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SubjectRe: cpu hotplug: possible_cpus broken (again?) next-20130607
Date
On Tuesday, June 11, 2013 09:07:37 PM Yinghai Lu wrote:
> On Wed, Jun 12, 2013 at 5:32 AM, Youquan Song
> <youquan.song@linux.intel.com> wrote:
> >> On 06/12/2013 05:03 AM, Youquan Song wrote:
> >> > +#ifdef CONFIG_SMP
> >> > + /* return when cpu number greater than maximum number of
> >> > CPUs */
> >> > + if (setup_max_cpus <= num_online_cpus() + 1) {
> >> > + cpu_hotplug_driver_unlock();
> >> > + return -EINVAL;
> >> > + }
> >> > +#endif
> >> > from_nid = cpu_to_node(cpuid);
> >> > ret = cpu_up(cpuid);
> >>
> >> Your patch is line-wrapped.
> >>
> >> Also, the #ifdef is unnecessary. If CONFIG_SMP is off:
> >>
> >> static const unsigned int setup_max_cpus = NR_CPUS;
> >> #define num_online_cpus() 1U
> >>
> >> The compiler will take care of optimizing out the the if() without the
> >> explicit #ifdef.
> >>
> >> Also, the +1 looks goofy to me. Doesn't this do the same thing (and
> >> isn't it much easier to read)?
> >>
> >> if (num_online_cpus() >= setup_max_cpus)
> >>
> >
> > Thanks. Here is a formal patch for it. please review and try.
> >
> > Subject: [PATCH] core: Fix maxcpus boot option broken
> >
> > maxcpus boot option to limit maximum number of CPUs on system, but this option
> > is broken at recent kernel. Though we use maxcpus to limit CPUs number, but
> > current kernel will register all of present CPUs in sysfs.
> > udev will enumerate all registered cpu at sysfs, and it will bring up the CPU
> > if the CPU is offline. So the maxcpus option is broken.
> >
> > This patch will limit the online cpus number not over limitation of maxcpus
> > option. So it will keep the maxcpus limitation when udev enumeration
> > or other intention of bring up CPUs over the limitation by method like
> > echo 1 > /sys/devices/system/cpu/online
>
> Interesting, you are changing long standing meaning of maxcpus=
>
> We always use maxcpus=1 to have one cpu up, and later in user space
> to online other cpus like
> echo 1 > /sys/devices/system/cpuX/online.
>
> aka maxcpus= is a soft limit or initial online nr.
>
> we already have nr_cpus= for hard limit.
>
> So need to drop
> commit 3e275a5ba367ab74b3a4e49114307baed989fcac
> Author: Youquan Song <youquan.song@intel.com>
> Date: Fri Jun 7 10:07:08 2013 +1000
>
> drivers/base/cpu.c: fix maxcpus boot option

Agreed.

Thanks,
Rafael


> Greg,
> Can you drop that 3e275a5ba36 from your drivers/core tree ?
>
> Thanks
>
> Yinghai
--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.


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