288 lines
13 KiB
ReStructuredText
288 lines
13 KiB
ReStructuredText
====================================================================
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Interaction of Suspend code (S3) with the CPU hotplug infrastructure
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====================================================================
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(C) 2011 - 2014 Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
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I. Differences between CPU hotplug and Suspend-to-RAM
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======================================================
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How does the regular CPU hotplug code differ from how the Suspend-to-RAM
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infrastructure uses it internally? And where do they share common code?
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Well, a picture is worth a thousand words... So ASCII art follows :-)
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[This depicts the current design in the kernel, and focusses only on the
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interactions involving the freezer and CPU hotplug and also tries to explain
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the locking involved. It outlines the notifications involved as well.
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But please note that here, only the call paths are illustrated, with the aim
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of describing where they take different paths and where they share code.
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What happens when regular CPU hotplug and Suspend-to-RAM race with each other
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is not depicted here.]
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On a high level, the suspend-resume cycle goes like this::
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|Freeze| -> |Disable nonboot| -> |Do suspend| -> |Enable nonboot| -> |Thaw |
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|tasks | | cpus | | | | cpus | |tasks|
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More details follow::
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Suspend call path
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-----------------
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Write 'mem' to
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/sys/power/state
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sysfs file
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v
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Acquire system_transition_mutex lock
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v
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Send PM_SUSPEND_PREPARE
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notifications
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v
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Freeze tasks
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v
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freeze_secondary_cpus()
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/* start */
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v
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Acquire cpu_add_remove_lock
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v
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Iterate over CURRENTLY
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online CPUs
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| ----------
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v | L
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======> _cpu_down() |
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| [This takes cpuhotplug.lock |
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Common | before taking down the CPU |
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code | and releases it when done] | O
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| While it is at it, notifications |
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| are sent when notable events occur, |
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======> by running all registered callbacks. |
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| | O
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v |
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Note down these cpus in | P
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frozen_cpus mask ----------
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v
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Disable regular cpu hotplug
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by increasing cpu_hotplug_disabled
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v
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Release cpu_add_remove_lock
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v
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/* freeze_secondary_cpus() complete */
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v
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Do suspend
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Resuming back is likewise, with the counterparts being (in the order of
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execution during resume):
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* thaw_secondary_cpus() which involves::
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| Acquire cpu_add_remove_lock
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| Decrease cpu_hotplug_disabled, thereby enabling regular cpu hotplug
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| Call _cpu_up() [for all those cpus in the frozen_cpus mask, in a loop]
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| Release cpu_add_remove_lock
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v
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* thaw tasks
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* send PM_POST_SUSPEND notifications
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* Release system_transition_mutex lock.
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It is to be noted here that the system_transition_mutex lock is acquired at the
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very beginning, when we are just starting out to suspend, and then released only
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after the entire cycle is complete (i.e., suspend + resume).
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::
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Regular CPU hotplug call path
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-----------------------------
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Write 0 (or 1) to
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/sys/devices/system/cpu/cpu*/online
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sysfs file
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v
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cpu_down()
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v
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Acquire cpu_add_remove_lock
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v
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If cpu_hotplug_disabled > 0
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return gracefully
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v
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======> _cpu_down()
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| [This takes cpuhotplug.lock
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Common | before taking down the CPU
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code | and releases it when done]
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| While it is at it, notifications
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| are sent when notable events occur,
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======> by running all registered callbacks.
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v
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Release cpu_add_remove_lock
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[That's it!, for
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regular CPU hotplug]
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So, as can be seen from the two diagrams (the parts marked as "Common code"),
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regular CPU hotplug and the suspend code path converge at the _cpu_down() and
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_cpu_up() functions. They differ in the arguments passed to these functions,
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in that during regular CPU hotplug, 0 is passed for the 'tasks_frozen'
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argument. But during suspend, since the tasks are already frozen by the time
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the non-boot CPUs are offlined or onlined, the _cpu_*() functions are called
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with the 'tasks_frozen' argument set to 1.
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[See below for some known issues regarding this.]
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Important files and functions/entry points:
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-------------------------------------------
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- kernel/power/process.c : freeze_processes(), thaw_processes()
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- kernel/power/suspend.c : suspend_prepare(), suspend_enter(), suspend_finish()
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- kernel/cpu.c: cpu_[up|down](), _cpu_[up|down](),
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[disable|enable]_nonboot_cpus()
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II. What are the issues involved in CPU hotplug?
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------------------------------------------------
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There are some interesting situations involving CPU hotplug and microcode
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update on the CPUs, as discussed below:
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[Please bear in mind that the kernel requests the microcode images from
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userspace, using the request_firmware() function defined in
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drivers/base/firmware_loader/main.c]
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a. When all the CPUs are identical:
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This is the most common situation and it is quite straightforward: we want
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to apply the same microcode revision to each of the CPUs.
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To give an example of x86, the collect_cpu_info() function defined in
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arch/x86/kernel/microcode_core.c helps in discovering the type of the CPU
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and thereby in applying the correct microcode revision to it.
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But note that the kernel does not maintain a common microcode image for the
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all CPUs, in order to handle case 'b' described below.
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b. When some of the CPUs are different than the rest:
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In this case since we probably need to apply different microcode revisions
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to different CPUs, the kernel maintains a copy of the correct microcode
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image for each CPU (after appropriate CPU type/model discovery using
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functions such as collect_cpu_info()).
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c. When a CPU is physically hot-unplugged and a new (and possibly different
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type of) CPU is hot-plugged into the system:
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In the current design of the kernel, whenever a CPU is taken offline during
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a regular CPU hotplug operation, upon receiving the CPU_DEAD notification
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(which is sent by the CPU hotplug code), the microcode update driver's
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callback for that event reacts by freeing the kernel's copy of the
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microcode image for that CPU.
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Hence, when a new CPU is brought online, since the kernel finds that it
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doesn't have the microcode image, it does the CPU type/model discovery
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afresh and then requests the userspace for the appropriate microcode image
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for that CPU, which is subsequently applied.
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For example, in x86, the mc_cpu_callback() function (which is the microcode
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update driver's callback registered for CPU hotplug events) calls
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microcode_update_cpu() which would call microcode_init_cpu() in this case,
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instead of microcode_resume_cpu() when it finds that the kernel doesn't
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have a valid microcode image. This ensures that the CPU type/model
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discovery is performed and the right microcode is applied to the CPU after
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getting it from userspace.
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d. Handling microcode update during suspend/hibernate:
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Strictly speaking, during a CPU hotplug operation which does not involve
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physically removing or inserting CPUs, the CPUs are not actually powered
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off during a CPU offline. They are just put to the lowest C-states possible.
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Hence, in such a case, it is not really necessary to re-apply microcode
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when the CPUs are brought back online, since they wouldn't have lost the
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image during the CPU offline operation.
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This is the usual scenario encountered during a resume after a suspend.
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However, in the case of hibernation, since all the CPUs are completely
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powered off, during restore it becomes necessary to apply the microcode
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images to all the CPUs.
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[Note that we don't expect someone to physically pull out nodes and insert
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nodes with a different type of CPUs in-between a suspend-resume or a
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hibernate/restore cycle.]
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In the current design of the kernel however, during a CPU offline operation
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as part of the suspend/hibernate cycle (cpuhp_tasks_frozen is set),
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the existing copy of microcode image in the kernel is not freed up.
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And during the CPU online operations (during resume/restore), since the
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kernel finds that it already has copies of the microcode images for all the
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CPUs, it just applies them to the CPUs, avoiding any re-discovery of CPU
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type/model and the need for validating whether the microcode revisions are
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right for the CPUs or not (due to the above assumption that physical CPU
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hotplug will not be done in-between suspend/resume or hibernate/restore
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cycles).
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III. Known problems
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===================
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Are there any known problems when regular CPU hotplug and suspend race
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with each other?
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Yes, they are listed below:
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1. When invoking regular CPU hotplug, the 'tasks_frozen' argument passed to
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the _cpu_down() and _cpu_up() functions is *always* 0.
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This might not reflect the true current state of the system, since the
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tasks could have been frozen by an out-of-band event such as a suspend
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operation in progress. Hence, the cpuhp_tasks_frozen variable will not
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reflect the frozen state and the CPU hotplug callbacks which evaluate
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that variable might execute the wrong code path.
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2. If a regular CPU hotplug stress test happens to race with the freezer due
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to a suspend operation in progress at the same time, then we could hit the
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situation described below:
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* A regular cpu online operation continues its journey from userspace
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into the kernel, since the freezing has not yet begun.
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* Then freezer gets to work and freezes userspace.
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* If cpu online has not yet completed the microcode update stuff by now,
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it will now start waiting on the frozen userspace in the
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TASK_UNINTERRUPTIBLE state, in order to get the microcode image.
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* Now the freezer continues and tries to freeze the remaining tasks. But
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due to this wait mentioned above, the freezer won't be able to freeze
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the cpu online hotplug task and hence freezing of tasks fails.
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As a result of this task freezing failure, the suspend operation gets
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aborted.
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