Documentation/power/suspend-and-cpuhotplug.rst

   1 ====================================================================
   2 Interaction of Suspend code (S3) with the CPU hotplug infrastructure
   3 ====================================================================
   4
   5 (C) 2011 - 2014 Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
   6
   7
   8 I. Differences between CPU hotplug and Suspend-to-RAM
   9 ======================================================
  10
  11 How does the regular CPU hotplug code differ from how the Suspend-to-RAM
  12 infrastructure uses it internally? And where do they share common code?
  13
  14 Well, a picture is worth a thousand words... So ASCII art follows :-)
  15
  16 [This depicts the current design in the kernel, and focusses only on the
  17 interactions involving the freezer and CPU hotplug and also tries to explain
  18 the locking involved. It outlines the notifications involved as well.
  19 But please note that here, only the call paths are illustrated, with the aim
  20 of describing where they take different paths and where they share code.
  21 What happens when regular CPU hotplug and Suspend-to-RAM race with each other
  22 is not depicted here.]
  23
  24 On a high level, the suspend-resume cycle goes like this::
  25
  26   |Freeze| -> |Disable nonboot| -> |Do suspend| -> |Enable nonboot| -> |Thaw |
  27   |tasks |    |     cpus      |    |          |    |     cpus     |    |tasks|
  28
  29
  30 More details follow::
  31
  32                                 Suspend call path
  33                                 -----------------
  34
  35                                   Write 'mem' to
  36                                 /sys/power/state
  37                                     sysfs file
  38                                         |
  39                                         v
  40                                Acquire system_transition_mutex lock
  41                                         |
  42                                         v
  43                              Send PM_SUSPEND_PREPARE
  44                                    notifications
  45                                         |
  46                                         v
  47                                    Freeze tasks
  48                                         |
  49                                         |
  50                                         v
  51                               freeze_secondary_cpus()
  52                                    /* start */
  53                                         |
  54                                         v
  55                             Acquire cpu_add_remove_lock
  56                                         |
  57                                         v
  58                              Iterate over CURRENTLY
  59                                    online CPUs
  60                                         |
  61                                         |
  62                                         |                ----------
  63                                         v                          | L
  64              ======>               _cpu_down()                     |
  65             |              [This takes cpuhotplug.lock             |
  66   Common    |               before taking down the CPU             |
  67    code     |               and releases it when done]             | O
  68             |            While it is at it, notifications          |
  69             |            are sent when notable events occur,       |
  70              ======>     by running all registered callbacks.      |
  71                                         |                          | O
  72                                         |                          |
  73                                         |                          |
  74                                         v                          |
  75                             Note down these cpus in                | P
  76                                 frozen_cpus mask         ----------
  77                                         |
  78                                         v
  79                            Disable regular cpu hotplug
  80                         by increasing cpu_hotplug_disabled
  81                                         |
  82                                         v
  83                             Release cpu_add_remove_lock
  84                                         |
  85                                         v
  86                        /* freeze_secondary_cpus() complete */
  87                                         |
  88                                         v
  89                                    Do suspend
  90
  91
  92
  93 Resuming back is likewise, with the counterparts being (in the order of
  94 execution during resume):
  95
  96 * thaw_secondary_cpus() which involves::
  97
  98    |  Acquire cpu_add_remove_lock
  99    |  Decrease cpu_hotplug_disabled, thereby enabling regular cpu hotplug
 100    |  Call _cpu_up() [for all those cpus in the frozen_cpus mask, in a loop]
 101    |  Release cpu_add_remove_lock
 102    v
 103
 104 * thaw tasks
 105 * send PM_POST_SUSPEND notifications
 106 * Release system_transition_mutex lock.
 107
 108
 109 It is to be noted here that the system_transition_mutex lock is acquired at the
 110 very beginning, when we are just starting out to suspend, and then released only
 111 after the entire cycle is complete (i.e., suspend + resume).
 112
 113 ::
 114
 115
 116
 117                           Regular CPU hotplug call path
 118                           -----------------------------
 119
 120                                 Write 0 (or 1) to
 121                        /sys/devices/system/cpu/cpu*/online
 122                                     sysfs file
 123                                         |
 124                                         |
 125                                         v
 126                                     cpu_down()
 127                                         |
 128                                         v
 129                            Acquire cpu_add_remove_lock
 130                                         |
 131                                         v
 132                           If cpu_hotplug_disabled > 0
 133                                 return gracefully
 134                                         |
 135                                         |
 136                                         v
 137              ======>                _cpu_down()
 138             |              [This takes cpuhotplug.lock
 139   Common    |               before taking down the CPU
 140    code     |               and releases it when done]
 141             |            While it is at it, notifications
 142             |           are sent when notable events occur,
 143              ======>    by running all registered callbacks.
 144                                         |
 145                                         |
 146                                         v
 147                           Release cpu_add_remove_lock
 148                                [That's it!, for
 149                               regular CPU hotplug]
 150
 151
 152
 153 So, as can be seen from the two diagrams (the parts marked as "Common code"),
 154 regular CPU hotplug and the suspend code path converge at the _cpu_down() and
 155 _cpu_up() functions. They differ in the arguments passed to these functions,
 156 in that during regular CPU hotplug, 0 is passed for the 'tasks_frozen'
 157 argument. But during suspend, since the tasks are already frozen by the time
 158 the non-boot CPUs are offlined or onlined, the _cpu_*() functions are called
 159 with the 'tasks_frozen' argument set to 1.
 160 [See below for some known issues regarding this.]
 161
 162
 163 Important files and functions/entry points:
 164 -------------------------------------------
 165
 166 - kernel/power/process.c : freeze_processes(), thaw_processes()
 167 - kernel/power/suspend.c : suspend_prepare(), suspend_enter(), suspend_finish()
 168 - kernel/cpu.c: cpu_[up|down](), _cpu_[up|down](),
 169   [disable|enable]_nonboot_cpus()
 170
 171
 172
 173 II. What are the issues involved in CPU hotplug?
 174 ------------------------------------------------
 175
 176 There are some interesting situations involving CPU hotplug and microcode
 177 update on the CPUs, as discussed below:
 178
 179 [Please bear in mind that the kernel requests the microcode images from
 180 userspace, using the request_firmware() function defined in
 181 drivers/base/firmware_loader/main.c]
 182
 183
 184 a. When all the CPUs are identical:
 185
 186    This is the most common situation and it is quite straightforward: we want
 187    to apply the same microcode revision to each of the CPUs.
 188    To give an example of x86, the collect_cpu_info() function defined in
 189    arch/x86/kernel/microcode_core.c helps in discovering the type of the CPU
 190    and thereby in applying the correct microcode revision to it.
 191    But note that the kernel does not maintain a common microcode image for the
 192    all CPUs, in order to handle case 'b' described below.
 193
 194
 195 b. When some of the CPUs are different than the rest:
 196
 197    In this case since we probably need to apply different microcode revisions
 198    to different CPUs, the kernel maintains a copy of the correct microcode
 199    image for each CPU (after appropriate CPU type/model discovery using
 200    functions such as collect_cpu_info()).
 201
 202
 203 c. When a CPU is physically hot-unplugged and a new (and possibly different
 204    type of) CPU is hot-plugged into the system:
 205
 206    In the current design of the kernel, whenever a CPU is taken offline during
 207    a regular CPU hotplug operation, upon receiving the CPU_DEAD notification
 208    (which is sent by the CPU hotplug code), the microcode update driver's
 209    callback for that event reacts by freeing the kernel's copy of the
 210    microcode image for that CPU.
 211
 212    Hence, when a new CPU is brought online, since the kernel finds that it
 213    doesn't have the microcode image, it does the CPU type/model discovery
 214    afresh and then requests the userspace for the appropriate microcode image
 215    for that CPU, which is subsequently applied.
 216
 217    For example, in x86, the mc_cpu_callback() function (which is the microcode
 218    update driver's callback registered for CPU hotplug events) calls
 219    microcode_update_cpu() which would call microcode_init_cpu() in this case,
 220    instead of microcode_resume_cpu() when it finds that the kernel doesn't
 221    have a valid microcode image. This ensures that the CPU type/model
 222    discovery is performed and the right microcode is applied to the CPU after
 223    getting it from userspace.
 224
 225
 226 d. Handling microcode update during suspend/hibernate:
 227
 228    Strictly speaking, during a CPU hotplug operation which does not involve
 229    physically removing or inserting CPUs, the CPUs are not actually powered
 230    off during a CPU offline. They are just put to the lowest C-states possible.
 231    Hence, in such a case, it is not really necessary to re-apply microcode
 232    when the CPUs are brought back online, since they wouldn't have lost the
 233    image during the CPU offline operation.
 234
 235    This is the usual scenario encountered during a resume after a suspend.
 236    However, in the case of hibernation, since all the CPUs are completely
 237    powered off, during restore it becomes necessary to apply the microcode
 238    images to all the CPUs.
 239
 240    [Note that we don't expect someone to physically pull out nodes and insert
 241    nodes with a different type of CPUs in-between a suspend-resume or a
 242    hibernate/restore cycle.]
 243
 244    In the current design of the kernel however, during a CPU offline operation
 245    as part of the suspend/hibernate cycle (cpuhp_tasks_frozen is set),
 246    the existing copy of microcode image in the kernel is not freed up.
 247    And during the CPU online operations (during resume/restore), since the
 248    kernel finds that it already has copies of the microcode images for all the
 249    CPUs, it just applies them to the CPUs, avoiding any re-discovery of CPU
 250    type/model and the need for validating whether the microcode revisions are
 251    right for the CPUs or not (due to the above assumption that physical CPU
 252    hotplug will not be done in-between suspend/resume or hibernate/restore
 253    cycles).
 254
 255
 256 III. Known problems
 257 ===================
 258
 259 Are there any known problems when regular CPU hotplug and suspend race
 260 with each other?
 261
 262 Yes, they are listed below:
 263
 264 1. When invoking regular CPU hotplug, the 'tasks_frozen' argument passed to
 265    the _cpu_down() and _cpu_up() functions is *always* 0.
 266    This might not reflect the true current state of the system, since the
 267    tasks could have been frozen by an out-of-band event such as a suspend
 268    operation in progress. Hence, the cpuhp_tasks_frozen variable will not
 269    reflect the frozen state and the CPU hotplug callbacks which evaluate
 270    that variable might execute the wrong code path.
 271
 272 2. If a regular CPU hotplug stress test happens to race with the freezer due
 273    to a suspend operation in progress at the same time, then we could hit the
 274    situation described below:
 275
 276     * A regular cpu online operation continues its journey from userspace
 277       into the kernel, since the freezing has not yet begun.
 278     * Then freezer gets to work and freezes userspace.
 279     * If cpu online has not yet completed the microcode update stuff by now,
 280       it will now start waiting on the frozen userspace in the
 281       TASK_UNINTERRUPTIBLE state, in order to get the microcode image.
 282     * Now the freezer continues and tries to freeze the remaining tasks. But
 283       due to this wait mentioned above, the freezer won't be able to freeze
 284       the cpu online hotplug task and hence freezing of tasks fails.
 285
 286    As a result of this task freezing failure, the suspend operation gets
 287    aborted.