linux-kernel - Re: [PATCH 00/16] dyn_array and nr

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Date:	Fri, 01 Aug 2008 18:41:27 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Yinghai Lu" <yhlu.kernel@...il.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>, hpa <hpa@...or.com>,
	"Dhaval Giani" <dhaval@...ux.vnet.ibm.com>,
	"Mike Travis" <travis@....com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/16] dyn_array and nr_irqs support v2

"Yinghai Lu" <yhlu.kernel@...il.com> writes:

>>> Increase NR_IRQS to 512 for x86_64?
>>
>> x86_32 has it set to 1024 so 512 is too small.  I think your patch
>> which essentially restores the old behavior is the right way to go for
>> this merge window.  I just want to carefully look at it and ensure we
>> are restoring the old heuristics.  On a lot of large machines we wind
>> up having irqs for pci slots that are never filled with cards.
>
> it seems 32bit summit need NR_IRQS=256, NR_IRQ_VECTOR=1024

Yes.  Which is 1024 irq sources/gsis only 1/4 used so it will fit into 256 irqs.

On x86_64 we have removed the confusing and brittle irq compression
code.  So to handle that many irqs we would need 1024 irqs.

I expect modern big systems that can only run x86_64 are larger still.

>> You have noticed how much of those arrays I have collapsed into irq_cfg
>> on x86_64.  We can ultimately do the same on x86_32.  The
>> tricky one is irq_2_pin.  I believe the proper solution is to just
>> dynamically allocate entries and place a pointer in irq_cfg.  Although
>> we may be able to simply a place a single entry in irq_cfg.

> so there will be irq_desc and irq_cfg lists?
Or we place irq_desc in irq_cfg.

> wonder if helper to get irq_desc and irq_cfg for one irq_no could be bottleneck?

Nah.  We lookup whatever it we need in the 256 entry vector_irq table.
I expect we can do the container_of trick beyond that.

If the helper which we should only see on the slow path is a bottleneck
we can easily turn organize irq_desc into a tree structure.  Ultimately
I think we want drivers to have a struct irq *irq pointer but we need
to get the arch backend working first.

> PS: cpumask_t domain in irq_cfg need to updated... it wast 512bytes
> when NR_CPUS=4096
> could change it to unsigned int. logical mode (flat, x2apic logical) it as mask
> and (physical flat mode, and x2apic physical) it is cpu number.

Certainly there is the potential to simplify things.

>> I agree with your sentiment if we can actually allocate the irqs by
>> demand instead of preallocating them based on worst case usage we
>> should use much less memory.
>
> yes.
>
>>
>> I figure that keeping any type of nr_irqs around you are requiring
>> us to estimate the worst case number of irqs we need to deal with.
>
> need to comprise flexibility and performance..., or say waste some
> space to get some performance...

The thing is there is no good upper bound of how many irqs we can see
short of of NR_PCI_DEVICES*4096

>> The challenge is that we have hot plug devices with MSI-X capabilities
>> on them.  Just one of those could add 4K irqs (worst case).  256 or
>> so I have actually heard hardware guys talking about.

> good know. so one cpu handle one card? or need 16 cpus serve one
> cards? or they got new cpu to NR_VECTORS  with 32bit?

Yes.  Currently for the current worst case it requires 16 cpus.
The biggest I have heard a card using at this point is 256 irqs.
At lot of the goal in those cards is so they can have 2 irqs per cpu.
1 rx irq and 1 tx irq.  Allowing them to implement per cpu queues.

> then need to keep struct irq_desc, can not put everything into it.

Yes.  But we can put all the arch specific code in irq_cfg, and put
irq_desc in irq_cfg.

>> But even one msi vector on a pci card that doesn't have normal irqs could
>> mess up a tightly sized nr_irqs based soley on acpi_madt probing.
>
> v2 double that last_gsi_end

Which is usable, but no where near as nice as not having a fixed upper bound.

>> Sorry I was referring to the MSI-X source vector number which is a 12
>> bit index into an array of MSI-X vectors on the pci device, not the
>> vector we receive the irq at on the pci card.
>
> cpu is going to check that vectors in addition to vectors in IDT?

No. The destination cpu and destination vector number are encoded in
the MSI message.  Each MSI-X source ``vector'' has a different MSI message.

So on my wish list is to stably encode the MSI interurrpt numbers.  And
using a sparse irq address space I can.  As it only takes 28 bits to hold
the complete bus + device + function + msi source [ 0-4095 ] 

Eric
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