Re: [PATCH V2] MIPS: CPU#0 is not hotpluggable

From: Huacai Chen <chenhuacai@gmail.com>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	"open list:MIPS" <linux-mips@vger.kernel.org>,
	Fuxin Zhang <zhangfx@lemote.com>,
	Zhangjin Wu <wuzhangjin@gmail.com>,
	Jiaxun Yang <jiaxun.yang@flygoat.com>,
	stable <stable@vger.kernel.org>
Subject: Re: [PATCH V2] MIPS: CPU#0 is not hotpluggable
Date: Sat, 25 Jul 2020 16:29:28 +0800	[thread overview]
Message-ID: <CAAhV-H7C3-uro-UD6voeamcECQHo1PYNBiyGyHLPDyEAJUm98w@mail.gmail.com> (raw)
In-Reply-To: <20200725074521.GA347597@kroah.com>

Hi, Greg,

On Sat, Jul 25, 2020 at 3:45 PM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Sat, Jul 25, 2020 at 02:57:31PM +0800, Huacai Chen wrote:
> > Hi Greg,
> >
> > On Sat, Jul 25, 2020 at 2:49 PM Greg KH <gregkh@linuxfoundation.org> wrote:
> > >
> > > On Sat, Jul 25, 2020 at 02:37:52PM +0800, Huacai Chen wrote:
> > > > Hi, Thomas,
> > > >
> > > > What do you think about this patch? Other archs also do the same thing
> > > > except those support hotplug CPU#0.
> > > >
> > > > grep hotpluggable arch -rwI
> > > > arch/riscv/kernel/setup.c:        cpu->hotpluggable = cpu_has_hotplug(i);
> > > > arch/powerpc/kernel/sysfs.c:    BUG_ON(!c->hotpluggable);
> > > > arch/powerpc/kernel/sysfs.c:            c->hotpluggable = 1;
> > > > arch/powerpc/kernel/sysfs.c:        if (cpu_online(cpu) || c->hotpluggable) {
> > > > arch/arm/kernel/setup.c:        cpuinfo->cpu.hotpluggable =
> > > > platform_can_hotplug_cpu(cpu);
> > > > arch/sh/kernel/topology.c:        c->hotpluggable = 1;
> > > > arch/ia64/kernel/topology.c:     * CPEI target, then it is hotpluggable
> > > > arch/ia64/kernel/topology.c:        sysfs_cpus[num].cpu.hotpluggable = 1;
> > > > arch/xtensa/kernel/setup.c:        cpu->hotpluggable = !!i;
> > > > arch/s390/kernel/smp.c:    c->hotpluggable = 1;
> > > > arch/mips/kernel/topology.c:        c->hotpluggable = 1;
> > > > arch/arm64/kernel/cpuinfo.c: * In case the boot CPU is hotpluggable,
> > > > we record its initial state and
> > > > arch/arm64/kernel/setup.c:        cpu->hotpluggable = cpu_can_disable(i);
> > > > arch/x86/kernel/topology.c:        per_cpu(cpu_devices,
> > > > num).cpu.hotpluggable = 1;
> > > >
> > > > On Thu, Jul 16, 2020 at 6:38 PM Huacai Chen <chenhc@lemote.com> wrote:
> > > > >
> > > > > Now CPU#0 is not hotpluggable on MIPS, so prevent to create /sys/devices
> > > > > /system/cpu/cpu0/online which confuses some user-space tools.
> > >
> > > What userspace tools are confused by this?  They should be able to
> > > handle a cpu not being able to be removed, right?
> > It causes ltp's "hotplug" test fails, and ltp considers CPUs with a
> > "online" node be hotpluggable.
>
> Is that always true?
Yes, someone who meet the same problem report a bug to LTP, and LTP
maintainer said that this should be fixed in kernel.

Huacai