Re: [PATCH v2] riscv: add irq stack support

[Jisheng Zhang <jszhang@kernel.org>]

On Mon, Mar 07, 2022 at 08:19:35PM +0100, Arnd Bergmann wrote:
> On Mon, Mar 7, 2022 at 3:08 PM Jisheng Zhang <jszhang@kernel.org> wrote:
> >
> > Currently, IRQs are still handled on the kernel stack of the current
> > task on riscv platforms. If the task has a deep call stack at the time
> > of interrupt, and handling the interrupt also requires a deep stack,
> > it's possible to see stack overflow.
> >
> > Before this patch, the stack_max_size of a v5.17-rc1 kernel running on
> > a lichee RV board gave:
> > ~ # cat /sys/kernel/debug/tracing/stack_max_size
> > 3736
> >
> > After this patch,
> > ~ # cat /sys/kernel/debug/tracing/stack_max_size
> > 3176
> >
> > We reduce the max kernel stack usage by 560 bytes!
> >
> > From another side, after this patch, it's possible to reduce the
> > THREAD_SIZE to 8KB for RV64 platforms. This is especially useful for
> > those systems with small memory size, e.g the Allwinner D1S platform
> > which is RV64 but only has 64MB DDR.
> >
> > Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> 
> Very nice!
> 
> > diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> > index ed29e9c8f660..57c9b64e16a5 100644
> > --- a/arch/riscv/kernel/entry.S
> > +++ b/arch/riscv/kernel/entry.S
> > @@ -126,12 +126,39 @@ skip_context_tracking:
> >          */
> >         bge s4, zero, 1f
> >
> > -       la ra, ret_from_exception
> > +       /* preserve the sp */
> > +       move s0, sp
> >
> > -       /* Handle interrupts */
> >         move a0, sp /* pt_regs */
> > +
> > +       /*
> > +        * Compare sp with the base of the task stack.
> > +        * If the top ~(THREAD_SIZE - 1) bits match, we are on a task stack,
> > +        * and should switch to the irq stack.
> > +        */
> > +       REG_L t0, TASK_STACK(tp)
> > +       xor t0, t0, s0
> > +       li t1, ~(THREAD_SIZE - 1)
> > +       and t0, t0, t1
> > +       bnez t0, 2f
> > +
> > +       la t1, irq_stack
> > +       REG_L t2, TASK_TI_CPU(tp)
> > +       slli t2, t2, RISCV_LGPTR
> > +       add t1, t1, t2
> > +       REG_L t2, 0(t1)
> > +       li t1, IRQ_STACK_SIZE
> > +       /* switch to the irq stack */
> > +       add sp, t2, t1
> > +
> > +2:
> 
> What is the benefit of doing this in assembler? Is it measurably faster?
> 
> I see that arm64 does the same thing in C code, and it would be best to
> have a common implementation for doing this, in terms of maintainability.
> 

Hi Arnd,

Sorry for delay. The assembler code is mainly to cal the stack ptr then
change the SP to use the stack, which equals to arm64 call_on_irq_stack()
which is implemented in assembler too.

> > +
> > +       for_each_possible_cpu(cpu) {
> > +#ifdef CONFIG_VMAP_STACK
> > +               void *s = __vmalloc_node(IRQ_STACK_SIZE, THREAD_ALIGN,
> > +                                        THREADINFO_GFP, cpu_to_node(cpu),
> > +                                        __builtin_return_address(0));
> > +#else
> > +               void *s = (void *)__get_free_pages(GFP_KERNEL, get_order(IRQ_STACK_SIZE));
> > +#endif
> 
> On a related topic: is there a reason to still keep the non-VMAP_STACK

irq stack is 16KB on RV64 now, vmalloc doesn't gurantee physical
continuous pages, I want to keep the stack physical continuous
characteristic for !VMAP_STACK case.

Thanks

> code path around? I see that it currently is optional for 64-bit with MMU,
> but not available otherwise. The benefits should still outweigh the downside
> (virtual address space usage mainly) on 32-bit, especially when this allows
> a common implementation. Not sure about NOMMU, but I would guess
> that it's not a big issue to use the same code there as well, since nommu
> vmalloc just turns into a kmalloc as well.
> 


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