Re: [PATCH v2] arm64: gic: increase the number of IRQ descriptors

[Shanker Donthineni]

On 1/10/23 02:20, Marc Zyngier wrote:
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>
>
> On Mon, 09 Jan 2023 17:13:25 +0000,
> Shanker Donthineni <sdonthineni@xxxxxxxxxx> wrote:
> > > > > I'm happy to help with it, but I'm certainly not willing to accept any
> > > > > sort of new compile-time limit.
> > > >
> > > > Thanks for helping with a scalable solution instead of static
> > > > allocation.  Please include me whenever patches posted to LKML. I'm
> > > > happy to verify on NVIDIA server platforms and provide test
> > > > feedback.
> > >
> > > I offered to help you. I didn't offer to do the work for you! ;-)
> >
> > I've looked at the IDR/IDA API. There is no suitable function for
> > allocating contiguous IDs to replace bitmap API.
> >
> > __irq_alloc_descs():
> >
> >        mutex_lock(&sparse_irq_lock);
> >
> >        start = bitmap_find_next_zero_area(allocated_irqs, IRQ_BITMAP_BITS,
> >                                           from, cnt, 0);
> >        ret = -EEXIST;
> >
> > Is there any existing API that I can use for allocating contiguous IDs?
>
> I think you should address the problem the other way around, as there
> are lower hanging fruits:
>
> - turn the irq_desc_tree radix tree into a XArray
>
> - use the XArray mark feature to reimplement the irqs_resend bitmap
>
> Once you have done that, you have already halved the memory usage.
> To implement the allocated_irqs bitmap functionality, you have a
> bunch of options:
>
> - make the XArray an allocating XArray, and iterate over XA_FREE_MARK
>    to find the free range (see how the infiniband subsystem is doing
>    exactly that)
>
> - use another Xarray mark to annotate the allocated IRQs, find the
>    distance between two allocations, and use this range if the request
>    fits (a poor man's variation of the above)
>
> - use a sideband data structure such as the GICv3 LPI allocator, which
>    is already dealing with range allocation (I'd rather avoid that)
>
> - something else?

I'll also prototype using XArray data structure instead of IDR based.