question about the limited interrupt vector resource on a single cpu

question about the limited interrupt vector resource on a single cpu

[Pingfan Liu]

For x86, there is around 200 vectors left for external device on a
single logic cpu.
Is there any case that we exhaust them in real world, and is it worth to fix?

Thanks,
Pingfan

Re: question about the limited interrupt vector resource on a single cpu

[Thomas Gleixner]

On Wed, 6 Jun 2018, Pingfan Liu wrote:

> For x86, there is around 200 vectors left for external device on a
> single logic cpu.
>
> Is there any case that we exhaust them in real world, and is it worth to fix?

Exhaustion is unlikely, but how would you fix it? The vector space per CPU
is limited by hardware not software.

Thanks,

	tglx

Re: question about the limited interrupt vector resource on a single cpu

[Pingfan Liu]

On Wed, Jun 6, 2018 at 5:15 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Wed, 6 Jun 2018, Pingfan Liu wrote:
>
>> For x86, there is around 200 vectors left for external device on a
>> single logic cpu.
>>
>> Is there any case that we exhaust them in real world, and is it worth to fix?
>
> Exhaustion is unlikely, but how would you fix it? The vector space per CPU
> is limited by hardware not software.
>
Playing some trick on iommu, we can use it to identify the irq requester.

Regards,
Pingfan

Re: question about the limited interrupt vector resource on a single cpu

[Thomas Gleixner]

On Wed, 6 Jun 2018, Pingfan Liu wrote:

> On Wed, Jun 6, 2018 at 5:15 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > On Wed, 6 Jun 2018, Pingfan Liu wrote:
> >
> >> For x86, there is around 200 vectors left for external device on a
> >> single logic cpu.
> >>
> >> Is there any case that we exhaust them in real world, and is it worth to fix?
> >
> > Exhaustion is unlikely, but how would you fix it? The vector space per CPU
> > is limited by hardware not software.
> >
> Playing some trick on iommu, we can use it to identify the irq requester.

But what's the point? If you have more than 200 device interrupts on a
single CPU, then you surely have other problems than that.

What kind of problem are you trying to solve?

Thanks,

	tglx

Re: question about the limited interrupt vector resource on a single cpu

[Pingfan Liu]

On Wed, Jun 6, 2018 at 5:34 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Wed, 6 Jun 2018, Pingfan Liu wrote:
>
>> On Wed, Jun 6, 2018 at 5:15 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>> > On Wed, 6 Jun 2018, Pingfan Liu wrote:
>> >
>> >> For x86, there is around 200 vectors left for external device on a
>> >> single logic cpu.
>> >>
>> >> Is there any case that we exhaust them in real world, and is it worth to fix?
>> >
>> > Exhaustion is unlikely, but how would you fix it? The vector space per CPU
>> > is limited by hardware not software.
>> >
>> Playing some trick on iommu, we can use it to identify the irq requester.
>
> But what's the point? If you have more than 200 device interrupts on a
> single CPU, then you surely have other problems than that.
>
> What kind of problem are you trying to solve?
>
In kdump case,  normally we set nr_cpus=1 by default, and in some huge
system, it can exhaust vector space.
By this is easily to resolve by inc nr_cpus. Hence I post this
question to see whether there are other requirements to resolve such
case

Thanks,
Pingfan

Re: question about the limited interrupt vector resource on a single cpu

[Thomas Gleixner]

On Wed, 6 Jun 2018, Pingfan Liu wrote:
> On Wed, Jun 6, 2018 at 5:34 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > On Wed, 6 Jun 2018, Pingfan Liu wrote:
> >
> >> On Wed, Jun 6, 2018 at 5:15 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> >> > On Wed, 6 Jun 2018, Pingfan Liu wrote:
> >> >
> >> >> For x86, there is around 200 vectors left for external device on a
> >> >> single logic cpu.
> >> >>
> >> >> Is there any case that we exhaust them in real world, and is it worth to fix?
> >> >
> >> > Exhaustion is unlikely, but how would you fix it? The vector space per CPU
> >> > is limited by hardware not software.
> >> >
> >> Playing some trick on iommu, we can use it to identify the irq requester.
> >
> > But what's the point? If you have more than 200 device interrupts on a
> > single CPU, then you surely have other problems than that.
> >
> > What kind of problem are you trying to solve?
> >
> In kdump case,  normally we set nr_cpus=1 by default, and in some huge
> system, it can exhaust vector space.

Well. For kdump it's really not necessary to initialize all devices. So
this is more a configuration problem.

> By this is easily to resolve by inc nr_cpus. Hence I post this
> question to see whether there are other requirements to resolve such
> case

Not really. We've covered a lot of the issues with multi-queue devices and
suspend, where you can run into the same issue, via the managed interrupt
infrastructure.

Thanks,

	tglx