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SubjectRe: [PATCH v4 5/5] x86: initialize secondary CPU only if master CPU will wait for it
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On Mon, 2014-04-14 at 17:11 +0200, Igor Mammedov wrote:
> Hang is observed on virtual machines during CPU hotplug,
> especially in big guests with many CPUs. (It reproducible
> more often if host is over-committed).
>
> It happens because master CPU gives up waiting on
> secondary CPU and allows it to run wild. As result
> AP causes locking or crashing system. For example
> as described here: https://lkml.org/lkml/2014/3/6/257
>
> If master CPU have sent STARTUP IPI successfully,
> and AP signalled to master CPU that it's ready
> to start initialization, make master CPU wait
> indefinitely till AP is onlined.
> To ensure that AP won't ever run wild, make it
> wait at early startup till master CPU confirms its
> intention to wait for AP.

Please also add description that the master CPU times out when an AP
does not start initialization within 10 seconds.

> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
> v2:
> - ammend comment in cpu_init()
> v3:
> - leave timeouts in do_boot_cpu(), so that master CPU
> won't hang if AP doesn't respond, use cpu_initialized_mask
> as a way for AP to signal to master CPU that it's ready
> to start initialzation.
> v4:
> - move common code in cpu_init() for x32/x64 in shared
> helper function wait_for_master_cpu()
> - add WARN_ON(cpumask_test_and_set_cpu(cpu, cpu_initialized_mask))
> to wait_formaster_cpu()
> ---
> arch/x86/kernel/cpu/common.c | 27 +++++++-----
> arch/x86/kernel/smpboot.c | 98 +++++++++++++-----------------------------
> 2 files changed, 46 insertions(+), 79 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index a135239..a4bcbac 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1221,6 +1221,17 @@ static void dbg_restore_debug_regs(void)
> #define dbg_restore_debug_regs()
> #endif /* ! CONFIG_KGDB */
>
> +static void wait_for_master_cpu(int cpu)
> +{
> + /*
> + * wait for ACK from master CPU before continuing
> + * with AP initialization
> + */
> + WARN_ON(cpumask_test_and_set_cpu(cpu, cpu_initialized_mask));
> + while (!cpumask_test_cpu(cpu, cpu_callout_mask))
> + cpu_relax();
> +}
> +
> /*
> * cpu_init() initializes state that is per-CPU. Some data is already
> * initialized (naturally) in the bootstrap process, such as the GDT
> @@ -1236,16 +1247,17 @@ void cpu_init(void)
> struct task_struct *me;
> struct tss_struct *t;
> unsigned long v;
> - int cpu;
> + int cpu = stack_smp_processor_id();
> int i;
>
> + wait_for_master_cpu(cpu);
> +
> /*
> * Load microcode on this cpu if a valid microcode is available.
> * This is early microcode loading procedure.
> */
> load_ucode_ap();
>
> - cpu = stack_smp_processor_id();
> t = &per_cpu(init_tss, cpu);
> oist = &per_cpu(orig_ist, cpu);
>
> @@ -1257,9 +1269,6 @@ void cpu_init(void)
>
> me = current;
>
> - if (cpumask_test_and_set_cpu(cpu, cpu_initialized_mask))
> - panic("CPU#%d already initialized!\n", cpu);
> -
> pr_debug("Initializing CPU#%d\n", cpu);
>
> clear_in_cr4(X86_CR4_VME|X86_CR4_PVI|X86_CR4_TSD|X86_CR4_DE);
> @@ -1336,13 +1345,9 @@ void cpu_init(void)
> struct tss_struct *t = &per_cpu(init_tss, cpu);
> struct thread_struct *thread = &curr->thread;
>
> - show_ucode_info_early();
> + wait_for_master_cpu(cpu);
>
> - if (cpumask_test_and_set_cpu(cpu, cpu_initialized_mask)) {
> - printk(KERN_WARNING "CPU#%d already initialized!\n", cpu);
> - for (;;)
> - local_irq_enable();
> - }
> + show_ucode_info_early();
>
> printk(KERN_INFO "Initializing CPU#%d\n", cpu);
>
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index ae2fd97..44903ad 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -111,7 +111,6 @@ atomic_t init_deasserted;
> static void smp_callin(void)
> {
> int cpuid, phys_id;
> - unsigned long timeout;
>
> /*
> * If waken up by an INIT in an 82489DX configuration
> @@ -130,37 +129,6 @@ static void smp_callin(void)
> * (This works even if the APIC is not enabled.)
> */
> phys_id = read_apic_id();
> - if (cpumask_test_cpu(cpuid, cpu_callin_mask)) {
> - panic("%s: phys CPU#%d, CPU#%d already present??\n", __func__,
> - phys_id, cpuid);
> - }
> - pr_debug("CPU#%d (phys ID: %d) waiting for CALLOUT\n", cpuid, phys_id);
> -
> - /*
> - * STARTUP IPIs are fragile beasts as they might sometimes
> - * trigger some glue motherboard logic. Complete APIC bus
> - * silence for 1 second, this overestimates the time the
> - * boot CPU is spending to send the up to 2 STARTUP IPIs
> - * by a factor of two. This should be enough.
> - */
> -
> - /*
> - * Waiting 2s total for startup (udelay is not yet working)
> - */
> - timeout = jiffies + 2*HZ;
> - while (time_before(jiffies, timeout)) {
> - /*
> - * Has the boot CPU finished it's STARTUP sequence?
> - */
> - if (cpumask_test_cpu(cpuid, cpu_callout_mask))
> - break;
> - cpu_relax();
> - }
> -
> - if (!time_before(jiffies, timeout)) {
> - panic("%s: CPU%d started up but did not get a callout!\n",
> - __func__, cpuid);
> - }
>
> /*
> * the boot CPU has finished the init stage and is spinning
> @@ -750,8 +718,8 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
> unsigned long start_ip = real_mode_header->trampoline_start;
>
> unsigned long boot_error = 0;
> - int timeout;
> int cpu0_nmi_registered = 0;
> + unsigned long timeout;
>
> /* Just in case we booted with a single CPU. */
> alternatives_enable_smp();
> @@ -799,6 +767,14 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
> }
>
> /*
> + * AP might wait on cpu_callout_mask in cpu_init() with
> + * cpu_initialized_mask set if previous attempt to online
> + * it timed-out. Clear cpu_initialized_mask so that after
> + * INIT/SIPI it could start with a clean state.
> + */
> + cpumask_clear_cpu(cpu, cpu_initialized_mask);

I think smp_mb() should be added here to ensure that the target AP sees
this change.

> +
> + /*
> * Wake up a CPU in difference cases:
> * - Use the method in the APIC driver if it's defined
> * Otherwise,
> @@ -810,55 +786,41 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
> boot_error = wakeup_cpu_via_init_nmi(cpu, start_ip, apicid,
> &cpu0_nmi_registered);
>
> +
> if (!boot_error) {
> /*
> - * allow APs to start initializing.
> + * Wait 10s total for a response from AP
> */
> - pr_debug("Before Callout %d\n", cpu);
> - cpumask_set_cpu(cpu, cpu_callout_mask);
> - pr_debug("After Callout %d\n", cpu);
> + boot_error = -1;
> + timeout = jiffies + 10*HZ;
> + while (time_before(jiffies, timeout)) {
> + if (cpumask_test_cpu(cpu, cpu_initialized_mask)) {
> + /*
> + * Tell AP to proceed with initialization
> + */
> + cpumask_set_cpu(cpu, cpu_callout_mask);
> + boot_error = 0;
> + break;
> + }
> + udelay(100);
> + schedule();
> + }
> + }

When 10s passed, the master could set a new flag, ex.
cpu_callout_error_mask, which wait_for_master_cpu() checks and call
play_dead() when it is set. This avoids AP to spin forever when 10s
becomes not long enough. But it does not have to be part of this
patchset, though.

> + if (!boot_error) {
> /*
> - * Wait 5s total for a response
> + * Wait till AP completes initial initialization

We should generally avoid such wait w/o a timeout condition, but since
native_cpu_up() waits till cpu_online(cpu) anyway after this point, this
seems OK... I wonder if we need touch_nmi_watchdog(), though.

Thanks,
-Toshi




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