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

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Message-Id: <CFF333D1-E771-4C50-902A-444DAE76C163@gmail.com>
Date:	Thu, 17 Sep 2015 22:22:26 +0900
From:	Jungseok Lee <jungseoklee85@...il.com>
To:	Catalin Marinas <Catalin.Marinas@....com>
Cc:	will.deacon@....com, linux-arm-kernel@...ts.infradead.org,
	mark.rutland@....com, takahiro.akashi@...aro.org,
	James Morse <james.morse@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64: Introduce IRQ stack

On Sep 17, 2015, at 10:17 PM, Jungseok Lee wrote:
> On Sep 17, 2015, at 8:17 PM, Catalin Marinas wrote:
> 
> Hi Catalin,
> 
>> On Sun, Sep 13, 2015 at 02:42:17PM +0000, Jungseok Lee wrote:
>>> Currently, kernel context and interrupts are handled using a single
>>> kernel stack navigated by sp_el1. This forces many systems to use
>>> 16KB stack, not 8KB one. Low memory platforms naturally suffer from
>>> memory pressure accompanied by performance degradation.
>>> 
>>> This patch addresses the issue as introducing a separate percpu IRQ
>>> stack to handle both hard and soft interrupts with two ground rules:
>>> 
>>> - Utilize sp_el0 in EL1 context, which is not used currently
>>> - Do not complicate current_thread_info calculation
>>> 
>>> It is a core concept to trace struct thread_info using sp_el0 instead
>>> of sp_el1. This approach helps arm64 align with other architectures
>>> regarding object_is_on_stack() without additional complexity.
>> 
>> I'm still trying to understand how this patch works. I initially thought
>> that we would set SPSel = 0 while in kernel thread mode to make use of
>> SP_EL0 but I can't find any such code. Do you still use SP_EL1 all the
>> time and SP_EL0 just for temporary saving the thread stack?
> 
> Exactly.
> 
> My first approach was to set SPSel = 0 and implement EL1t Sync and IRQ.
> This idea originally comes from your comment [1]. A kernel thread could
> be handled easily and neatly, but it complicated current_thread_info
> calculation due to a user process.
> 
> Let's assume that a kernel thread uses SP_EL0 by default. When an interrupt
> comes in, a core jumps to EL1t IRQ. In case of a user process, a CPU goes
> into EL1h IRQ when an interrupt raises. To handle this scenario correctly,
> SPSel or spsr_el1 should be referenced. This reaches to quite big overhead
> in current_thread_info function.

This statement is described incorrectly. In case of user process, a CPU goes
into EL0 IRQ. Under this context, another interrupt could come in. At this
time, a core jumps to EL1h IRQ.

My original intention is to describe this situation.

Sorry for confusion.

Best Regards
Jungseok Lee
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