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                                struct res_counter *rc_parent)
  52
  53        Initializes the resource counter. As usual, should be the first
  54        routine called for a new counter.
  55
  56        The struct res_counter *parent can be used to define a hierarchical
  57        child -> parent relationship directly in the res_counter structure,
  58        NULL can be used to define no relationship.
  59
  60 c. int res_counter_charge(struct res_counter *rc, unsigned long val,
  61                                struct res_counter **limit_fail_at)
  62
  63        When a resource is about to be allocated it has to be accounted
  64        with the appropriate resource counter (controller should determine
  65        which one to use on its own). This operation is called "charging".
  66
  67        This is not very important which operation - resource allocation
  68        or charging - is performed first, but
  69          * if the allocation is performed first, this may create a
  70            temporary resource over-usage by the time resource counter is
  71            charged;
  72          * if the charging is performed first, then it should be uncharged
  73            on error path (if the one is called).
  74
  75        If the charging fails and a hierarchical dependency exists, the
  76        limit_fail_at parameter is set to the particular res_counter element
  77        where the charging failed.
  78
  79 d. int res_counter_charge_locked
  80                        (struct res_counter *rc, unsigned long val)
  81
  82        The same as res_counter_charge(), but it must not acquire/release the
  83        res_counter->lock internally (it must be called with res_counter->lock
  84        held).
  85
  86 e. void res_counter_uncharge[_locked]
  87                        (struct res_counter *rc, unsigned long val)
  88
  89        When a resource is released (freed) it should be de-accounted
  90        from the resource counter it was accounted to.  This is called
  91        "uncharging".
  92
  93        The _locked routines imply that the res_counter->lock is taken.
  94
  95 2.1 Other accounting routines
  96
  97    There are more routines that may help you with common needs, like
  98    checking whether the limit is reached or resetting the max_usage
  99    value. They are all declared in include/linux/res_counter.h.
 100
 101
 102
 1033. Analyzing the resource counter registrations
 104
 105 a. If the failcnt value constantly grows, this means that the counter's
 106    limit is too tight. Either the group is misbehaving and consumes too
 107    many resources, or the configuration is not suitable for the group
 108    and the limit should be increased.
 109
 110 b. The max_usage value can be used to quickly tune the group. One may
 111    set the limits to maximal values and either load the container with
 112    a common pattern or leave one for a while. After this the max_usage
 113    value shows the amount of memory the container would require during
 114    its common activity.
 115
 116    Setting the limit a bit above this value gives a pretty good
 117    configuration that works in most of the cases.
 118
 119 c. If the max_usage is much less than the limit, but the failcnt value
 120    is growing, then the group tries to allocate a big chunk of resource
 121    at once.
 122
 123 d. If the max_usage is much less than the limit, but the failcnt value
 124    is 0, then this group is given too high limit, that it does not
 125    require. It is better to lower the limit a bit leaving more resource
 126    for other groups.
 127
 128
 129
 1304. Communication with the control groups subsystem (cgroups)
 131
 132All the resource controllers that are using cgroups and resource counters
 133should provide files (in the cgroup filesystem) to work with the resource
 134counter fields. They are recommended to adhere to the following rules:
 135
 136 a. File names
 137
 138        Field name      File name
 139        ---------------------------------------------------
 140        usage           usage_in_<unit_of_measurement>
 141        max_usage       max_usage_in_<unit_of_measurement>
 142        limit           limit_in_<unit_of_measurement>
 143        failcnt         failcnt
 144        lock            no file :)
 145
 146 b. Reading from file should show the corresponding field value in the
 147    appropriate format.
 148
 149 c. Writing to file
 150
 151        Field           Expected behavior
 152        ----------------------------------
 153        usage           prohibited
 154        max_usage       reset to usage
 155        limit           set the limit
 156        failcnt         reset to zero
 157
 158
 159
 1605. Usage example
 161
 162 a. Declare a task group (take a look at cgroups subsystem for this) and
 163    fold a res_counter into it
 164
 165        struct my_group {
 166                struct res_counter res;
 167
 168                <other fields>
 169        }
 170
 171 b. Put hooks in resource allocation/release paths
 172
 173        int alloc_something(...)
 174        {
 175                if (res_counter_charge(res_counter_ptr, amount) < 0)
 176                        return -ENOMEM;
 177
 178                <allocate the resource and return to the caller>
 179        }
 180
 181        void release_something(...)
 182        {
 183                res_counter_uncharge(res_counter_ptr, amount);
 184
 185                <release the resource>
 186        }
 187
 188    In order to keep the usage value self-consistent, both the
 189    "res_counter_ptr" and the "amount" in release_something() should be
 190    the same as they were in the alloc_something() when the releasing
 191    resource was allocated.
 192
 193 c. Provide the way to read res_counter values and set them (the cgroups
 194    still can help with it).
 195
 196 c. Compile and run :)
 197
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