Re: [PATCH] x86/hotplug: Silence APIC only after all irq's are migrated

[Raj, Ashok]

Hi Randy,


On Fri, Aug 14, 2020 at 04:25:32PM -0700, Randy Dunlap wrote:
> On 8/14/20 2:38 PM, Ashok Raj wrote:
> > When offlining CPU's, fixup_irqs() migrates all interrupts away from the
> 
>                  CPUs,

Thanks for catching these. I'll fix all these suggested changes in my next rev
Once i get additional feedback from Thomas.


> 
> > outgoing CPU to an online CPU. Its always possible the device sent an
> 
>                                  It's
> 
> > interrupt to the previous CPU destination. Pending interrupt bit in IRR in
> > lapic identifies such interrupts. apic_soft_disable() will not capture any
> 
>   LAPIC
> 
> > new interrupts in IRR. This causes interrupts from device to be lost during
> > cpu offline. The issue was found when explicitly setting MSI affinity to a
> 
>   CPU
> 
> > CPU and immediately offlining it. It was simple to recreate with a USB
> > ethernet device and doing I/O to it while the CPU is offlined. Lost
> > interrupts happen even when Interrupt Remapping is enabled.
> > 
> > Current code does apic_soft_disable() before migrating interrupts.
> > 
> > native_cpu_disable()
> > {
> > 	...
> > 	apic_soft_disable();
> > 	cpu_disable_common();
> > 	  --> fixup_irqs(); // Too late to capture anything in IRR.
> > }
> > 
> > Just fliping the above call sequence seems to hit the IRR checks
> 
>        flipping
> 
> > and the lost interrupt is fixed for both legacy MSI and when
> > interrupt remapping is enabled.
> > 
> > 
> > Fixes: 60dcaad5736f ("x86/hotplug: Silence APIC and NMI when CPU is dead")
> > Link: https://lore.kernel.org/lkml/875zdarr4h.fsf@xxxxxxxxxxxxxxxxxxxxxxx/
> > Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
> > 
> > To: linux-kernel@xxxxxxxxxxxxxxx
> > To: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Sukumar Ghorai <sukumar.ghorai@xxxxxxxxx>
> > Cc: Srikanth Nandamuri <srikanth.nandamuri@xxxxxxxxx>
> > Cc: Evan Green <evgreen@xxxxxxxxxxxx>
> > Cc: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
> > Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> >  arch/x86/kernel/smpboot.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > index ffbd9a3d78d8..278cc9f92f2f 100644
> > --- a/arch/x86/kernel/smpboot.c
> > +++ b/arch/x86/kernel/smpboot.c
> > @@ -1603,13 +1603,20 @@ int native_cpu_disable(void)
> >  	if (ret)
> >  		return ret;
> >  
> > +	cpu_disable_common();
> >  	/*
> >  	 * Disable the local APIC. Otherwise IPI broadcasts will reach
> >  	 * it. It still responds normally to INIT, NMI, SMI, and SIPI
> > -	 * messages.
> > +	 * messages. Its important to do apic_soft_disable() after
> 
> 	             It's
> 
> > +	 * fixup_irqs(), because fixup_irqs() called from cpu_disable_common()
> > +	 * depends on IRR being set. After apic_soft_disable() CPU preserves
> > +	 * currently set IRR/ISR but new interrupts will not set IRR.
> > +	 * This causes interrupts sent to outgoing cpu before completion
> 
> 	                                           CPU
> 
> > +	 * of irq migration to be lost. Check SDM Vol 3 "10.4.7.2 Local
> 
> 	      IRQ
> 
> > +	 * APIC State after It Has been Software Disabled" section for more
> > +	 * details.
> >  	 */
> >  	apic_soft_disable();
> > -	cpu_disable_common();
> >  
> >  	return 0;
> >  }
> > 
> 
> thanks.
> -- 
> ~Randy
>