Messages in this thread | | | Date | Thu, 9 Apr 2009 15:38:04 -0700 | Subject | Re: [PATCH 2/3] [BUGFIX] x86/x86_64: fix CPU offlining triggered inactive device IRQ interrruption | From | Yinghai Lu <> |
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On Thu, Apr 9, 2009 at 12:17 PM, Gary Hade <garyhade@us.ibm.com> wrote: > On Wed, Apr 08, 2009 at 04:59:35PM -0700, Yinghai Lu wrote: >> 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... > > OK, I see the 'vector_lock' spin_lock_irqsave/spin_unlock_irqrestore > surrounding the __assign_irq_vector call in assign_irq_vector. > >> > so cpu_online_mask will not changed during, > > I understand that this 'vector_lock' acquisition prevents > multiple simultaneous executions of __assign_irq_vector but > does that really prevent another thread executing outside > __assign_irq_vector (or outside other 'vector_lock' serialized > code) from modifying cpu_online_mask? > > Isn't it really 'cpu_add_remove_lock' (also held when > __assign_irq_vector() is called in the context of a CPU add > or remove) that is used for this purpose? > >> > why do you need to check that again in __assign_irq_vector ? > > Because that is where the cfg->move_in_progress flag was > being set. > > Is there some reason that the content of cpu_online_mask > cannot be trusted at this location? > > If all the CPUs in the old domain are offline doesn't > that imply that we got to that location in response to > a CPU offline request? > >> > >> looks like you need to clear move_in_progress in fixup_irqs() > > This would be a difficult since I believe the code is > currently partitioned in a manner that prevents access to > irq_cfg records from functions defined in arch/x86/kernel/irq_32.c > and arch/x86/kernel/irq_64.c. It also doesn't feel right to > allow cfg->move_in_progress to be set in __assign_irq_vector > and then clear it in fixup_irqs().
it looks before fixup_irqs() cpu_online_mask get updated, and before irq_complete_move get called.
so we could fixup_irqs to clear move_in_progress and cleanup percpu vector_irq ...
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