Re: [RFC][PATCH] Setting /proc/irq/[n]/smp_affinity failure on 8cpu systems

[John Blackwood]

> On Wed, 2009-12-02 at 08:19 -0800, John Blackwood wrote:
> > > I would like to mention an irq set affinity issue occurs on an
> > > older Dell PowerEdge 6650 system:
> > >     - 8 cpus (4 are hyper-threaded),
> > >     - uses the struct apic 'apic_default' support
> > >     - 32 bit (x86) kernel
> > >
> > > This issue is that once you change the irq smp_affinity of an irq
> > > to be less than all cpus in the system, you can never change really
> > > the
> > > irq smp_affinity back to be all cpus in the system (0xff) again,
> > > even though no error status is returned on the "/bin/echo ff >
> > > /proc/irq/[n]/smp_affinity" operation.
> > >
> > > This is due to that fact that BAD_APICID has the same value as
> > > all cpus (0xff) on 32bit kernels, and thus the value returned from
> > > set_desc_affinity() via the cpu_mask_to_apicid_and() function is
> > > treated
> > > as a failure in set_ioapic_affinity_irq_desc(), and no affinity
> > > changes
> > > are made.
> > >
> > > It seems like changing the BAD_APICID value from 0xFF to 0xFFFF for
> > > CONFIG_X86_32 might be one possible solution, and it seems to function
> > > properly for this Dell PowerEdge 6650 system.
>
> John, Thanks for the root cause and analysis. As you noticed, 0xff is a
> valid logical apicid. Similarly 0xffff is a valid logical x2apic cluster
> id. So while your patch fixes your issue, it doesn't completely fix the
> issue. Can you please check if the appended patch works and ack if it
> passes your test?
>

Hi Suresh,

Yes, your patch works on the Dell PowerEdge 6650 system.

Thanks for your help and quick reply.

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