Messages in this thread | | | Subject | Re: [PATCH 5/5] x86: update nr_irqs according cpu num | From | (Eric W. Biederman) | Date | Fri, 08 Jan 2010 12:10:27 -0800 |
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"H. Peter Anvin" <hpa@zytor.com> writes:
> On 01/08/2010 11:11 AM, Eric W. Biederman wrote: >> Yinghai Lu <yinghai@kernel.org> writes: >> >>> that is max number on run time. >> >> Ouch! Unless I misread this code this will leave nr_irqs at >> NR_IRQS_LEGACY. aka 16.
I goofed and misread this. I was looking at nr_irqs_gsi which is initialized to 16.
We actually initialize nr_irqs to NR_IRQS, which has an unfortunately convoluted formula, that winds up being 8*NR_CPUS or 32 *MAX_IO_APICS. in the extreme cases.
Since there are still arrays sized at NR_IRQS (bleh) we can not increase nr_irqs to be greater than NR_IRQS.
So YN can you do the simple thing here and simply remove arch_probe_nr_irqs(). Sane code doesn't care how big nr_irqs is and code that does care needs to be fixed.
>> Let's do something stupid and simple. >> nr_irqs = nr_cpus_ids * 256; /* Semi-arbitrary number */ > > This would be 1048576 on the biggest machines we currently support. > Now, the number of IRQ *vectors* is limited to > (224-system vectors)*(cpu count), so one could argue that if there is > anything that is not semi-arbitrary it would be that number, but that > doesn't account for vector sharing.
Except we have irq sources that we know about that are never utilized, Think of unconnected inputs to ioapics.
I don't know if we ever actually perform vector sharing. The only case I recall where the code could share vectors is if the firmware tables told us to irq sources were the same interrupt. I don't think that happens. We do have the remains of support for vector sharing in the code but I don't think it is utilized. MSI interrupts certainly can not share vectors.
My point with the semi-arbitrary number is that we should not think of nr_irqs as something defined by the resources of the receivers of interrupts. NR_IRQS has never been that. nr_irqs really is a limit on how many interrupt sources we have.
> Do we have any place which requires nr_irqs to be *stable*, or can we > simply treat it as a high water mark for IRQ numbers used?
We have several loops that walk through the irq descriptors and look for an unbound irq. Which means having nr_irqs as a high water mark is not going to work today.
>> Ideally we would set "nr_irqs = 0x7fffffff;" but we have just enough >> places using nr_irqs that I think those loops would get painful if we >> were to do that. > > Ideally we should presumably get rid of nr_irqs completely?
Yes. It was enough of a pain the first pass at it that we wound up with nr_irqs, a value that can vary at boot time.
Once YH's radix tree changes get it in. A war on NR_IRQS and nr_irqs seems appropriate.
Eric
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