linux-kernel - Re: [PATCH v6 04/12] x86/smpboot.c: Don't offline CPU0 if any irq can not be migrated out of it and remove CPU0 check in smp

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Message-ID: <alpine.LFD.2.02.1205111352330.3231@ionos>
Date:	Fri, 11 May 2012 14:05:42 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Fenghua Yu <fenghua.yu@...el.com>
cc:	Ingo Molnar <mingo@...e.hu>, H Peter Anvin <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Asit K Mallick <asit.k.mallick@...el.com>,
	Tony Luck <tony.luck@...el.com>,
	Arjan Dan De Ven <arjan@...ux.intel.com>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Len Brown <len.brown@...el.com>,
	"Srivatssa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Chen Gong <gong.chen@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-pm <linux-pm@...r.kernel.org>, x86 <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v6 04/12] x86/smpboot.c: Don't offline CPU0 if any irq
 can not be migrated out of it and remove CPU0 check in smp_callin()

On Thu, 10 May 2012, Fenghua Yu wrote:
> From: Fenghua Yu <fenghua.yu@...el.com>
> 
> Don't offline CPU0 if any irq can not be migrated out of it.
> 
> Call identify_boot_cpu_online() for CPU0 in smp_callin() and continue to online
> CPU0 in native_cpu_up().
> 
> Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
> ---
>  arch/x86/kernel/smpboot.c |   42 +++++++++++++++++++++++++++++++++++-------
>  1 files changed, 35 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index e543e02..fa3822e 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -121,8 +121,8 @@ EXPORT_PER_CPU_SYMBOL(cpu_info);
>  atomic_t init_deasserted;
>  
>  /*
> - * Report back to the Boot Processor.
> - * Running on AP.
> + * Report back to the Boot Processor during boot time or to the caller processor
> + * during CPU online.
>   */
>  static void __cpuinit smp_callin(void)
>  {
> @@ -210,6 +210,13 @@ static void __cpuinit smp_callin(void)
>  	pr_debug("Stack at about %p\n", &cpuid);
>  
>  	/*
> +	 * This function won't run on the BSP during boot time. It run
> +	 * on BSP only when BSP is offlined and onlined again.
> +	 */
> +	if (cpuid == 0)
> +		identify_boot_cpu_online();

No, this is not how we do things. 

If this is required, then hell we do it at boot time and not at a
random place in the cpu hotplug code.

> +
> +	/*
>  	 * This must be done before setting cpu_online_mask
>  	 * or calling notify_cpu_starting.
>  	 */
> @@ -778,7 +785,7 @@ int __cpuinit native_cpu_up(unsigned int cpu, struct task_struct *tidle)
>  
>  	pr_debug("++++++++++++++++++++=_---CPU UP  %u\n", cpu);
>  
> -	if (apicid == BAD_APICID || apicid == boot_cpu_physical_apicid ||
> +	if (apicid == BAD_APICID ||
>  	    !physid_isset(apicid, phys_cpu_present_map) ||
>  	    !apic->apic_id_valid(apicid)) {
>  		printk(KERN_ERR "%s: bad cpu %d\n", __func__, cpu);
> @@ -1224,12 +1231,33 @@ int native_cpu_disable(void)
>  	 * Perhaps use cpufreq to drop frequency, but that could go
>  	 * into generic code.
>  	 *
> -	 * We won't take down the boot processor on i386 due to some
> +	 * We won't take down the boot processor on x86 if some
>  	 * interrupts only being able to be serviced by the BSP.
> -	 * Especially so if we're not using an IOAPIC	-zwane
> +	 * Especially so if we're not using an IOAPIC
>  	 */
> -	if (cpu == 0)
> -		return -EBUSY;
> +	if (cpu == 0) {
> +		int irq;
> +		struct irq_desc *desc;
> +		struct irq_data *data;
> +		struct irq_chip *chip;
> +
> +		for_each_irq_desc(irq, desc) {
> +			raw_spin_lock(&desc->lock);
> +			if (!irq_has_action(irq)) {
> +				raw_spin_unlock(&desc->lock);
> +				continue;
> +			}
> +
> +			data = irq_desc_get_irq_data(desc);
> +			chip = irq_data_get_irq_chip(data);
> +			if (!chip->irq_set_affinity) {
> +				pr_debug("irq%d can't move out of BSP\n", irq);
> +				raw_spin_unlock(&desc->lock);
> +				return -EBUSY;
> +			}
> +			raw_spin_unlock(&desc->lock);
> +		}

This is as fricking wrong as it gets. We already know that at boot
time when we initialize the irq chips. There is no need to iterate
over the whole irq descriptors just to figure that out.

And of course you'd detect cascaded irq chips behind a PCIe master
interrupt as non migratable even if that's not affecting hotplug.


I haven't looked at the other patches, but given the gems in this one
I can't be bothered to see the other magic fixups which are just
hacked into the code at random places to make it somehow work.

NAK.

	tglx
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