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

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Message-ID: <6984b38d-0b55-0e31-4096-209b2a0f7ef3@nvidia.com>
Date:   Tue, 10 Jan 2023 11:17:39 -0600
From:   Shanker Donthineni <sdonthineni@...dia.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64: gic: increase the number of IRQ descriptors



On 1/10/23 02:20, Marc Zyngier wrote:
> External email: Use caution opening links or attachments
> 
> 
> On Mon, 09 Jan 2023 17:13:25 +0000,
> Shanker Donthineni <sdonthineni@...dia.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?
> 

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