Re: [PATCH 2/3] [BUGFIX] x86/x86_64: fix CPU offlining triggered inactive device IRQ interrruption

[Yinghai Lu <yhlu.kernel@gmail.com>]

On Wed, Apr 8, 2009 at 4:58 PM, Yinghai Lu <yhlu.kernel@gmail.com> wrote:
> On Wed, Apr 8, 2009 at 4:37 PM, Gary Hade <garyhade@us.ibm.com> wrote:
>> On Wed, Apr 08, 2009 at 03:30:15PM -0700, Yinghai Lu wrote:
>>> On Wed, Apr 8, 2009 at 2:07 PM, Gary Hade <garyhade@us.ibm.com> wrote:
>>> > Impact: Eliminates a race that can leave the system in an
>>> >        unusable state
>>> >
>>> > During rapid offlining of multiple CPUs there is a chance
>>> > that an IRQ affinity move destination CPU will be offlined
>>> > before the IRQ affinity move initiated during the offlining
>>> > of a previous CPU completes.  This can happen when the device
>>> > is not very active and thus fails to generate the IRQ that is
>>> > needed to complete the IRQ affinity move before the move
>>> > destination CPU is offlined.  When this happens there is an
>>> > -EBUSY return from __assign_irq_vector() during the offlining
>>> > of the IRQ move destination CPU which prevents initiation of
>>> > a new IRQ affinity move operation to an online CPU.  This
>>> > leaves the IRQ affinity set to an offlined CPU.
>>> >
>>> > I have been able to reproduce the problem on some of our
>>> > systems using the following script.  When the system is idle
>>> > the problem often reproduces during the first CPU offlining
>>> > sequence.
>>> >
>>> > #!/bin/sh
>>> >
>>> > SYS_CPU_DIR=/sys/devices/system/cpu
>>> > VICTIM_IRQ=25
>>> > IRQ_MASK=f0
>>> >
>>> > iteration=0
>>> > while true; do
>>> >  echo $iteration
>>> >  echo $IRQ_MASK > /proc/irq/$VICTIM_IRQ/smp_affinity
>>> >  for cpudir in $SYS_CPU_DIR/cpu[1-9] $SYS_CPU_DIR/cpu??; do
>>> >    echo 0 > $cpudir/online
>>> >  done
>>> >  for cpudir in $SYS_CPU_DIR/cpu[1-9] $SYS_CPU_DIR/cpu??; do
>>> >    echo 1 > $cpudir/online
>>> >  done
>>> >  iteration=`expr $iteration + 1`
>>> > done
>>> >
>>> > The proposed fix takes advantage of the fact that when all
>>> > CPUs in the old domain are offline there is nothing to be done
>>> > by send_cleanup_vector() during the affinity move completion.
>>> > So, we simply avoid setting cfg->move_in_progress preventing
>>> > the above mentioned -EBUSY return from __assign_irq_vector().
>>> > This allows initiation of a new IRQ affinity move to a CPU
>>> > that is not going offline.
>>> >
>>> > Signed-off-by: Gary Hade <garyhade@us.ibm.com>
>>> >
>>> > ---
>>> >  arch/x86/kernel/apic/io_apic.c |   11 ++++++++---
>>> >  1 file changed, 8 insertions(+), 3 deletions(-)
>>> >
>>> > Index: linux-2.6.30-rc1/arch/x86/kernel/apic/io_apic.c
>>> > ===================================================================
>>> > --- linux-2.6.30-rc1.orig/arch/x86/kernel/apic/io_apic.c        2009-04-08 09:23:00.000000000 -0700
>>> > +++ linux-2.6.30-rc1/arch/x86/kernel/apic/io_apic.c     2009-04-08 09:23:16.000000000 -0700
>>> > @@ -363,7 +363,8 @@ set_extra_move_desc(struct irq_desc *des
>>> >        struct irq_cfg *cfg = desc->chip_data;
>>> >
>>> >        if (!cfg->move_in_progress) {
>>> > -               /* it means that domain is not changed */
>>> > +               /* it means that domain has not changed or all CPUs
>>> > +                * in old domain are offline */
>>> >                if (!cpumask_intersects(desc->affinity, mask))
>>> >                        cfg->move_desc_pending = 1;
>>> >        }
>>> > @@ -1262,8 +1263,11 @@ next:
>>> >                current_vector = vector;
>>> >                current_offset = offset;
>>> >                if (old_vector) {
>>> > -                       cfg->move_in_progress = 1;
>>> >                        cpumask_copy(cfg->old_domain, cfg->domain);
>>> > +                       if (cpumask_intersects(cfg->old_domain,
>>> > +                                              cpu_online_mask)) {
>>> > +                               cfg->move_in_progress = 1;
>>> > +                       }
>>> >                }
>>> >                for_each_cpu_and(new_cpu, tmp_mask, cpu_online_mask)
>>> >                        per_cpu(vector_irq, new_cpu)[vector] = irq;
>>> > @@ -2492,7 +2496,8 @@ static void irq_complete_move(struct irq
>>> >                if (likely(!cfg->move_desc_pending))
>>> >                        return;
>>> >
>>> > -               /* domain has not changed, but affinity did */
>>> > +               /* domain has not changed or all CPUs in old domain
>>> > +                * are offline, but affinity changed */
>>> >                me = smp_processor_id();
>>> >                if (cpumask_test_cpu(me, desc->affinity)) {
>>> >                        *descp = desc = move_irq_desc(desc, me);
>>> > --
>>>
>>> so you mean during __assign_irq_vector(), cpu_online_mask get updated?
>>
>> No, the CPU being offlined is removed from cpu_online_mask
>> earlier via a call to remove_cpu_from_maps() from
>> cpu_disable_common().  This happens just before fixup_irqs()
>> is called.
>>
>>> with your patch, how about that it just happen right after you check
>>> that second time.
>>>
>>> it seems we are missing some lock_vector_lock() on the remove cpu from
>>> online mask.
>>
>> The remove_cpu_from_maps() call in cpu_disable_common() is vector
>> lock protected:
>> void cpu_disable_common(void)
>> {
>>               < snip >
>>        /* It's now safe to remove this processor from the online map */
>>        lock_vector_lock();
>>        remove_cpu_from_maps(cpu);
>>        unlock_vector_lock();
>>        fixup_irqs();
>> }
>
>
> __assign_irq_vector always has vector_lock locked...
> so cpu_online_mask will not changed during, why do you need to check
> that again in __assign_irq_vector ?
>
looks like you need to clear move_in_progress in fixup_irqs()

YH