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

From: Nikita Yushchenko <nyushchenko@dev.rtsoft.ru>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: kuznetsovg@dev.rtsoft.ru,
	Vladimir Murzin <vladimir.murzin@arm.com>,
	Ian Campbell <ian.campbell@citrix.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Mason <slash.tmp@free.fr>, Will Deacon <will.deacon@arm.com>,
	Paul Kocialkowski <contact@paulk.fr>,
	linux-kernel@vger.kernel.org,
	Masahiro Yamada <yamada.m@jp.panasonic.com>,
	Pavel Machek <pavel@ucw.cz>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC/PATCH] arm: do not skip SMP init calls on SMP_ON_UP case
Date: Mon, 30 Nov 2015 11:25:15 +0300	[thread overview]
Message-ID: <565C07EB.1060306@dev.rtsoft.ru> (raw)
In-Reply-To: <56598C59.4070307@dev.rtsoft.ru>

28.11.2015 14:13, Nikita Yushchenko пишет:
>>> 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>>>