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 [off-list ref] wrote:

quoted

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