Date:	Wed, 2 Jul 2014 11:26:39 +0200
From:	Igor Mammedov <imammedo@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...hat.com, hpa@...or.com
Subject: Re: [PATCH v7] x86: initialize secondary CPU only if master CPU
 will wait for it

On Fri, 20 Jun 2014 14:23:11 +0200
Igor Mammedov <imammedo@...hat.com> 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. If AP doesn't respond in 10
> seconds, the master CPU will timeout and cancel
> AP onlining.
> 
> Signed-off-by: Igor Mammedov <imammedo@...hat.com>
ping,
may be it's better to take in for the next merge window

> ---
> v7:
>  - fix stuck boot with non SMP config
>  - fix stuck paravirtual Xen SMP boot with more than 1VCPU
>    and CPU hotplug
> v6:
>  - no changes
> v5:
>  - add smp_mb() after clearing cpu_initialized_mask in do_boot_cpu()
>  - add 10 sec timeout description into commit message.
> v4:
>  - move commont code in cpu_init() for x32/x64 in shared
>    helper function wait_formaster_cpu()
>  - add WARN_ON(cpumask_test_and_set_cpu(cpu, cpu_initialized_mask))
>    to wait_formaster_cpu()
> 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.
> v2:
>  - ammend comment in cpu_init()
> ---
>  arch/x86/kernel/cpu/common.c |   29 ++++++++-----
>  arch/x86/kernel/smpboot.c    |   99 +++++++++++++-----------------------------
>  arch/x86/xen/smp.c           |    2 +
>  3 files changed, 51 insertions(+), 79 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index ef1b93f..0b94e57 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1258,6 +1258,19 @@ static void dbg_restore_debug_regs(void)
>  #define dbg_restore_debug_regs()
>  #endif /* ! CONFIG_KGDB */
>  
> +static void wait_for_master_cpu(int cpu)
> +{
> +#ifdef CONFIG_SMP
> +	/*
> +	 * 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();
> +#endif
> +}
> +
>  /*
>   * cpu_init() initializes state that is per-CPU. Some data is already
>   * initialized (naturally) in the bootstrap process, such as the GDT
> @@ -1273,16 +1286,17 @@ 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);
>  
> @@ -1294,9 +1308,6 @@ 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);
> @@ -1373,13 +1384,9 @@ 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 5492798..a1472b7 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
> @@ -757,8 +725,8 @@ 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();
> @@ -806,6 +774,15 @@ 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);
> +	smp_mb();
> +
> +	/*
>  	 * Wake up a CPU in difference cases:
>  	 * - Use the method in the APIC driver if it's defined
>  	 * Otherwise,
> @@ -817,55 +794,41 @@ 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();
> +		}
> +	}
>  
> +	if (!boot_error) {
>  		/*
> -		 * Wait 5s total for a response
> +		 * Wait till AP completes initial initialization
>  		 */
> -		for (timeout = 0; timeout < 50000; timeout++) {
> -			if (cpumask_test_cpu(cpu, cpu_callin_mask))
> -				break;	/* It has booted */
> -			udelay(100);
> +		while (!cpumask_test_cpu(cpu, cpu_callin_mask)) {
>  			/*
>  			 * Allow other tasks to run while we wait for the
>  			 * AP to come online. This also gives a chance
>  			 * for the MTRR work(triggered by the AP coming online)
>  			 * to be completed in the stop machine context.
>  			 */
> +			udelay(100);
>  			schedule();
>  		}
> -
> -		if (cpumask_test_cpu(cpu, cpu_callin_mask)) {
> -			print_cpu_msr(&cpu_data(cpu));
> -			pr_debug("CPU%d: has booted.\n", cpu);
> -		} else {
> -			boot_error = 1;
> -			if (*trampoline_status == 0xA5A5A5A5)
> -				/* trampoline started but...? */
> -				pr_err("CPU%d: Stuck ??\n", cpu);
> -			else
> -				/* trampoline code not run */
> -				pr_err("CPU%d: Not responding\n", cpu);
> -			if (apic->inquire_remote_apic)
> -				apic->inquire_remote_apic(apicid);
> -		}
> -	}
> -
> -	if (boot_error) {
> -		/* Try to put things back the way they were before ... */
> -		numa_remove_cpu(cpu); /* was set by numa_add_cpu */
> -
> -		/* was set by do_boot_cpu() */
> -		cpumask_clear_cpu(cpu, cpu_callout_mask);
> -
> -		/* was set by cpu_init() */
> -		cpumask_clear_cpu(cpu, cpu_initialized_mask);
>  	}
>  
>  	/* mark "stuck" area as not stuck */
> diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
> index 7005974..3631e71 100644
> --- a/arch/x86/xen/smp.c
> +++ b/arch/x86/xen/smp.c
> @@ -360,6 +360,8 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
>  	struct desc_struct *gdt;
>  	unsigned long gdt_mfn;
>  
> +	/* used to tell cpu_init() that it can proceed with initialization */
> +	cpumask_set_cpu(cpu, cpu_callout_mask);
>  	if (cpumask_test_and_set_cpu(cpu, xen_cpu_initialized_map))
>  		return 0;
>  
> -- 
> 1.7.1
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@...r.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
> 
> 


-- 
Regards,
  Igor
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Powered by blists - more mailing lists