linux-kernel - Re: Adjust interrupt Priority for ARM64 GIC

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Message-ID: <51146530616bb8fdf23c637ff5bee44e@kernel.org>
Date:   Tue, 01 Sep 2020 08:48:02 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Dongjiu Geng <gengdongjiu@...wei.com>
Cc:     linux-kernel@...r.kernel.org, linuxarm@...wei.com
Subject: Re: Adjust interrupt Priority for ARM64 GIC

Hi Dongjiu,

In the future, please use my kernel.org address, as I don't work
for ARM anymore, and would have missed this email if I wasn't pointed
to it.

On 2020-08-14 18:10, Dongjiu Geng wrote:
> Hi Marc,
>    In the Linux kernel, we can not adjust the  interrupt Priority, For
> all the interrupts, the interrupt Priority are fixed to 0xa0.
> In some scenarios, it needs to change the Priority. so I want to
> upstream a serie patch to support to change the Priority through
> procfs. do you agree I upstream this feature? thanks~

No, that's not something I would ever consider, and for multiple
reasons:

- Linux only supports a single priority, meaning that interrupts are
   themselves aren't preemptable. Dealing with things like (pseudo) NMI
   is invasive enough, and I can't see a good reason to relax the
   single priority requirement.

- Building on top of the above, the whole scheduler and locking model
   relies on the non-preemptable property of an interrupt.

- I cannot see a good reason to leave the priority control to userspace.
   That's a sure recipe for userspace-controlled livelocks.

Now, I'm sure you want to introduce this for a reason, and you are not
explaining it ("some scenarios" doesn't quite cut it). If you care to
explain these "scenarios", maybe there is something we can do.

But please don't waste time implementing any sort of priority change,
there is no way I'll consider it as such.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...