[PATCHv4 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 <kernelf...@gmail.com>
---
 arch/powerpc/kernel/setup-common.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/setup-common.c 
b/arch/powerpc/kernel/setup-common.c
index 1a67344..6920b5e 100644
--- a/arch/powerpc/kernel/setup-common.c
+++ b/arch/powerpc/kernel/setup-common.c
@@ -462,6 +462,8 @@ void __init smp_setup_cpu_maps(void)
        struct device_node *dn;
        struct device_node *boot_dn = NULL;
        bool handling_bootdn = true;
+       int head_thread = 0;
+       int online_cnt = 0;
        int cpu = 0;
        int nthreads = 1;
 
@@ -499,13 +501,19 @@ void __init smp_setup_cpu_maps(void)
                        if (boot_cpuid < nthreads &&
                                be32_to_cpu(intserv[boot_cpuid]) == 
boot_cpuhwid) {
                                boot_dn = dn;
+                               /* choose a bunch of continous threads */
+                               if (boot_cpuid > nr_cpu_ids - 1) {
+                                       head_thread = boot_cpuid - nr_cpu_ids + 
1;
+                                       /* keep the mapping of logical and 
thread */
+                                       cpu = head_thread;
+                               }
                        }
                        if (boot_dn == NULL)
                                continue;
                } else if (dn == boot_dn)
                        continue;
 
-               for (j = 0; j < nthreads && cpu < nr_cpu_ids; j++) {
+               for (j = head_thread; j < nthreads && online_cnt < nr_cpu_ids; 
j++) {
                        bool avail;
 
                        DBG("    thread %d -> cpu %d (hard id %d)\n",
@@ -520,13 +528,15 @@ void __init smp_setup_cpu_maps(void)
                        set_hard_smp_processor_id(cpu, be32_to_cpu(intserv[j]));
                        set_cpu_possible(cpu, true);
                        cpu++;
+                       online_cnt++;
                }
 
-               if (cpu >= nr_cpu_ids) {
+               if (online_cnt >= nr_cpu_ids) {
                        of_node_put(dn);
                        break;
                }
                if (handling_bootdn) {
+                       head_thread = 0;
                        handling_bootdn = false;
                        goto again;
                }
-- 
2.7.4