation/cgroups/resource_counter.txt
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   1
   2                The Resource Counter
   3
   4The resource counter, declared at include/linux/res_counter.h,
   5is supposed to facilitate the resource management by controllers
   6by providing common stuff for accounting.
   7
   8This "stuff" includes the res_counter structure and routines
   9to work with it.
  10
  11
  12
  131. Crucial parts of the res_counter structure
  14
  15 a. unsigned long long usage
  16
  17        The usage value shows the amount of a resource that is consumed
  18        by a group at a given time. The units of measurement should be
  19        determined by the controller that uses this counter. E.g. it can
  20        be bytes, items or any other unit the controller operates on.
  21
  22 b. unsigned long long max_usage
  23
  24        The maximal value of the usage over time.
  25
  26        This value is useful when gathering statistical information about
  27        the particular group, as it shows the actual resource requirements
  28        for a particular group, not just some usage snapshot.
  29
  30 c. unsigned long long limit
  31
  32        The maximal allowed amount of resource to consume by the group. In
  33        case the group requests for more resources, so that the usage value
  34        would exceed the limit, the resource allocation is rejected (see
  35        the next section).
  36
  37 d. unsigned long long failcnt
  38
  39        The failcnt stands for "failures counter". This is the number of
  40        resource allocation attempts that failed.
  41
  42 c. spinlock_t lock
  43
  44        Protects changes of the above values.
  45
  46
  47
  482. Basic accounting routines
  49
  50 a. void res_counter_init(struct res_counter *rc)
  51
  52        Initializes the resource counter. As usual, should be the first
  53        routine called for a new counter.
  54
  55 b. int res_counter_charge[_locked]
  56                        (struct res_counter *rc, unsigned long val)
  57
  58        When a resource is about to be allocated it has to be accounted
  59        with the appropriate resource counter (controller should determine
  60        which one to use on its own). This operation is called "charging".
  61
  62        This is not very important which operation - resource allocation
  63        or charging - is performed first, but
  64          * if the allocation is performed first, this may create a
  65            temporary resource over-usage by the time resource counter is
  66            charged;
  67          * if the charging is performed first, then it should be uncharged
  68            on error path (if the one is called).
  69
  70 c. void res_counter_uncharge[_locked]
  71                        (struct res_counter *rc, unsigned long val)
  72
  73        When a resource is released (freed) it should be de-accounted
  74        from the resource counter it was accounted to.  This is called
  75        "uncharging".
  76
  77    The _locked routines imply that the res_counter->lock is taken.
  78
  79
  80 2.1 Other accounting routines
  81
  82    There are more routines that may help you with common needs, like
  83    checking whether the limit is reached or resetting the max_usage
  84    value. They are all declared in include/linux/res_counter.h.
  85
  86
  87
  883. Analyzing the resource counter registrations
  89
  90 a. If the failcnt value constantly grows, this means that the counter's
  91    limit is too tight. Either the group is misbehaving and consumes too
  92    many resources, or the configuration is not suitable for the group
  93    and the limit should be increased.
  94
  95 b. The max_usage value can be used to quickly tune the group. One may
  96    set the limits to maximal values and either load the container with
  97    a common pattern or leave one for a while. After this the max_usage
  98    value shows the amount of memory the container would require during
  99    its common activity.
 100
 101    Setting the limit a bit above this value gives a pretty good
 102    configuration that works in most of the cases.
 103
 104 c. If the max_usage is much less than the limit, but the failcnt value
 105    is growing, then the group tries to allocate a big chunk of resource
 106    at once.
 107
 108 d. If the max_usage is much less than the limit, but the failcnt value
 109    is 0, then this group is given too high limit, that it does not
 110    require. It is better to lower the limit a bit leaving more resource
 111    for other groups.
 112
 113
 114
 1154. Communication with the control groups subsystem (cgroups)
 116
 117All the resource controllers that are using cgroups and resource counters
 118should provide files (in the cgroup filesystem) to work with the resource
 119counter fields. They are recommended to adhere to the following rules:
 120
 121 a. File names
 122
 123        Field name      File name
 124        ---------------------------------------------------
 125        usage           usage_in_<unit_of_measurement>
 126        max_usage       max_usage_in_<unit_of_measurement>
 127        limit           limit_in_<unit_of_measurement>
 128        failcnt         failcnt
 129        lock            no file :)
 130
 131 b. Reading from file should show the corresponding field value in the
 132    appropriate format.
 133
 134 c. Writing to file
 135
 136        Field           Expected behavior
 137        ----------------------------------
 138        usage           prohibited
 139        max_usage       reset to usage
 140        limit           set the limit
 141        failcnt         reset to zero
 142
 143
 144
 1455. Usage example
 146
 147 a. Declare a task group (take a look at cgroups subsystem for this) and
 148    fold a res_counter into it
 149
 150        struct my_group {
 151                struct res_counter res;
 152
 153                <other fields>
 154        }
 155
 156 b. Put hooks in resource allocation/release paths
 157
 158        int alloc_something(...)
 159        {
 160                if (res_counter_charge(res_counter_ptr, amount) < 0)
 161                        return -ENOMEM;
 162
 163                <allocate the resource and return to the caller>
 164        }
 165
 166        void release_something(...)
 167        {
 168                res_counter_uncharge(res_counter_ptr, amount);
 169
 170                <release the resource>
 171        }
 172
 173    In order to keep the usage value self-consistent, both the
 174    "res_counter_ptr" and the "amount" in release_something() should be
 175    the same as they were in the alloc_something() when the releasing
 176    resource was allocated.
 177
 178 c. Provide the way to read res_counter values and set them (the cgroups
 179    still can help with it).
 180
 181 c. Compile and run :)
 182
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