linux-kernel - Re: disabled APICs being counted as processors ?

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Message-ID: <20140126092912.GA31643@gmail.com>
Date:	Sun, 26 Jan 2014 10:29:12 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Dave Jones <davej@...hat.com>, x86@...nel.org,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Yinghai Lu <yinghai@...nel.org>
Subject: Re: disabled APICs being counted as processors ?


* David Rientjes <rientjes@...gle.com> wrote:

> On Sun, 26 Jan 2014, Ingo Molnar wrote:
> 
> > > I don't think the "ACPI: LAPIC (... disabled)" lines are problematic, they 
> > > are simply reporting the acpi processor id and apic id for processors that 
> > > do not have their enabled flag set.  The acpi spec allows for these to 
> > > exist without the enabled flag set when the processor isn't present at all 
> > > because the kernel will make no attempt to use it.
> > > 
> > > That said, I think the "smpboot: 8 Processors exceeds NR_CPUS limit 
> > > of 4" line is unnecessary since, as you said, these processors don't 
> > > physically exist.  I betcha that's because you have 
> > > CONFIG_HOTPLUG_CPU enabled and it's counting the disabled cpus that 
> > > were found when acpi_register_lapic() was done.  The warning is only 
> > > really meaningful for cpus in cpu_possible_map, which aren't set for 
> > > your disabled four, in the hotplug case where NR_CPUS is too small.
> > 
> > No, this message is printed in prefill_possible_map() which 
> > _generates_ cpu_possible_map, so '8' is the number of bits in 
> > cpu_possible_map.
> > 
> 
> Yeah, because I bet Dave has CONFIG_HOTPLUG_CPU enabled and it's adding 
> this to the number of possible cpus when in reality, per the spec, these 
> cpus aren't possible at all because their enable bit isn't set in their 
> lapic flags.

Yeah, I suspect Dave has a distro-ish .config on his desktop, and 
distros generally enable all things hot-plug.

> > So the problem is that the counting of disabled but hotpluggable 
> > CPUs is over-eager.
> 
> In the kernel, yeah, and we don't distinguish between physically 
> absent processors that have lapic entries and physically present but 
> disabled processors.

Correct. Is there a robust distinction possible between the two?

> > --- a/arch/x86/kernel/smpboot.c
> > +++ b/arch/x86/kernel/smpboot.c
> > @@ -1223,10 +1223,7 @@ __init void prefill_possible_map(void)
> >  	i = setup_max_cpus ?: 1;
> >  	if (setup_possible_cpus == -1) {
> >  		possible = num_processors;
> > -#ifdef CONFIG_HOTPLUG_CPU
> > -		if (setup_max_cpus)
> > -			possible += disabled_cpus;
> > -#else
> > +#ifndef CONFIG_HOTPLUG_CPU
> >  		if (possible > i)
> >  			possible = i;
> >  #endif
> 
> Yeah, this should suppress the warning for Dave.  This way, the only way 
> the log reports the number of "hotplug CPUs" is because we used 
> possible_cpus.

Not just that, it also reduces the number of possible CPUs, which 
should reduce percpu memory allocation overhead, amongst other things, 
right?

> I think you should also just do "total_cpus = possible" though and 
> forget about disabled_cpus or /sys/devices/system/cpu/offline is 
> still going to show him 4-7.

Agreed.

> This function could benefit from a cleanup at the same time, it's 
> not looking good:
> 
>  - "i" is a horribly named variable that stores the value so at least
>    one cpu is possible when "nosmp" is used,
> 
>  - what's with the
> 
>    #ifdef CONFIG_HOTPLUG_CPU
> 	if (!setup_max_cpus)
>    #endif ?
> 
>    if I do "maxcpus=4 nr_cpus=6 possible_cpus=8" what's the expected
>    behavior?  We're not only testing for "nosmp" use here, "possible"
>    should still be 4, and
> 
>  - the warning references "max_cpus" but the kernel command line option
>    is "maxcpus"

Ack.

I wouldn't object to someone sending a changelogged, tested patch that 
does all that. Maybe two patches: first the cleanups, then the CPU 
count trimming. Just in case it regresses ...

Thanks,

	Ingo
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