[PATCH 5.16 174/285] arch/arm64: Fix topology initialization for core scheduling

Greg Kroah-Hartman
2022-04-12T08:29:55Z

From: Phil Auld <pauld@redhat.com>

[ Upstream commit 5524cbb1bfcdff0cad0aaa9f94e6092002a07259 ]

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@redhat.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lore.kernel.org/r/20220331153926.25742-1-pauld@redhat.com
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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);

/*
--
2.35.1