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. int res_counter_charge_locked
  80                         (struct res_counter *rc, unsigned long val, bool force)
  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). The force parameter indicates whether we can bypass the limit.
  85
  86  e. u64 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". The return value of this function indicate the amount
  92         of charges still present in the counter.
  93
  94         The _locked routines imply that the res_counter->lock is taken.
  95
  96  f. u64 res_counter_uncharge_until
  97                 (struct res_counter *rc, struct res_counter *top,
  98                  unsinged long val)
  99
 100         Almost same as res_cunter_uncharge() but propagation of uncharge
 101         stops when rc == top. This is useful when kill a res_coutner in
 102         child cgroup.
 103
 104  2.1 Other accounting routines
 105
 106     There are more routines that may help you with common needs, like
 107     checking whether the limit is reached or resetting the max_usage
 108     value. They are all declared in include/linux/res_counter.h.
 109
 110
 111
 112 3. Analyzing the resource counter registrations
 113
 114  a. If the failcnt value constantly grows, this means that the counter's
 115     limit is too tight. Either the group is misbehaving and consumes too
 116     many resources, or the configuration is not suitable for the group
 117     and the limit should be increased.
 118
 119  b. The max_usage value can be used to quickly tune the group. One may
 120     set the limits to maximal values and either load the container with
 121     a common pattern or leave one for a while. After this the max_usage
 122     value shows the amount of memory the container would require during
 123     its common activity.
 124
 125     Setting the limit a bit above this value gives a pretty good
 126     configuration that works in most of the cases.
 127
 128  c. If the max_usage is much less than the limit, but the failcnt value
 129     is growing, then the group tries to allocate a big chunk of resource
 130     at once.
 131
 132  d. If the max_usage is much less than the limit, but the failcnt value
 133     is 0, then this group is given too high limit, that it does not
 134     require. It is better to lower the limit a bit leaving more resource
 135     for other groups.
 136
 137
 138
 139 4. Communication with the control groups subsystem (cgroups)
 140
 141 All the resource controllers that are using cgroups and resource counters
 142 should provide files (in the cgroup filesystem) to work with the resource
 143 counter fields. They are recommended to adhere to the following rules:
 144
 145  a. File names
 146
 147         Field name      File name
 148         ---------------------------------------------------
 149         usage           usage_in_<unit_of_measurement>
 150         max_usage       max_usage_in_<unit_of_measurement>
 151         limit           limit_in_<unit_of_measurement>
 152         failcnt         failcnt
 153         lock            no file :)
 154
 155  b. Reading from file should show the corresponding field value in the
 156     appropriate format.
 157
 158  c. Writing to file
 159
 160         Field           Expected behavior
 161         ----------------------------------
 162         usage           prohibited
 163         max_usage       reset to usage
 164         limit           set the limit
 165         failcnt         reset to zero
 166
 167
 168
 169 5. Usage example
 170
 171  a. Declare a task group (take a look at cgroups subsystem for this) and
 172     fold a res_counter into it
 173
 174         struct my_group {
 175                 struct res_counter res;
 176
 177                 <other fields>
 178         }
 179
 180  b. Put hooks in resource allocation/release paths
 181
 182         int alloc_something(...)
 183         {
 184                 if (res_counter_charge(res_counter_ptr, amount) < 0)
 185                         return -ENOMEM;
 186
 187                 <allocate the resource and return to the caller>
 188         }
 189
 190         void release_something(...)
 191         {
 192                 res_counter_uncharge(res_counter_ptr, amount);
 193
 194                 <release the resource>
 195         }
 196
 197     In order to keep the usage value self-consistent, both the
 198     "res_counter_ptr" and the "amount" in release_something() should be
 199     the same as they were in the alloc_something() when the releasing
 200     resource was allocated.
 201
 202  c. Provide the way to read res_counter values and set them (the cgroups
 203     still can help with it).
 204
 205  c. Compile and run :)