Subject: Re: [PATCH 1/3] powerpc/smp: Fix a crash while booting kvm guest with nr_cpus=2

From: Srikar Dronamraju <[email-masked]>
Date: Mon, 23 Aug 2021 15:34:09 +0530

* Gautham R Shenoy <[email-masked]> [2021-08-23 11:41:22]:

> On Sat, Aug 21, 2021 at 02:54:17PM +0530, Srikar Dronamraju wrote:
> > Aneesh reported a crash with a fairly recent upstream kernel when
> > booting kernel whose commandline was appended with nr_cpus=2
> > 
> > 1:mon> e
> > cpu 0x1: Vector: 300 (Data Access) at [c000000008a67bd0]
> >     pc: c00000000002557c: cpu_to_chip_id+0x3c/0x100
> >     lr: c000000000058380: start_secondary+0x460/0xb00
> >     sp: c000000008a67e70
> >    msr: 8000000000001033
> >    dar: 10
> >  dsisr: 80000
> >   current = 0xc00000000891bb00
> >   paca    = 0xc0000018ff981f80   irqmask: 0x03   irq_happened: 0x01
> >     pid   = 0, comm = swapper/1
> > Linux version 5.13.0-rc3-15704-ga050a6d2b7e8 (kvaneesh@ltc-boston8) (gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0, GNU ld (GNU Binutils for Ubuntu) 2.34) #433 SMP Tue May 25 02:38:49 CDT 2021
> > 1:mon> t
> > [link register   ] c000000000058380 start_secondary+0x460/0xb00
> > [c000000008a67e70] c000000008a67eb0 (unreliable)
> > [c000000008a67eb0] c0000000000589d4 start_secondary+0xab4/0xb00
> > [c000000008a67f90] c00000000000c654 start_secondary_prolog+0x10/0x14
> > 
> > Current code assumes that num_possible_cpus() is always greater than
> > threads_per_core. However this may not be true when using nr_cpus=2 or
> > similar options. Handle the case where num_possible_cpus is smaller than
> > threads_per_core.
> >
> > Cc: [email-masked]
> > Cc: Aneesh Kumar K.V <[email-masked]>
> > Cc: Nathan Lynch <[email-masked]>
> > Cc: Michael Ellerman <[email-masked]>
> > Cc: Ingo Molnar <[email-masked]>
> > Cc: Peter Zijlstra <[email-masked]>
> > Cc: Valentin Schneider <[email-masked]>
> > Cc: Gautham R Shenoy <[email-masked]>
> > Cc: Vincent Guittot <[email-masked]>
> > Fixes: c1e53367dab1 ("powerpc/smp: Cache CPU to chip lookup")
> > Reported-by: Aneesh Kumar K.V <[email-masked]>
> > Debugged-by: Michael Ellerman <[email-masked]>
> > Signed-off-by: Srikar Dronamraju <[email-masked]>
> > ---
> >  arch/powerpc/kernel/smp.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > index 6c6e4d934d86..3d6874fe1937 100644
> > --- a/arch/powerpc/kernel/smp.c
> > +++ b/arch/powerpc/kernel/smp.c
> > @@ -1074,7 +1074,7 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
> >  	}
> > 
> >  	if (cpu_to_chip_id(boot_cpuid) != -1) {
> > -		int idx = num_possible_cpus() / threads_per_core;
> > +		int idx = max((int)num_possible_cpus() / threads_per_core, 1);
> 
> I think this code was assuming that num_possible_cpus() is a multiple
> of threads_per_core.
> 
> So, on a system with threads_per_core=8, if we pass nr_cpus=10, we
> will still get idx=1. Thus, we will allocate only one entry in
> chip_id_lookup_table[] even though there are two cores and
> chip_id_lookup_table[] is expected to have one entry per core.
> 
> Is this a valid scenario ? If yes, should we use
> 
>    idx = DIV_ROUND_UP(num_possible_cpus, threads_per_core);
> 

Yes, this can be done.
will resend this patch with this change.

> 
> > 
> >  		/*
> >  		 * All threads of a core will all belong to the same core,
> > -- 
> > 2.18.2
> > 
> 
> --
> Thanks and Regards
> gautham.

-- 
Thanks and Regards
Srikar Dronamraju