Re: [PATCH] riscv: Improve exception and system call latency

Jisheng Zhang
2023-12-25T10:21:49Z

On Sun, Dec 24, 2023 at 08:00:18PM -0800, Anton Blanchard wrote:
> Many CPUs implement return address branch prediction as a stack. The
> RISCV architecture refers to this as a return address stack (RAS). If
> this gets corrupted then the CPU will mispredict at least one but
> potentally many function returns.
>
> There are two issues with the current RISCV exception code:
>
> - We are using the alternate link stack (x5/t0) for the indirect branch
> which makes the hardware think this is a function return. This will
> corrupt the RAS.
>
> - We modify the return address of handle_exception to point to
> ret_from_exception. This will also corrupt the RAS.
>
> Testing the null system call latency before and after the patch:
>
> Visionfive2 (StarFive JH7110 / U74)
> baseline: 189.87 ns
> patched: 176.76 ns
>
> Lichee pi 4a (T-Head TH1520 / C910)
> baseline: 666.58 ns
> patched: 636.90 ns
>
> Just over 7% on the U74 and just over 4% on the C910.

Nice improvement!

>
> Signed-off-by: Anton Blanchard <antonb@tenstorrent.com>
> ---
>
> This introduces some complexity in the stackframe walk code. PowerPC
> resolves the multiple exception exit paths issue by placing a value into
> the exception stack frame (basically the word "REGS") that the stack frame
> code can look for. Perhaps something to look at.
>
> arch/riscv/kernel/entry.S | 21 ++++++++++++++-------
> arch/riscv/kernel/stacktrace.c | 14 +++++++++++++-
> 2 files changed, 27 insertions(+), 8 deletions(-)
>
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 54ca4564a926..89af35edbf6c 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -84,7 +84,6 @@ SYM_CODE_START(handle_exception)
> scs_load_current_if_task_changed s5
>
> move a0, sp /* pt_regs */
> - la ra, ret_from_exception
>
> /*
> * MSB of cause differentiates between
> @@ -93,7 +92,10 @@ SYM_CODE_START(handle_exception)
> bge s4, zero, 1f
>
> /* Handle interrupts */
> - tail do_irq
> + call do_irq
> +.globl ret_from_irq_exception
> +ret_from_irq_exception:
> + j ret_from_exception
> 1:
> /* Handle other exceptions */
> slli t0, s4, RISCV_LGPTR
> @@ -101,11 +103,16 @@ SYM_CODE_START(handle_exception)
> la t2, excp_vect_table_end
> add t0, t1, t0
> /* Check if exception code lies within bounds */
> - bgeu t0, t2, 1f
> - REG_L t0, 0(t0)
> - jr t0
> -1:
> - tail do_trap_unknown
> + bgeu t0, t2, 3f
> + REG_L t1, 0(t0)
> +2: jalr ra,t1

can be simplified to
jalr t1

with the above change,
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>

> +.globl ret_from_other_exception
> +ret_from_other_exception:
> + j ret_from_exception
> +3:
> +
> + la t1, do_trap_unknown
> + j 2b
> SYM_CODE_END(handle_exception)
>
> /*
> diff --git a/arch/riscv/kernel/stacktrace.c b/arch/riscv/kernel/stacktrace.c
> index 64a9c093aef9..b9cd131bbc4c 100644
> --- a/arch/riscv/kernel/stacktrace.c
> +++ b/arch/riscv/kernel/stacktrace.c
> @@ -17,6 +17,18 @@
> #ifdef CONFIG_FRAME_POINTER
>
> extern asmlinkage void ret_from_exception(void);
> +extern asmlinkage void ret_from_irq_exception(void);
> +extern asmlinkage void ret_from_other_exception(void);
> +
> +static inline bool is_exception_frame(unsigned long pc)
> +{
> + if ((pc == (unsigned long)ret_from_exception) ||
> + (pc == (unsigned long)ret_from_irq_exception) ||
> + (pc == (unsigned long)ret_from_other_exception))
> + return true;
> +
> + return false;
> +}
>
> void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
> bool (*fn)(void *, unsigned long), void *arg)
> @@ -62,7 +74,7 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
> fp = frame->fp;
> pc = ftrace_graph_ret_addr(current, NULL, frame->ra,
> &frame->ra);
> - if (pc == (unsigned long)ret_from_exception) {
> + if (is_exception_frame(pc)) {
> if (unlikely(!__kernel_text_address(pc) || !fn(arg, pc)))
> break;
>
> --
> 2.25.1
>
>
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