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