Documentation/cgroups/resource_counter.txt

   1
   2                 The Resource Counter
   3
   4 The resource counter, declared at include/linux/res_counter.h,
   5 is supposed to facilitate the resource management by controllers
   6 by providing common stuff for accounting.
   7
   8 This "stuff" includes the res_counter structure and routines
   9 to work with it.
  10
  11
  12
  13 1. 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
  48 2. 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. u64 res_counter_uncharge(struct res_counter *rc, unsigned long val)
  80
  81         When a resource is released (freed) it should be de-accounted
  82         from the resource counter it was accounted to.  This is called
  83         "uncharging". The return value of this function indicate the amount
  84         of charges still present in the counter.
  85
  86         The _locked routines imply that the res_counter->lock is taken.
  87
  88  e. u64 res_counter_uncharge_until
  89                 (struct res_counter *rc, struct res_counter *top,
  90                  unsigned long val)
  91
  92         Almost same as res_counter_uncharge() but propagation of uncharge
  93         stops when rc == top. This is useful when kill a res_counter in
  94         child cgroup.
  95
  96  2.1 Other accounting routines
  97
  98     There are more routines that may help you with common needs, like
  99     checking whether the limit is reached or resetting the max_usage
 100     value. They are all declared in include/linux/res_counter.h.
 101
 102
 103
 104 3. Analyzing the resource counter registrations
 105
 106  a. If the failcnt value constantly grows, this means that the counter's
 107     limit is too tight. Either the group is misbehaving and consumes too
 108     many resources, or the configuration is not suitable for the group
 109     and the limit should be increased.
 110
 111  b. The max_usage value can be used to quickly tune the group. One may
 112     set the limits to maximal values and either load the container with
 113     a common pattern or leave one for a while. After this the max_usage
 114     value shows the amount of memory the container would require during
 115     its common activity.
 116
 117     Setting the limit a bit above this value gives a pretty good
 118     configuration that works in most of the cases.
 119
 120  c. If the max_usage is much less than the limit, but the failcnt value
 121     is growing, then the group tries to allocate a big chunk of resource
 122     at once.
 123
 124  d. If the max_usage is much less than the limit, but the failcnt value
 125     is 0, then this group is given too high limit, that it does not
 126     require. It is better to lower the limit a bit leaving more resource
 127     for other groups.
 128
 129
 130
 131 4. Communication with the control groups subsystem (cgroups)
 132
 133 All the resource controllers that are using cgroups and resource counters
 134 should provide files (in the cgroup filesystem) to work with the resource
 135 counter fields. They are recommended to adhere to the following rules:
 136
 137  a. File names
 138
 139         Field name      File name
 140         ---------------------------------------------------
 141         usage           usage_in_<unit_of_measurement>
 142         max_usage       max_usage_in_<unit_of_measurement>
 143         limit           limit_in_<unit_of_measurement>
 144         failcnt         failcnt
 145         lock            no file :)
 146
 147  b. Reading from file should show the corresponding field value in the
 148     appropriate format.
 149
 150  c. Writing to file
 151
 152         Field           Expected behavior
 153         ----------------------------------
 154         usage           prohibited
 155         max_usage       reset to usage
 156         limit           set the limit
 157         failcnt         reset to zero
 158
 159
 160
 161 5. Usage example
 162
 163  a. Declare a task group (take a look at cgroups subsystem for this) and
 164     fold a res_counter into it
 165
 166         struct my_group {
 167                 struct res_counter res;
 168
 169                 <other fields>
 170         }
 171
 172  b. Put hooks in resource allocation/release paths
 173
 174         int alloc_something(...)
 175         {
 176                 if (res_counter_charge(res_counter_ptr, amount) < 0)
 177                         return -ENOMEM;
 178
 179                 <allocate the resource and return to the caller>
 180         }
 181
 182         void release_something(...)
 183         {
 184                 res_counter_uncharge(res_counter_ptr, amount);
 185
 186                 <release the resource>
 187         }
 188
 189     In order to keep the usage value self-consistent, both the
 190     "res_counter_ptr" and the "amount" in release_something() should be
 191     the same as they were in the alloc_something() when the releasing
 192     resource was allocated.
 193
 194  c. Provide the way to read res_counter values and set them (the cgroups
 195     still can help with it).
 196
 197  c. Compile and run :)