linux-kernel - Re: [PATCH 1/2] genirq/msi, platform-msi: Adjust return value of msi_domain_prepare

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Date:   Mon, 29 May 2023 11:27:29 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Huacai Chen <chenhuacai@...il.com>, Marc Zyngier <maz@...nel.org>
Cc:     Huacai Chen <chenhuacai@...ngson.cn>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org, loongson-kernel@...ts.loongnix.cn,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Jiaxun Yang <jiaxun.yang@...goat.com>
Subject: Re: [PATCH 1/2] genirq/msi, platform-msi: Adjust return value of
 msi_domain_prepare_irqs()

On Sun, May 28 2023 at 20:07, Huacai Chen wrote:
> On Sun, May 28, 2023 at 3:47 PM Marc Zyngier <maz@...nel.org> wrote:
>>
>> Being able to allocate MSIs is not a guarantee, and is always
>> opportunistic. If some drivers badly fail because the they don't get
>> the number of MSIs they need, then they need fixing.
>
> Yes, I know allocating MSIs is not a guarantee, and most existing
> drivers will fallback to use legacy irqs when failed. However, as I
> replied in an early mail, we want to do some proactive throttling in
> the loongson-pch-msi irqchip driver, rather than consume msi vectors
> aggressively. For example, if we have two NICs, we want both of them
> to get 32 msi vectors; not one exhaust all available vectors, and the
> other fallback to use legacy irq.

By default you allow up to 256 interrupts to be allocated, right? So to
prevent vector exhaustion, the admin needs to reboot the machine and set
a command line parameter to limit this, right? As that parameter is not
documented the admin is going to dice a number. That's impractical and
just a horrible bandaid.

Thanks,

        tglx