linux-kernel - Re: cpu hotplug: possible_cpus broken (again?) next-20130607

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Message-ID: <4005805.V3phb1agMs@vostro.rjw.lan>
Date:	Wed, 12 Jun 2013 13:02:32 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Youquan Song <youquan.song@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave@...1.net>, Toshi Kani <toshi.kani@...com>,
	Youquan Song <youquan.song@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: cpu hotplug: possible_cpus broken (again?) next-20130607

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@...ux.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@...el.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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