Subject: [RFC/PATCH] arm: do not skip SMP init calls on SMP_ON_UP case

From: [email-masked] (Nikita Yushchenko)
Date: Mon, 30 Nov 2015 11:25:15 +0300

28.11.2015 14:13, Nikita Yushchenko ?????:
>>> Not sure I understand logic behind this. With the current code,
>>> resulting cpu_possible_mask depends on CONFIG_SMP_ON_UP:
>>> - if it is set, cpu_possible_mask contains (0 1), as initialized in
>>> arm_dt_init_cpu_maps()
>>> - if it is not set, cpu_possible_mask contains (0), since
>>> imx_smp_init_cpus() removes 1 from there.
>>
>> Right, adding debug to arch/arm/kernel/setup.c, just before the
>> "if (is_smp())" shows:
>>
>> is_smp() 0 possible 3 present 1 online 1
>>
>> which is totally wrong: if is_smp() is false, we should not be setting
>> up any possible CPUs.  See a patch below to fix that.
>>
>> However, this doesn't matter much, because the code in setup.c won't
>> initialise the SMP operations struct ...
> 
> But cpu start code is not the only place in the kernel that uses cpu_present_mask.
> 
> Are you sure that running with invalid cpu_present_mask has no side effects?

At least LTP suite does not like it:
while running /opt/ltp/runtest/cpuhotplug, we see things like the above

<<<test_start>>>
tag=cpuhotplug02 stime=1446628761
cmdline="cpuhotplug02.sh -c 1 -l 1"
contacts=""
analysis=exit
<<<test_output>>>
Name:   cpuhotplug02
Date:   Wed Nov  4 09:19:21 UTC 2015
Desc:   What happens to a process when its CPU is offlined?

CPU is 1
/opt/ltp/testcases/bin/cpuhotplug_hotplug.sh: line 76: echo: write error:
Function not implemented
cpuhotplug02 1 TBROK : CPU1 cannot be onlined
<<<execution_status>>>
initiation_status="ok"
duration=1 termination_type=exited termination_id=2 corefile=no
cutime=4 cstime=6
<<<test_end>>>