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

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Message-Id: <3C2C78B3-4669-4DBA-98DC-362EB762FD9C@gmail.com>
Date:	Thu, 17 Sep 2015 22:17:52 +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 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.

I always keep my mind on simplicity of the function. Thus, I've decided to
give up the approach.

[1] https://lkml.org/lkml/2015/5/25/454

Best Regards
Jungseok Lee
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