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

From: Marc Zyngier <maz@kernel.org>
To: Shanker Donthineni <sdonthineni@nvidia.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, James Morse <james.morse@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] arm64: gic: increase the number of IRQ descriptors
Date: Tue, 10 Jan 2023 08:20:38 +0000	[thread overview]
Message-ID: <87pmbmer6x.wl-maz@kernel.org> (raw)
In-Reply-To: <ed471259-5d32-e93b-20d6-42a32b8b3d89@nvidia.com>

On Mon, 09 Jan 2023 17:13:25 +0000,
Shanker Donthineni <sdonthineni@nvidia.com> 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?

It should be fairly straightforward to perform the conversion in
place.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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