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SubjectRe: [bisected] x86 boot still broken on -rc2
On Mon,  4 Dec 2017 11:45:21 -0500, Prarit Bhargava wrote:
> On 12/04/2017 08:13 AM, Prarit Bhargava wrote:
> > x86: Booting SMP configuration:
> > .... node #0, CPUs: #1 #2 #3 #4
> > .... node #1, CPUs: #5 #6 #7 #8 #9
> > .... node #0, CPUs: #10 #11 #12 #13 #14
> > .... node #1, CPUs: #15 #16 #17 #18 #19
> > smp: Brought up 2 nodes, 20 CPUs
> > smpboot: Max logical packages: 1
> >
> > which means that the calculation of logical packages is wrong because
> >
> > ncpus = cpu_data(0).booted_cores * smp_num_siblings;
> > ncpus = 10 * 2;
> > ncpus = 20;
> >
> > smp_num_siblings is defined as "The number of threads in a core" which
> > should be 1 if HT/SMT is disabled.
> >
> > It looks like my patch has exposed a bug in the
> > smp_num_siblings calculation. I'm still debugging ...
>
> The bug is that smp_num_siblings has been incorrectly calculated as the
> *maximum* number of threads in a core, and not the actual number of threads in
> a core on systems which have a CPUID level greater than 0xb. (see
> arch/x86/kernel/cpu/topology.c:59)
>
> That will take some time to investigate and come up with a proper solution and
> fix. In the meantime, the patch below will fix the problem in the short-term.
> I've tested the patch using SMT enabled, SMT disabled, maxcpus=1 and nr_cpus=1.

Thanks Prarit, the work around does the job! Indeed, I have SMT
disabled.

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