[PATCHv5 2/3] powerpc, cpu: handling the special case when boot_cpuid greater than nr_cpus

[Pingfan Liu]

For kexec -p, after boot_cpuid is mapping into the range of [0,
threads_per_core), then if nr_cpus is small, we will have the bitmap
[0,..., nr_cpus, ..., boot_cpuid, ...). This patch chooses cpus inside
the range of [boot_cpuid - nr_cpus +1, ..., boot_cpuid] to be online.

With this patch and the next, on a P9 machine with thread_per_core=4,
and set nr_cpus=2 for the crash kernel.
After
    taskset -c 11 sh -c "echo c > /proc/sysrq-trigger"
Then
    kdump:/sys/devices/system/cpu# cat possible
    2-3
    kdump:/sys/devices/system/cpu# cat present
    2-3
    kdump:/sys/devices/system/cpu# cat online
    2-3

Signed-off-by: Pingfan Liu <kernelfans at gmail.com>
---
 arch/powerpc/kernel/setup-common.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
index a683ed1..f3aaf9f 100644
--- a/arch/powerpc/kernel/setup-common.c
+++ b/arch/powerpc/kernel/setup-common.c
@@ -468,6 +468,7 @@ void __init smp_setup_cpu_maps(void)
 	int online_num = 0;
 	int next_cpu = boot_dn_nthreads;
 	bool btdn_handled = false;
+	int first_threadid = 0;
 
 	DBG("smp_setup_cpu_maps()\n");
 
@@ -497,7 +498,12 @@ void __init smp_setup_cpu_maps(void)
 
 		if (boot_cpuid < nthreads &&
 			be32_to_cpu(intserv[boot_cpuid]) == boot_cpuhwid) {
-			cpu = 0;
+			/* choose a bunch of continous threads */
+			if (boot_cpuid > nr_cpu_ids - 1) {
+				first_threadid = boot_cpuid - nr_cpu_ids + 1;
+			}
+			/* keep the mapping of logical and thread */
+			cpu = first_threadid;
 			btdn_handled = true;
 		} else if ( !btdn_handled &&
 				nr_cpu_ids - online_num <= boot_dn_nthreads)
@@ -509,7 +515,8 @@ void __init smp_setup_cpu_maps(void)
 		}
 
 
-		for (j = 0; j < nthreads && online_num < nr_cpu_ids; j++) {
+		for (j = first_threadid;
+			j < nthreads && online_num < nr_cpu_ids; j++) {
 			bool avail;
 
 			DBG("    thread %d -> cpu %d (hard id %d)\n",
-- 
2.7.4