Re: [PATCH] arch/arm64: Fix topology initialization for core scheduling

[Phil Auld <pauld@redhat.com>]

On Tue, Mar 29, 2022 at 04:02:22PM +0200 Dietmar Eggemann wrote:
> On 22/03/2022 17:03, Phil Auld wrote:
> > Some arm64 rely on store_cpu_topology() to setup the real topology.
> > This needs to be done before the call to notify_cpu_starting() which
> > tell the scheduler about the cpu otherwise the core scheduling data
> > structures are setup in a way that does not match the actual topology.
> > 
> > Without this change stress-ng (which enables core scheduling in its prctl 
> > tests) causes a warning and then a crash (trimmed for legibility):
> > 
> > [ 1853.805168] ------------[ cut here ]------------
> > [ 1853.809784] task_rq(b)->core != rq->core
> > [ 1853.809792] WARNING: CPU: 117 PID: 0 at kernel/sched/fair.c:11102 cfs_prio_less+0x1b4/0x1c4
> > ...
> > [ 1854.015210] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
> > ...
> > [ 1854.231256] Call trace:
> > [ 1854.233689]  pick_next_task+0x3dc/0x81c
> > [ 1854.237512]  __schedule+0x10c/0x4cc
> > [ 1854.240988]  schedule_idle+0x34/0x54
> > 
> > Fixes: 9edeaea1bc45 ("sched: Core-wide rq->lock")
> > Signed-off-by: Phil Auld <pauld@redhat.com>
> > ---
> > This is a similar issue to 
> >   f2703def339c ("MIPS: smp: fill in sibling and core maps earlier") 
> > which fixed it for MIPS.
> 
> I assume this is for a machine which relies on MPIDR-based setup
> (package_id == -1)? I.e. it doesn't have proper ACPI/(DT) data for
> topology setup.

Yes, that's my understanding. No PPTT.

> 
> Tried on a ThunderX2 by disabling parse_acpi_topology() but then I end
> up with a machine w/o SMT, so `stress-ng --prctl N` doesn't show this issue.
>
> Which machine were you using?

This instance is an HPE Apollo 70 set to smt-4.  I believe it's ThunderX2
chips.

ARM (CN9980-2200LG4077-Y21-G) 


Thanks,
Phil

> 
> >  arch/arm64/kernel/smp.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> > index 27df5c1e6baa..3b46041f2b97 100644
> > --- a/arch/arm64/kernel/smp.c
> > +++ b/arch/arm64/kernel/smp.c
> > @@ -234,6 +234,7 @@ asmlinkage notrace void secondary_start_kernel(void)
> >  	 * Log the CPU info before it is marked online and might get read.
> >  	 */
> >  	cpuinfo_store_cpu();
> > +	store_cpu_topology(cpu);
> >  
> >  	/*
> >  	 * Enable GIC and timers.
> > @@ -242,7 +243,6 @@ asmlinkage notrace void secondary_start_kernel(void)
> >  
> >  	ipi_setup(cpu);
> >  
> > -	store_cpu_topology(cpu);
> >  	numa_add_cpu(cpu);
> >  
> >  	/*
> 

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