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Date: Wed, 8 Apr 2009 15:30:15 -0700
Message-ID: <86802c440904081530i1b83e19ayddebd8b2f6d413af@mail.gmail.com>
Subject: Re: [PATCH 2/3] [BUGFIX] x86/x86_64: fix CPU offlining triggered 
	inactive device IRQ interrruption
From: Yinghai Lu <yhlu.kernel@gmail.com>
To: Gary Hade <garyhade@us.ibm.com>
Cc: mingo@elte.hu, mingo@redhat.com, tglx@linutronix.de, hpa@zytor.com,
       x86@kernel.org, linux-kernel@vger.kernel.org, lcm@us.ibm.com
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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?
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.

YH
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