With CONFIG_DEBUG_PER_CPU_MAPS and CONFIG_CPUMASK_OFFSTACK enabled fixes the following WARN_ON_ONCE() for booting with nr_cpus=1:
[ 0.000000] Linux version 4.9.0 ([email protected]) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-4) (GCC) ) #36 SMP Mon Dec 12 18:05:46 MSK 2016 [ 0.000000] Command line: BOOT_IMAGE=/vmlinuz-4.9.0 root=/dev/mapper/vz-root ro crashkernel=auto rd.lvm.lv=vz/root rd.lvm.lv=vz/swap console=ttyS0,115200 vsyscall=none nr_cpus=1 [ 0.000000] smpboot: 4 Processors exceeds NR_CPUS limit of 1 [ 0.000000] smpboot: Allowing 1 CPUs, 0 hotplug CPUs [ 0.000000] ------------[ cut here ]------------ [ 0.000000] WARNING: CPU: 0 PID: 0 at ./include/linux/cpumask.h:121 cpumask_check.part.2+0x1c/0x1e [ 0.000000] Modules linked in: [ 0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.9.0 #36 [ 0.000000] Call Trace: [ 0.000000] [<ffffffff8136664b>] dump_stack+0x67/0x9c [ 0.000000] [<ffffffff81063e21>] __warn+0xd1/0xf0 [ 0.000000] [<ffffffff81063f0d>] warn_slowpath_null+0x1d/0x20 [ 0.000000] [<ffffffff8105f4ca>] cpumask_check.part.2+0x1c/0x1e [ 0.000000] [<ffffffff8104008e>] cpumask_clear_cpu+0x2e/0x40 [ 0.000000] [<ffffffff81f88a62>] prefill_possible_map+0x15c/0x16a [ 0.000000] [<ffffffff81f802c6>] setup_arch+0xba7/0xc33 [ 0.000000] [<ffffffff81f78c59>] start_kernel+0x63/0x448 [ 0.000000] [<ffffffff81f7858c>] x86_64_start_reservations+0x2a/0x2c [ 0.000000] [<ffffffff81f78678>] x86_64_start_kernel+0xea/0xed [ 0.000000] ---[ end trace 5876da8d2ace83fb ]--- nr_cpu_ids is set to possible two lines futher - omit checking in set_cpu_possible() cycles. Cc: Thomas Gleixner <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: Rusty Russell <[email protected]> Cc: "H. Peter Anvin" <[email protected]> Cc: Jan Beulich <[email protected]> Cc: [email protected] Signed-off-by: Dmitry Safonov <[email protected]> --- arch/x86/kernel/smpboot.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index 42f5eb7b4f6c..17167bec7c61 100644 --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -1459,6 +1459,9 @@ __init void prefill_possible_map(void) pr_info("Allowing %d CPUs, %d hotplug CPUs\n", possible, max_t(int, possible - num_processors, 0)); + /* Avoid WARN() in set_cpu_possible()=>cpumask_check() */ + nr_cpu_ids = NR_CPUS; + for (i = 0; i < possible; i++) set_cpu_possible(i, true); for (; i < NR_CPUS; i++) -- 2.10.2
