linux-kernel - Re: [PATCH v2] arm64: Introduce IRQ stack

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Message-Id: <FE4C26C5-C759-4924-96A5-9D5F6A78CD74@gmail.com>
Date:	Sat, 19 Sep 2015 17:44:37 +0900
From:	Jungseok Lee <jungseoklee85@...il.com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	mark.rutland@....com, will.deacon@....com,
	linux-kernel@...r.kernel.org, takahiro.akashi@...aro.org,
	James Morse <james.morse@....com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2] arm64: Introduce IRQ stack

On Sep 19, 2015, at 12:31 AM, Catalin Marinas wrote:
> On Fri, Sep 18, 2015 at 04:03:02PM +0100, Catalin Marinas wrote:
>> On Fri, Sep 18, 2015 at 09:57:56PM +0900, Jungseok Lee wrote:
>>> On Sep 18, 2015, at 1:21 AM, Catalin Marinas wrote:
>>>> So, without any better suggestion for current_thread_info(), I'm giving
>>>> up the idea of using SPSel == 0 in the kernel. I'll look at your patch
>>>> in more detail. BTW, I don't think we need the any count for the irq
>>>> stack as we don't re-enter the same IRQ stack.
>>> 
>>> Another interrupt could come in since IRQ is enabled when handling softirq
>>> according to the following information which are self-evident.
>>> 
>>> (Am I missing something?)
>> 
>> No. I had the wrong impression that we switch to the softirqd stack for
>> softirqs but you are right, if we run them on the same stack the IRQs
>> are enabled and they can be re-entered before we return from this
>> exception handler.
>> 
>> I've seen other architectures implementing a dedicated softirq stack but
>> for now we should just re-use the current IRQ stack.
>> 
>>> In my first approach using SPSel = 0, current_thread_info function was inefficient
>>> in order to handle this case correctly.
>> 
>> I agree. And we don't have any other scratch register left as we use
>> tpidr_el1 for per-cpu areas.
>> 
>>> BTW, in this context, it is only meaningful to decide whether a current interrupt
>>> is re-enterrant or not. Its actual value is not important, but I could not figure
>>> out a better implementation than this one yet. Any suggestions are welcome!
>> 
>> James' idea of checking the lower SP bits instead of a count may work.
> 
> Another thought (it seems that x86 does something similar): we know the
> IRQ stack is not re-entered until interrupts are enabled in
> __do_softirq. If we enable __ARCH_HAS_DO_SOFTIRQ, we can implement an
> arm64-specific do_softirq_own_stack() which increments a counter before
> calling __do_softirq. The difference from your patch is that
> irq_stack_entry only reads such counter, doesn't need to write it.
> 
> Yet another idea is to reserve some space in the lower address part of
> the stack with a "stack type" information. It still requires another
> read, so I think the x86 approach is probably better.

I've realized both hardirq and softirq should be handled on a separate stack
in order to reduce kernel stack size, which is a principal objective of this
patch. (If I'm not missing something) It is not possible to get a big win
with implementing do_softirq_own_stack() since hardirq is handled using a task
stack. This prevents a size of kernel stack from being decreased.

However, it would be meaningful to separate hard IRQ stack and soft IRQ one
as the next step.

Best Regards
Jungseok Lee--
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