Messages in this thread | | | From | Arnd Bergmann <> | Date | Mon, 7 Mar 2022 20:19:35 +0100 | Subject | Re: [PATCH v2] riscv: add irq stack support |
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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.
> + > + 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 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.
Arnd
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