linux-kernel - Re: [PATCH 0/5] MIPS: Add per-cpu IRQ stack

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Date:   Wed, 7 Dec 2016 09:30:23 +0000
From:   Matt Redfearn <matt.redfearn@...tec.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
CC:     Ralf Baechle <ralf@...ux-mips.org>, <linux-mips@...ux-mips.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Marcin Nowakowski <marcin.nowakowski@...tec.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Petr Mladek <pmladek@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrea Gelmini <andrea.gelmini@...ma.net>,
        James Hogan <james.hogan@...tec.com>,
        Paul Burton <paul.burton@...tec.com>,
        Jiri Slaby <jslaby@...e.cz>,
        "Maciej W. Rozycki" <macro@...tec.com>,
        Aaron Tomlin <atomlin@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/5] MIPS: Add per-cpu IRQ stack

Hi Jason,


On 06/12/16 22:09, Jason A. Donenfeld wrote:
> Hi Matt,
>
> Thanks for submitting this. A happy OpenWRT/WireGuard user has
> reported to me that this patch set frees ~1300 bytes of stack on a
> small MIPS router. This kind of savings should allow me to reintroduce
> my crypto operations to be on the stack, rather than the conditional
> MIPS kmallocing, which will be a performance win.

Excellent.

>
> By the way, it looks like x86 and SPARC have separately allocated hard
> IRQ and soft IRQ stacks. ARM has only one for both, similar to this
> MIPS patchset.

The separate stack for soft-IRQs could be added as well, but of course 
there is a tradeoff against the additional memory consumed by it. I 
believe that most soft IRQs will now be handled on the new irq stack, 
because HAVE_IRQ_EXIT_ON_IRQ_STACK causes them to be invoked within 
irq_exit, while still executing on the irq stack

Thanks,
Matt

>
> Jason
>
> On Fri, Dec 2, 2016 at 2:39 PM, Matt Redfearn <matt.redfearn@...tec.com> wrote:
>> This series adds a separate stack for each CPU wihin the system to use
>> when handling IRQs. Previously IRQs were handled on the kernel stack of
>> the current task. If that task was deep down a call stack at the point
>> of the interrupt, and handling the interrupt required a deep IRQ stack,
>> then there was a likelihood of stack overflow. Since the kernel stack
>> is in normal unmapped memory, overflowing it can lead to silent
>> corruption of other kernel data, with weird and wonderful results.
>>
>> Before this patch series, ftracing the maximum stack size of a v4.9-rc6
>> kernel running on a Ci40 board gave:
>> 4996
>>
>> And with this series:
>> 4084
>>
>> Handling interrupts on a separate stack reduces the maximum kernel stack
>> usage in this configuration by ~900 bytes.
>>
>> Since do_IRQ is now invoked on a separate stack, we select
>> HAVE_IRQ_EXIT_ON_IRQ_STACK so that softirqs will also be executed on the
>> irq stack rather than attempting to switch with do_softirq_own_stack().
>>
>> This series has been tested on MIPS Boston, Malta and SEAD3 platforms,
>> Pistachio on the Creator Ci40 board and Cavium Octeon III.
>>
>>
>>
>> Matt Redfearn (5):
>>    MIPS: Introduce irq_stack
>>    MIPS: Stack unwinding while on IRQ stack
>>    MIPS: Only change $28 to thread_info if coming from user mode
>>    MIPS: Switch to the irq_stack in interrupts
>>    MIPS: Select HAVE_IRQ_EXIT_ON_IRQ_STACK
>>
>>   arch/mips/Kconfig                  |  1 +
>>   arch/mips/include/asm/irq.h        | 12 ++++++
>>   arch/mips/include/asm/stackframe.h | 10 +++++
>>   arch/mips/kernel/asm-offsets.c     |  1 +
>>   arch/mips/kernel/genex.S           | 81 +++++++++++++++++++++++++++++++++++---
>>   arch/mips/kernel/irq.c             | 11 ++++++
>>   arch/mips/kernel/process.c         | 15 ++++++-
>>   7 files changed, 125 insertions(+), 6 deletions(-)
>>
>> --
>> 2.7.4
>>