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From:   Dongli Zhang <dongli.zhang@oracle.com>
To:     kvm@vger.kernel.org, linux-kselftest@vger.kernel.org
Cc:     pbonzini@redhat.com, shuah@kernel.org, seanjc@google.com,
        linux-kernel@vger.kernel.org
Subject: [PATCH 1/1] selftests: KVM: set affinity of VM to right CPUs
Date:   Fri, 24 Sep 2021 16:30:37 -0700
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The nr_cpus = CPU_COUNT(&possible_mask) is the number of available CPUs in
possible_mask. As a result, the "cpu = i % nr_cpus" may always return CPU
that is not available in possible_mask.

Suppose the server has 8 CPUs. The below Failure is encountered immediately
if the task is bound to CPU 5 and 6.

==== Test Assertion Failure ====
  rseq_test.c:228: i > (NR_TASK_MIGRATIONS / 2)
  pid=10127 tid=10127 errno=4 - Interrupted system call
     1	0x00000000004018e5: main at rseq_test.c:227
     2	0x00007fcc8fc66bf6: ?? ??:0
     3	0x0000000000401959: _start at ??:?
  Only performed 4 KVM_RUNs, task stalled too much?

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
---
 tools/testing/selftests/kvm/rseq_test.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/kvm/rseq_test.c b/tools/testing/selftests/kvm/rseq_test.c
index c5e0dd664a7b..41df5173970c 100644
--- a/tools/testing/selftests/kvm/rseq_test.c
+++ b/tools/testing/selftests/kvm/rseq_test.c
@@ -10,6 +10,7 @@
 #include <signal.h>
 #include <syscall.h>
 #include <sys/ioctl.h>
+#include <sys/sysinfo.h>
 #include <asm/barrier.h>
 #include <linux/atomic.h>
 #include <linux/rseq.h>
@@ -43,6 +44,18 @@ static bool done;
 
 static atomic_t seq_cnt;
 
+static int get_max_cpu_idx(void)
+{
+	int nproc = get_nprocs_conf();
+	int i, max = -ENOENT;
+
+	for (i = 0; i < nproc; i++)
+		if (CPU_ISSET(i, &possible_mask))
+			max = i;
+
+	return max;
+}
+
 static void guest_code(void)
 {
 	for (;;)
@@ -61,10 +74,13 @@ static void *migration_worker(void *ign)
 {
 	cpu_set_t allowed_mask;
 	int r, i, nr_cpus, cpu;
+	int max_cpu_idx;
 
 	CPU_ZERO(&allowed_mask);
 
-	nr_cpus = CPU_COUNT(&possible_mask);
+	max_cpu_idx = get_max_cpu_idx();
+	TEST_ASSERT(max_cpu_idx >= 0, "Invalid possible_mask");
+	nr_cpus = max_cpu_idx + 1;
 
 	for (i = 0; i < NR_TASK_MIGRATIONS; i++) {
 		cpu = i % nr_cpus;
-- 
2.17.1