On Thu, Jul 16, 2015 at 11:17:17AM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <[email protected]>
>
> Currently the code to bring up secondary CPUs only checks
> for cpu_online before it proceeds with launching the per-cpu
> threads for the freshly booted remote CPU.
>
> But the code to move these threads to the new CPU checks for
> cpu_active to do so. If this check fails the threads end up
> on the wrong CPU, causing warnings and bugs like:
>
> WARNING: CPU: 0 PID: 1 at ../kernel/workqueue.c:4417
> workqueue_cpu_up_callback
>
> and/or:
>
> kernel BUG at ../kernel/smpboot.c:135!
>
> The reason is that the cpu_active bit for the new CPU
> becomes visible significantly later than the cpu_online bit.
I see
void set_cpu_online(unsigned int cpu, bool online)
{
if (online) {
cpumask_set_cpu(cpu, to_cpumask(cpu_online_bits));
cpumask_set_cpu(cpu, to_cpumask(cpu_active_bits));
} else {
which is called in start_secondary().
Do you mean that setting the bit in cpu_active_mask gets delayed soo
much? Because it comes right after setting the bit in cpu_online_mask.
> The reasons could be that the kernel runs in a KVM guest,
> where the vCPU thread gets preempted when the cpu_online bit
> is set, but with cpu_active still clear.
>
> But this could also happen on bare-metal systems with lots
> of CPUs. We have observed this issue on an 88 core x86
> system on bare-metal.
>
> To fix this issue, wait before the remote CPU is online
> *and* active before launching the per-cpu threads.
>
> Signed-off-by: Joerg Roedel <[email protected]>
> ---
> arch/x86/kernel/smpboot.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index d3010aa..30b7b8b 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1006,7 +1006,7 @@ int native_cpu_up(unsigned int cpu, struct task_struct
> *tidle)
> check_tsc_sync_source(cpu);
> local_irq_restore(flags);
>
> - while (!cpu_online(cpu)) {
> + while (!cpu_online(cpu) || !cpu_active(cpu)) {
> cpu_relax();
> touch_nmi_watchdog();
Maybe we should just swap the calls in set_cpu_online() instead? I.e.,
if (online) {
cpumask_set_cpu(cpu, to_cpumask(cpu_active_bits));
cpumask_set_cpu(cpu, to_cpumask(cpu_online_bits));
}
?
I see cpu_online() being called much more than cpu_active()...
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
--
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
