[PATCH] powerpc/cpu: nr_cpu_ids should be aligned on threads_per_core

[Pingfan Liu <kernelfans@gmail.com>]

With p8 subcore mode, if nr_cpus is not aligned on threads_per_core,
then we will hit the following panic:
  [    0.310006] Unpacking initramfs...
  [    1.105015] Freeing initrd memory: 35968K
  [    1.105466] rtas_flash: no firmware flash support
  [    1.105688] Unable to handle kernel paging request for data at address 0x00001035
  [    1.105768] Faulting instruction address: 0xc000000000099bb8
  [    1.105837] Oops: Kernel access of bad area, sig: 11 [#1]
  [    1.105890] LE SMP NR_CPUS=2048 NUMA PowerNV
  [    1.105947] Modules linked in:
  [    1.105992] CPU: 8 PID: 11 Comm: migration/8 Not tainted 4.17.0-rc4+ #26
  [    1.106059] NIP:  c000000000099bb8 LR: c0000000000ac554 CTR: c000000000099b70
  [    1.106139] REGS: c000000ff649f860 TRAP: 0380   Not tainted  (4.17.0-rc4+)
  [    1.106205] MSR:  9000000000001033 <SF,HV,ME,IR,DR,RI,LE>  CR: 28004022  XER: 00000000
  [    1.106293] CFAR: c0000000000ac550 SOFTE: 3
  [    1.106293] GPR00: c0000000000ac554 c000000ff649fae0 c0000000012e6d00 0000000000000060
  [    1.106293] GPR04: 0000000000000001 c0000000000ac914 9000000000001033 c000003fffffff80
  [    1.106293] GPR08: 0000000000000000 0000000000000001 0000000000000000 9000000000001003
  [    1.106293] GPR12: c000000000099b70 c000000001700000 c000000000141c28 c000000ffa201180
  [    1.106293] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
  [    1.106293] GPR20: 0000000000000000 0000000000000000 0000000000000001 c000000fffd13840
  [    1.106293] GPR24: c000000fffd13838 0000000000000000 0000000000000001 0000000000000010
  [    1.106293] GPR28: 0000000000000008 0000000000000001 c000000001370548 000000000000000c
  [    1.106984] NIP [c000000000099bb8] icp_native_cause_ipi+0x48/0x70
  [    1.107053] LR [c0000000000ac554] pnv_cause_ipi+0x54/0x80
  [    1.107106] Call Trace:
  [    1.107137] [c000000ff649fae0] [c0000000000478b4] doorbell_try_core_ipi+0x74/0x90 (unreliable)
  [    1.107232] [c000000ff649fb10] [c0000000000ac554] pnv_cause_ipi+0x54/0x80
  [    1.107301] [c000000ff649fb40] [c00000000004ccfc] smp_muxed_ipi_message_pass+0x6c/0x90
  [    1.107384] [c000000ff649fb80] [c0000000000acbe8] cpu_do_split+0x2b8/0x2f0
  [    1.107453] [c000000ff649fbf0] [c0000000000acdb8] cpu_update_split_mode+0x68/0x204
  [    1.107536] [c000000ff649fc40] [c000000000204ab8] multi_cpu_stop+0x1c8/0x200
  [    1.107617] [c000000ff649fca0] [c000000000204eb4] cpu_stopper_thread+0x114/0x1f0
  [    1.107699] [c000000ff649fd60] [c000000000148650] smpboot_thread_fn+0x290/0x2a0
  [    1.107780] [c000000ff649fdc0] [c000000000141d84] kthread+0x164/0x1b0
  [    1.107850] [c000000ff649fe30] [c00000000000b628] ret_from_kernel_thread+0x5c/0xb4
  [    1.107929] Instruction dump:
  [    1.107971] 7c7f1b78 60000000 60000000 3d420004 394ab070 7be31f24 39200001 e90a0000
  [    1.108058] 3d42001b 394a2650 7d08182a 7d4a182a <99281035> 39000004 7c0004ac 990a000c
  [    1.108150] ---[ end trace 83dad19c162dc306 ]---
  [    1.110137]
  [    2.110227] Kernel panic - not syncing: Fatal exception

This is caused by that during subcore_init, it IPIs all cpu inside the core, but due to the limitation
imposed by nr_cpus, there are no mem allocated for percpu cpu_sibling_map.
To avoid a complicated fix, this patch just forces the nr_cpu_ids to be aligned on thread_per_code
at the cost of a bit more memory.

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
---
 arch/powerpc/kernel/prom.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 9dbed48..358adb4 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c
@@ -315,6 +315,7 @@ static int __init early_init_dt_scan_cpus(unsigned long node,
 	int len;
 	int found = -1;
 	int found_thread = 0;
+	unsigned int nr_cpus_aligned;
 
 	/* We are scanning "cpu" nodes only */
 	if (type == NULL || strcmp(type, "cpu") != 0)
@@ -361,7 +362,13 @@ static int __init early_init_dt_scan_cpus(unsigned long node,
 	/* Not the boot CPU */
 	if (found < 0)
 		return 0;
-
+	/* to work around p8 subcore mode */
+	nr_cpus_aligned = _ALIGN_UP(nr_cpu_ids, nthreads);
+	if (nr_cpus_aligned != nr_cpu_ids) {
+		pr_info("nr_cpus is forced to align up on threads_per_core,"
+			"from %u to %u\n", nr_cpu_ids, nr_cpus_aligned);
+		nr_cpu_ids = nr_cpus_aligned;
+	}
 	DBG("boot cpu: logical %d physical %d\n", found,
 	    be32_to_cpu(intserv[found_thread]));
 	boot_cpuid = found;
-- 
2.7.4