[PATCH v3] arch/arm64: Fix topology initialization for core scheduling

[Phil Auld]

Arm64 systems rely on store_cpu_topology() to call update_siblings_masks()
to transfer the toplogy to the various cpu masks. This needs to be done 
before the call to notify_cpu_starting() which tells the scheduler about 
each cpu found, otherwise the core scheduling data structures are setup 
in a way that does not match the actual topology.

With smt_mask not setup correctly we bail on `cpumask_weight(smt_mask) == 1` 
for !leaders in:

 notify_cpu_starting()
   cpuhp_invoke_callback_range()
     sched_cpu_starting()
       sched_core_cpu_starting()

which leads to rq->core not being correctly set for !leader-rq's.

Without this change stress-ng (which enables core scheduling in its prctl 
tests in newer versions -- i.e. with PR_SCHED_CORE support) 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 at redhat.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann at arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann at arm.com>

---
This is a similar issue to 
  f2703def339c ("MIPS: smp: fill in sibling and core maps earlier") 
which fixed it for MIPS. 

v2: Fixed the commit message. No code change.

 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);
 
 	/*
-- 
2.18.0