[PATCH v3] selftests: powerpc: Fix CPU affinity for child process

[Harish]

On systems with large number of cpus, test fails trying to set
affinity by calling sched_setaffinity() with smaller size for
affinity mask. This patch fixes it by making sure that the size of
allocated affinity mask is dependent on the number of CPUs as
reported by get_nprocs().

Fixes: 00b7ec5c9cf3 ("selftests/powerpc: Import Anton's context_switch2 
benchmark")
Reported-by: Shirisha Ganta <shiga...@in.ibm.com>
Signed-off-by: Sandipan Das <sandi...@linux.ibm.com>
Signed-off-by: Harish <har...@linux.ibm.com>
---
v2: 
https://lore.kernel.org/linuxppc-dev/20200609034005.520137-1-har...@linux.ibm.com/

Changes from v2:
- Interchanged size and ncpus as suggested by Satheesh
- Revert the exit code as suggested by Satheesh
- Added NULL check for the affinity mask as suggested by Kamalesh
- Freed the affinity mask allocation after affinity is set
  as suggested by Kamalesh
- Changed "cpu set" to "affinity mask" in the commit message

---
 .../powerpc/benchmarks/context_switch.c       | 21 ++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/powerpc/benchmarks/context_switch.c 
b/tools/testing/selftests/powerpc/benchmarks/context_switch.c
index a2e8c9da7fa5..d50cc05df495 100644
--- a/tools/testing/selftests/powerpc/benchmarks/context_switch.c
+++ b/tools/testing/selftests/powerpc/benchmarks/context_switch.c
@@ -19,6 +19,7 @@
 #include <limits.h>
 #include <sys/time.h>
 #include <sys/syscall.h>
+#include <sys/sysinfo.h>
 #include <sys/types.h>
 #include <sys/shm.h>
 #include <linux/futex.h>
@@ -104,8 +105,9 @@ static void start_thread_on(void *(*fn)(void *), void *arg, 
unsigned long cpu)
 
 static void start_process_on(void *(*fn)(void *), void *arg, unsigned long cpu)
 {
-       int pid;
-       cpu_set_t cpuset;
+       int pid, ncpus;
+       cpu_set_t *cpuset;
+       size_t size;
 
        pid = fork();
        if (pid == -1) {
@@ -116,14 +118,23 @@ static void start_process_on(void *(*fn)(void *), void 
*arg, unsigned long cpu)
        if (pid)
                return;
 
-       CPU_ZERO(&cpuset);
-       CPU_SET(cpu, &cpuset);
+       ncpus = get_nprocs();
+       size = CPU_ALLOC_SIZE(ncpus);
+       cpuset = CPU_ALLOC(ncpus);
+       if (!cpuset) {
+               perror("malloc");
+               exit(1);
+       }
+       CPU_ZERO_S(size, cpuset);
+       CPU_SET_S(cpu, size, cpuset);
 
-       if (sched_setaffinity(0, sizeof(cpuset), &cpuset)) {
+       if (sched_setaffinity(0, size, cpuset)) {
                perror("sched_setaffinity");
+               CPU_FREE(cpuset);
                exit(1);
        }
 
+       CPU_FREE(cpuset);
        fn(arg);
 
        exit(0);
-- 
2.24.1