Re: [PATCH v2] abort secondary CPU bring-up gracefully if do_boot_cpu timed out on cpu_callin_mask

[Prarit Bhargava <prarit@redhat.com>]

On 03/06/2014 09:10 AM, Igor Mammedov wrote:
> Hang is observed on virtual machines during CPU hotplug, especially
> in big guests with many CPUs. (It happens more often if host is
> over-committed).
> 
> Patch is present in RHEL6 since 2012 and it fixes issue there,
> it also fixes issue in upstream kernel according to tests.
> 
> Below is detailed description of the problem:
> -----
> Master CPU may timeout before cpu_callin_mask is set and cancel
> booting CPU, but being onlined CPU still continues to boot, sets
> cpu_active_mask (CPU_STARTING notifiers) and spins in
> check_tsc_sync_target() for master cpu to arrive. Following attempt
> to online another cpu hangs in stop_machine, initiated from here:
> 
> smp_callin ->
>   smp_store_cpu_info ->
>     identify_secondary_cpu ->
>       mtrr_ap_init -> set_mtrr_from_inactive_cpu
> 
> stop_machine waits on completion of stop_work on all CPUs from
> cpu_active_mask including a failed CPU that spins in check_tsc_sync_target().
> 
> Issue is fixed if being onlined CPU continues to boot and calls
> notify_cpu_starting(cpuid) only when master CPU waits for it to
> come online. If master CPU times out on cpu_callin_mask and goes via
> cancel path, the being onlined CPU should gracefully shutdown itself.
> 
> Patch introduces cpu_may_complete_boot_mask to notify a being onlined
> CPU that it may call notify_cpu_starting(cpuid) and continue to boot
> when master CPU goes via normal boot path and going to wait till the
> being onlined CPU completes its initialization.
> 
> - normal boot sequence will look like:
>     master CPU1                         being onlined CPU2
> 
>  * wait for CPU2 in cpu_callin_mask
> ---------------------------------------------------------------------
>                                         * set CPU2 in cpu_callin_mask
>                                         * wait till CPU1 set CPU2 bit
>                                         in cpu_may_complete_boot_mask
> ---------------------------------------------------------------------
>  * set CPU2 bit in
>    cpu_may_complete_boot_mask
>  * return from do_boot_cpu() and
>    wait in
>      - check_tsc_sync_source() or
>      - while (!cpu_online(CPU2))
> ---------------------------------------------------------------------
>                                         * call notify_cpu_starting()
>                                           and continue CPU2 initialization
>                                         * mark itself as ONLINE
> ---------------------------------------------------------------------
>  * return to _cpu_up and call
>    cpu_notify(CPU_ONLINE, ...)
> 
> - cancel/error path will look like:
>     master CPU1                         being onlined CPU2
> 
>  * time out on cpu_callin_mask
> ---------------------------------------------------------------------
>                                        * set CPU2 in cpu_callin_mask
>                                        * wait till CPU2 is set in
>                                          cpu_may_complete_boot_mask or
>                                          cleared in cpu_callout_mask
> ---------------------------------------------------------------------
>  * clear CPU2 in cpu_callout_mask
>  and return with error
> ---------------------------------------------------------------------
>                                        * do cleanups and play_dead()
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
> v2:
>  * updated commit message with description of which systems are affected
>    but bug and under which conditions.
> ---
>  arch/x86/include/asm/cpumask.h |    1 +
>  arch/x86/kernel/cpu/common.c   |    2 ++
>  arch/x86/kernel/smpboot.c      |   37 +++++++++++++++++++++++++++++++++++--
>  3 files changed, 38 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/cpumask.h b/arch/x86/include/asm/cpumask.h
> index 61c852f..eacd269 100644
> --- a/arch/x86/include/asm/cpumask.h
> +++ b/arch/x86/include/asm/cpumask.h
> @@ -7,6 +7,7 @@ extern cpumask_var_t cpu_callin_mask;
>  extern cpumask_var_t cpu_callout_mask;
>  extern cpumask_var_t cpu_initialized_mask;
>  extern cpumask_var_t cpu_sibling_setup_mask;
> +extern cpumask_var_t cpu_may_complete_boot_mask;
>  
>  extern void setup_cpu_local_masks(void);
>  
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 8e28bf2..4797111 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -50,6 +50,7 @@
>  cpumask_var_t cpu_initialized_mask;
>  cpumask_var_t cpu_callout_mask;
>  cpumask_var_t cpu_callin_mask;
> +cpumask_var_t cpu_may_complete_boot_mask;
>  
>  /* representing cpus for which sibling maps can be computed */
>  cpumask_var_t cpu_sibling_setup_mask;
> @@ -61,6 +62,7 @@ void __init setup_cpu_local_masks(void)
>  	alloc_bootmem_cpumask_var(&cpu_callin_mask);
>  	alloc_bootmem_cpumask_var(&cpu_callout_mask);
>  	alloc_bootmem_cpumask_var(&cpu_sibling_setup_mask);
> +	alloc_bootmem_cpumask_var(&cpu_may_complete_boot_mask);
>  }
>  
>  static void default_init(struct cpuinfo_x86 *c)
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index a32da80..4e9a63b 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -104,6 +104,8 @@ EXPORT_PER_CPU_SYMBOL(cpu_info);
>  
>  atomic_t init_deasserted;
>  
> +static void remove_siblinginfo(int cpu);
> +
>  /*
>   * Report back to the Boot Processor during boot time or to the caller processor
>   * during CPU online.
> @@ -202,12 +204,38 @@ static void smp_callin(void)
>  	set_cpu_sibling_map(raw_smp_processor_id());
>  	wmb();
>  
> -	notify_cpu_starting(cpuid);
> -
>  	/*
>  	 * Allow the master to continue.
>  	 */
>  	cpumask_set_cpu(cpuid, cpu_callin_mask);
> +
> +	/*
> +	* Wait for signal from master CPU to continue or abort.
> +	*/
> +	while (!cpumask_test_cpu(cpuid, cpu_may_complete_boot_mask) &&
> +		cpumask_test_cpu(cpuid, cpu_callout_mask)) {
> +		cpu_relax();
> +	}

It is possible that you loop indefinitely in here ... but I suppose that's okay
because other places in the cpu_up() path also loop indefinitely.

> +
> +	/* die if master cancelled cpu_up */
> +	if (!cpumask_test_cpu(cpuid, cpu_may_complete_boot_mask))
> +		goto die;

Can this really happen?  AFAICT the master cpu does in do_boot_cpu() ... (sorry
for the cut-and-paste)


                /*
                 * Wait 5s total for a response
                 */
                for (timeout = 0; timeout < 50000; timeout++) {
                        if (cpumask_test_cpu(cpu, cpu_callin_mask))
                                break;  /* It has booted */
                        udelay(100);
                        /*
                         * 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.
                         */
                        schedule();
                }

and if !cpumask_test_cpu(cpu, cpu_callin_mask) the master will set boot_error =
1, which would result in the cpu_may_complete_boot_mask mask being cleared.  But
you're not patching that path, or maybe I'm missing something.  Either way it is
also prone to races through the code.

> +
> +	notify_cpu_starting(cpuid);
> +	return;
> +
> +die:
> +#ifdef CONFIG_HOTPLUG_CPU
> +	/* was set by smp_store_cpu_info->...->numa_add_cpu */
> +	numa_remove_cpu(cpuid);
> +	remove_siblinginfo(cpuid);
> +	clear_local_APIC();
> +	/* was set by cpu_init() */
> +	cpumask_clear_cpu(cpuid, cpu_initialized_mask);
> +	cpumask_clear_cpu(cpuid, cpu_callin_mask);
> +	play_dead();
> +#endif

Is the above necessary or should we just leave the state as-is and panic.  I
think having the information around might be useful in determining what went wrong.

> +	panic("%s: Failed to online CPU%d!\n", __func__, cpuid);
>  }
>  
>  static int cpu0_logical_apicid;
> @@ -827,6 +855,8 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
>  		}
>  
>  		if (cpumask_test_cpu(cpu, cpu_callin_mask)) {
> +			/* Signal AP that it may continue to boot */
> +			cpumask_set_cpu(cpu, cpu_may_complete_boot_mask);
>  			print_cpu_msr(&cpu_data(cpu));
>  			pr_debug("CPU%d: has booted.\n", cpu);
>  		} else {
> @@ -1085,6 +1115,7 @@ void __init native_smp_prepare_cpus(unsigned int max_cpus)
>  	 */
>  	smp_store_boot_cpu_info(); /* Final full version of the data */
>  	cpumask_copy(cpu_callin_mask, cpumask_of(0));
> +	cpumask_copy(cpu_may_complete_boot_mask, cpumask_of(0));
>  	mb();
>  
>  	current_thread_info()->cpu = 0;  /* needed? */
> @@ -1294,6 +1325,8 @@ static void __ref remove_cpu_from_maps(int cpu)
>  	cpumask_clear_cpu(cpu, cpu_callin_mask);
>  	/* was set by cpu_init() */
>  	cpumask_clear_cpu(cpu, cpu_initialized_mask);
> +	/* set by do_boot_cpu() */
> +	cpumask_clear_cpu(cpu, cpu_may_complete_boot_mask);
>  	numa_remove_cpu(cpu);
>  }
>