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SubjectRe: [PATCH] Documentation/power: Update docs about suspend and CPU hotplug
On 10/12/2011 03:32 AM, Rafael J. Wysocki wrote:
> On Tuesday, October 11, 2011, Srivatsa S. Bhat wrote:
>> Update the documentation about the interaction between the suspend (S3) call
>> path and the CPU hotplug infrastructure.
>> This patch focusses only on the activities of the freezer, cpu hotplug and
>> the notifications involved. It outlines how regular CPU hotplug differs from
>> the way it is invoked during suspend and also tries to explain the locking
>> involved.
>>
>> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
>> ---
>>
>> Documentation/power/00-INDEX | 2
>> Documentation/power/suspend-and-cpuhotplug.txt | 113 ++++++++++++++++++++++++
>> 2 files changed, 115 insertions(+), 0 deletions(-)
>> create mode 100644 Documentation/power/suspend-and-cpuhotplug.txt
>>
>> diff --git a/Documentation/power/00-INDEX b/Documentation/power/00-INDEX
>> index 45e9d4a..a4d682f 100644
>> --- a/Documentation/power/00-INDEX
>> +++ b/Documentation/power/00-INDEX
>> @@ -26,6 +26,8 @@ s2ram.txt
>> - How to get suspend to ram working (and debug it when it isn't)
>> states.txt
>> - System power management states
>> +suspend-and-cpuhotplug.txt
>> + - Explains the interaction between Suspend-to-RAM (S3) and CPU hotplug
>> swsusp-and-swap-files.txt
>> - Using swap files with software suspend (to disk)
>> swsusp-dmcrypt.txt
>> diff --git a/Documentation/power/suspend-and-cpuhotplug.txt b/Documentation/power/suspend-and-cpuhotplug.txt
>> new file mode 100644
>> index 0000000..d0ba411
>> --- /dev/null
>> +++ b/Documentation/power/suspend-and-cpuhotplug.txt
>> @@ -0,0 +1,113 @@
>> +Interaction of Suspend code (S3) with the CPU hotplug infrastructure
>> + (C) 2011 Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>, GPL
>> +
>> +
>> +I. How does the Suspend-to-RAM code interact with CPU hotplug infrastructure?
>> +
>> +Well, a picture speaks more than a thousand words... So ASCII art follows :-)
>> +
>> +[This depicts the current design in the kernel, and focusses only on the
>> +interactions between suspend call paths involving the freezer and cpu hotplug
>> +and also tries to explain the locking involved. It also outlines the
>> +notifications involved.]
>> +
>> +On a high level, the suspend-resume cycle goes like this:
>> +
>> +|Freeze| -> |Disable nonboot| -> |Do suspend| -> |Enable nonboot| -> |Thaw |
>> +|tasks | | cpus | | | | cpus | |tasks|
>> +
>> +
>> +More details follow:
>> +
>> +Regular CPU hotplug Suspend call path
>> +------------------- ---------------------------
>> +
>> +Write 0 (or 1) to Write 'mem' to
>> +/sys/devices/system/cpu/cpu*/online /sys/power/state
>> + sysfs file syfs file
>> + | |
>> + | v
>> + | Acquire pm_mutex lock
>> + | |
>> + | v
>> + | Send PM_SUSPEND_PREPARE notifications
>> + | |
>> + | v
>> + | Freeze tasks
>
> OK, so something appears to be missing here. Namely, the task writing to
> /sys/devices/system/cpu/cpu*/online should be frozen at this point or
> suspend should be aborted. I suppose neither of these happens and I wonder
> why exactly.
>

I have a couple of clarifications to make here:
* Firstly, this picture is not meant to represent what happens when regular
cpu hotplug and suspend run together. That race condition has not been
brought out here. What it does try to explain is, how the regular cpu
hotplug path is different from suspend, and where they share common code.
Please don't think about timing/race condition when reading it. Its just
meant to explain the call path and locking involved.

* Secondly, this picture explains the *current* design, and *not* the mutual
exclusion design I have proposed between regular cpu hotplug and suspend.
The reason being, this doc was written to help everyone understand the
current locking schemes, to help evaluate my proposal for a different
scheme (mutual exclusion).

Now, coming to your point, if that task writing to the sysfs file has not
been frozen, then the current kernel doesn't abort suspend, which is why we are
encountering problems, and which is exactly what my patchset tries to solve.
Link to my patchset:
http://thread.gmane.org/gmane.linux.documentation/3414/focus=3414

>
>> + | |
>> + | |
>> + v v
>> + cpu_down() disable_nonboot_cpus() /*start*/
>> + | |
>> + v v
>> +Acquire cpu_add_remove_lock Acquire cpu_add_remove_lock
>> + | |
>> + v v
>> +If cpu_hotplug_disabled is 1 Iterate over CURRENTLY online CPUs
>> + return gracefully |
>> + | |
>> + | | ----
>> + v v |
>> + \ / |
>> + -------- -------- |
>> + \ / |
>> + -------- -------- |L
>> + \____/ |
>> + | |
>> + v |O
>> + _cpu_down() |
>> + [This takes cpuhotplug.lock |
>> + before taking down the CPU |
>> + and releases it when done] |O
>> + While it is at it, notifications |
>> + are sent when notable events occur, |
>> + by running all registered callbacks. |
>> + | |O
>> + / \ |
>> + / \ |
>> + < > |
>> + _______________________/ \_____________________ |P
>> + | | |
>> + v v |
>> +Release cpu_add_remove_lock Note down these cpus in |
>> +[That's it!, for frozen_cpus mask ----
>> + regular CPU hotplug] |
>> + v
>> + Disable regular cpu hotplug
>> + by setting cpu_hotplug_disabled=1
>> + |
>> + v
>> + Release cpu_add_remove_lock
>> + |
>> + v
>> + /* disable_nonboot_cpus() complete */
>> + |
>> + v
>> + Do suspend
>> +
>> +
>> +Resuming back is likewise, with the counterparts being (in the order of
>> +execution during resume):
>> +* enable_nonboot_cpus() which involves:
>> + | Acquire cpu_add_remove_lock
>> + | Reset cpu_hotplug_disabled to 0, thereby enabling regular cpu hotplug
>> + | Call _cpu_up() [for all those cpus in the frozen_cpus mask, in a loop]
>> + | Release cpu_add_remove_lock
>> + v
>> +
>> +* thaw tasks
>> +* send PM_POST_SUSPEND notifications
>> +* Release pm_mutex lock.
>> +
>> +It is to be noted here that the pm_mutex lock is acquired at the very
>> +beginning, when we are just starting out to suspend, and then released only
>> +after the entire cycle is complete (i.e., suspend + resume).
>> +
>> +
>> +Important files and functions/entry points:
>> +------------------------------------------
>> +
>> +kernel/power/process.c : freeze_processes(), thaw_processes()
>> +kernel/power/suspend.c : suspend_prepare(), suspend_enter(), suspend_finish()
>> +kernel/cpu.c: cpu_[up|down](), _cpu_[up|down](), [disable|enable]_nonboot_cpus()
>> +
>>
>>
>>
>


--
Regards,
Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Linux Technology Center,
IBM India Systems and Technology Lab



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