Hi guys,
The question
------------------
What are the possible drawbacks of enabling CONFIG_HOTPLUG_CPU for CPU
that does not have hardware support for hot-plug?
The question I'd like to ask is architecture agnostic, but the described
behavior
is observed on MPCore Cortex-A9 CPU with Linux 3.4.59.
The issue
-------------
When Linux Kernel compiled in SMP mode, and CONFIG_HOTPLUG_CPU is not set,
and booted on single core CPU, then warning messages
"... task blocked for more than 120 seconds ..." starts popping up in dmesg log.
For example:
INFO: task ksoftirqd/1:9 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
To make the message disappear, CONFIG_HOTPLUG_CPU should be enabled.
Following the example of "ksoftirqd", the root cause of the issue is that
there are as many "ksoftirqd" threads created as CONFIG_NR_CPU
(see cpu_present_mask in kernel/cpu.c file). See below some details on
how "ksoftirqd" task is created and why it is not killed.
Now, the "ksoftirqd" task is *not* killed and just stays around in task queue
till the scheduler shouts "... task blocked ...".
Details:
----------
The first "ksoftirqd" task (for CPU[0]) is created as part of executing
the registered early_initcall function spawn_ksoftirqd() in the below
flow:
start_kernel() ---> rest_init() ---> kernel_init() --->
do_pre_smp_initcalls() ---> spawd_ksoftirqd() --->
cpu_callback(... CPU_ONLINE ...)
The "ksoftirqd" tasks for CPU[1 .. N-1] are created in the different
flow.
First of all, cpu_callback() from kernel/softirq.c ("ksoftirqd" task is
created in this callback) is registered through CPU notifier in
spawn_ksoftirqd(). Then this callback is called in the below flow:
start_kernel() ---> rest_init() ---> kernel_init() --->
smp_init() ---> for_each_present_cpu(cpu) { cpu_up(cpu) --->
_cpu_up() ---> __cpu_nofity(CPU_UP_PREPARE) (here "ksoftirqd" task
is created).
Right after that, CPU[x] is attempted to be enabled (using __cpu_up)
and,
if __cpu_up(cpu) fails, then "ksoftirqd" task is killed using
__cpu_notify(CPU_UP_CANCELLED) some lines below.
Now, the code that actually kills the task (using kthread_stop) is
wrapped up
with #ifdef CONFIG_HOTPLUG_CPU (see kernel/softirq.c, function
cpu_callback).
The solution
-----------------
The easy solution is to enable CONFIG_HOTPLUG_CPU which enables
the compilation of the code that kills "ksoftirqd" task.
What is the possible drawback of enabling CONFIG_HOTPLUG_CPU for CPU
that does not have hardware support hot-plug?
Thanks,
--- KostaZ
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