[PATCHv7 4/4] powerpc/setup: alloc extra paca_ptrs to hold boot_cpuid

[Pingfan Liu]

paca_ptrs should be large enough to hold the boot_cpuid, hence, its
lower boundary is set to the bigger one between boot_cpuid+1 and
nr_cpus.

On the other hand, some kernel component: -1. the timer assumes cpu0
online since the timer_list->flags subfield 'TIMER_CPUMASK' is zero if
not initialized to a proper present cpu.  -2. power9_idle_stop() assumes
the primary thread's paca is allocated.

Hence lift nr_cpu_ids from one to two to ensure cpu0 is onlined, if the
boot cpu is not cpu0.

Result:
When nr_cpus=1, taskset -c 14 bash -c 'echo c > /proc/sysrq-trigger'
the kdump kernel brings up two cpus.
While when taskset -c 4 bash -c 'echo c > /proc/sysrq-trigger',
the kdump kernel brings up one cpu.

Signed-off-by: Pingfan Liu <piliu at redhat.com>
Cc: Michael Ellerman <mpe at ellerman.id.au>
Cc: Nicholas Piggin <npiggin at gmail.com>
Cc: Christophe Leroy <christophe.leroy at csgroup.eu>
Cc: Mahesh Salgaonkar <mahesh at linux.ibm.com>
Cc: Wen Xiong <wenxiong at us.ibm.com>
Cc: Baoquan He <bhe at redhat.com>
Cc: Ming Lei <ming.lei at redhat.com>
Cc: kexec at lists.infradead.org
To: linuxppc-dev at lists.ozlabs.org
---
 arch/powerpc/kernel/paca.c | 10 ++++++----
 arch/powerpc/kernel/prom.c |  9 ++++++---
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/kernel/paca.c b/arch/powerpc/kernel/paca.c
index cda4e00b67c1..91e2401de1bd 100644
--- a/arch/powerpc/kernel/paca.c
+++ b/arch/powerpc/kernel/paca.c
@@ -242,9 +242,10 @@ static int __initdata paca_struct_size;
 
 void __init allocate_paca_ptrs(void)
 {
-	paca_nr_cpu_ids = nr_cpu_ids;
+	int n = (boot_cpuid + 1) > nr_cpu_ids ? (boot_cpuid + 1) : nr_cpu_ids;
 
-	paca_ptrs_size = sizeof(struct paca_struct *) * nr_cpu_ids;
+	paca_nr_cpu_ids = n;
+	paca_ptrs_size = sizeof(struct paca_struct *) * n;
 	paca_ptrs = memblock_alloc_raw(paca_ptrs_size, SMP_CACHE_BYTES);
 	if (!paca_ptrs)
 		panic("Failed to allocate %d bytes for paca pointers\n",
@@ -287,13 +288,14 @@ void __init allocate_paca(int cpu)
 void __init free_unused_pacas(void)
 {
 	int new_ptrs_size;
+	int n = (boot_cpuid + 1) > nr_cpu_ids ? (boot_cpuid + 1) : nr_cpu_ids;
 
-	new_ptrs_size = sizeof(struct paca_struct *) * nr_cpu_ids;
+	new_ptrs_size = sizeof(struct paca_struct *) * n;
 	if (new_ptrs_size < paca_ptrs_size)
 		memblock_phys_free(__pa(paca_ptrs) + new_ptrs_size,
 				   paca_ptrs_size - new_ptrs_size);
 
-	paca_nr_cpu_ids = nr_cpu_ids;
+	paca_nr_cpu_ids = n;
 	paca_ptrs_size = new_ptrs_size;
 
 #ifdef CONFIG_PPC_64S_HASH_MMU
diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 87272a2d8c10..15c994f54bf9 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c
@@ -362,9 +362,12 @@ static int __init early_init_dt_scan_cpus(unsigned long node,
 			 */
 			boot_cpuid = i;
 			found = true;
-			/* This works around the hole in paca_ptrs[]. */
-			if (nr_cpu_ids < nthreads)
-				set_nr_cpu_ids(nthreads);
+			/*
+			 * Ideally, nr_cpus=1 can be achieved if each kernel
+			 * component does not assume cpu0 is onlined.
+			 */
+			if (boot_cpuid != 0 && nr_cpu_ids < 2)
+				set_nr_cpu_ids(2);
 		}
 #ifdef CONFIG_SMP
 		/* logical cpu id is always 0 on UP kernels */
-- 
2.31.1