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

[Matt Redfearn]

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@xxxxxxxxxx> 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