Re: [PATCH v2] riscv: Add tracepoints for SBI calls and returns

[Alexandre Ghiti]

Hi Samuel,

On Fri, Mar 22, 2024 at 12:01 AM Samuel Holland
<samuel.holland@xxxxxxxxxx> wrote:
>
> These are useful for measuring the latency of SBI calls. The SBI HSM
> extension is excluded because those functions are called from contexts
> such as cpuidle where instrumentation is not allowed.
>
> Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>
> ---
>
> Changes in v2:
>  - Remove some blank lines
>
>  arch/riscv/include/asm/trace.h | 54 ++++++++++++++++++++++++++++++++++
>  arch/riscv/kernel/sbi.c        |  7 +++++
>  2 files changed, 61 insertions(+)
>  create mode 100644 arch/riscv/include/asm/trace.h
>
> diff --git a/arch/riscv/include/asm/trace.h b/arch/riscv/include/asm/trace.h
> new file mode 100644
> index 000000000000..6151cee5450c
> --- /dev/null
> +++ b/arch/riscv/include/asm/trace.h
> @@ -0,0 +1,54 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#undef TRACE_SYSTEM
> +#define TRACE_SYSTEM riscv
> +
> +#if !defined(_TRACE_RISCV_H) || defined(TRACE_HEADER_MULTI_READ)
> +#define _TRACE_RISCV_H
> +
> +#include <linux/tracepoint.h>
> +
> +TRACE_EVENT_CONDITION(sbi_call,
> +       TP_PROTO(int ext, int fid),
> +       TP_ARGS(ext, fid),
> +       TP_CONDITION(ext != SBI_EXT_HSM),
> +
> +       TP_STRUCT__entry(
> +               __field(int, ext)
> +               __field(int, fid)
> +       ),
> +
> +       TP_fast_assign(
> +               __entry->ext = ext;
> +               __entry->fid = fid;
> +       ),
> +
> +       TP_printk("ext=0x%x fid=%d", __entry->ext, __entry->fid)
> +);
> +
> +TRACE_EVENT_CONDITION(sbi_return,
> +       TP_PROTO(int ext, long error, long value),
> +       TP_ARGS(ext, error, value),
> +       TP_CONDITION(ext != SBI_EXT_HSM),
> +
> +       TP_STRUCT__entry(
> +               __field(long, error)
> +               __field(long, value)
> +       ),
> +
> +       TP_fast_assign(
> +               __entry->error = error;
> +               __entry->value = value;
> +       ),
> +
> +       TP_printk("error=%ld value=0x%lx", __entry->error, __entry->value)
> +);
> +
> +#endif /* _TRACE_RISCV_H */
> +
> +#undef TRACE_INCLUDE_PATH
> +#undef TRACE_INCLUDE_FILE
> +
> +#define TRACE_INCLUDE_PATH asm
> +#define TRACE_INCLUDE_FILE trace
> +
> +#include <trace/define_trace.h>
> diff --git a/arch/riscv/kernel/sbi.c b/arch/riscv/kernel/sbi.c
> index e66e0999a800..a1d21d8f5293 100644
> --- a/arch/riscv/kernel/sbi.c
> +++ b/arch/riscv/kernel/sbi.c
> @@ -14,6 +14,9 @@
>  #include <asm/smp.h>
>  #include <asm/tlbflush.h>
>
> +#define CREATE_TRACE_POINTS
> +#include <asm/trace.h>
> +
>  /* default SBI version is 0.1 */
>  unsigned long sbi_spec_version __ro_after_init = SBI_SPEC_VERSION_DEFAULT;
>  EXPORT_SYMBOL(sbi_spec_version);
> @@ -31,6 +34,8 @@ struct sbiret sbi_ecall(int ext, int fid, unsigned long arg0,
>  {
>         struct sbiret ret;
>
> +       trace_sbi_call(ext, fid);
> +
>         register uintptr_t a0 asm ("a0") = (uintptr_t)(arg0);
>         register uintptr_t a1 asm ("a1") = (uintptr_t)(arg1);
>         register uintptr_t a2 asm ("a2") = (uintptr_t)(arg2);
> @@ -46,6 +51,8 @@ struct sbiret sbi_ecall(int ext, int fid, unsigned long arg0,
>         ret.error = a0;
>         ret.value = a1;
>
> +       trace_sbi_return(ext, ret.error, ret.value);
> +
>         return ret;
>  }
>  EXPORT_SYMBOL(sbi_ecall);
> --
> 2.43.1
>

sbi_ecall() is used by flush_icache_all() so I was a bit scared we
would need to patch 2 instructions using a tracepoint and then to use
flush_icache_all() which could bug if only one of the patched
instruction was seen. But I took a look at the disassembled code, and
tracepoints happen to only need to patch 1 nop into an unconditional
jump, which is not a problem then.

But then when disassembling, I noticed that the addition of the
tracepoint comes with "quite" some overhead:

Before:

Dump of assembler code for function sbi_ecall:
   0xffffffff800085e0 <+0>: add sp,sp,-32
   0xffffffff800085e2 <+2>: sd s0,24(sp)
   0xffffffff800085e4 <+4>: mv t1,a0
   0xffffffff800085e6 <+6>: add s0,sp,32
   0xffffffff800085e8 <+8>: mv t3,a1
   0xffffffff800085ea <+10>: mv a0,a2
   0xffffffff800085ec <+12>: mv a1,a3
   0xffffffff800085ee <+14>: mv a2,a4
   0xffffffff800085f0 <+16>: mv a3,a5
   0xffffffff800085f2 <+18>: mv a4,a6
   0xffffffff800085f4 <+20>: mv a5,a7
   0xffffffff800085f6 <+22>: mv a6,t3
   0xffffffff800085f8 <+24>: mv a7,t1
   0xffffffff800085fa <+26>: ecall
   0xffffffff800085fe <+30>: ld s0,24(sp)
   0xffffffff80008600 <+32>: add sp,sp,32
   0xffffffff80008602 <+34>: ret

After:

Dump of assembler code for function sbi_ecall:
   0xffffffff8000bbf2 <+0>: add sp,sp,-112
   0xffffffff8000bbf4 <+2>: sd s0,96(sp)
   0xffffffff8000bbf6 <+4>: sd s1,88(sp)
   0xffffffff8000bbf8 <+6>: sd s3,72(sp)
   0xffffffff8000bbfa <+8>: sd s4,64(sp)
   0xffffffff8000bbfc <+10>: sd s5,56(sp)
   0xffffffff8000bbfe <+12>: sd s6,48(sp)
   0xffffffff8000bc00 <+14>: sd s7,40(sp)
   0xffffffff8000bc02 <+16>: sd s8,32(sp)
   0xffffffff8000bc04 <+18>: sd s9,24(sp)
   0xffffffff8000bc06 <+20>: sd ra,104(sp)
   0xffffffff8000bc08 <+22>: sd s2,80(sp)
   0xffffffff8000bc0a <+24>: add s0,sp,112
   0xffffffff8000bc0c <+26>: mv s1,a0
   0xffffffff8000bc0e <+28>: mv s3,a1
   0xffffffff8000bc10 <+30>: mv s9,a2
   0xffffffff8000bc12 <+32>: mv s8,a3
   0xffffffff8000bc14 <+34>: mv s7,a4
   0xffffffff8000bc16 <+36>: mv s6,a5
   0xffffffff8000bc18 <+38>: mv s5,a6
   0xffffffff8000bc1a <+40>: mv s4,a7
   0xffffffff8000bc1c <+42>: nop
   0xffffffff8000bc20 <+46>: mv a0,s9
   0xffffffff8000bc22 <+48>: mv a1,s8
   0xffffffff8000bc24 <+50>: mv a2,s7
   0xffffffff8000bc26 <+52>: mv a3,s6
   0xffffffff8000bc28 <+54>: mv a4,s5
   0xffffffff8000bc2a <+56>: mv a5,s4
   0xffffffff8000bc2c <+58>: mv a6,s3
   0xffffffff8000bc2e <+60>: mv a7,s1
   0xffffffff8000bc30 <+62>: ecall
   0xffffffff8000bc34 <+66>: mv s4,a0
   0xffffffff8000bc36 <+68>: mv s3,a1
   0xffffffff8000bc38 <+70>: nop
   0xffffffff8000bc3c <+74>: ld ra,104(sp)
   0xffffffff8000bc3e <+76>: ld s0,96(sp)
   0xffffffff8000bc40 <+78>: ld s1,88(sp)
   0xffffffff8000bc42 <+80>: ld s2,80(sp)
   0xffffffff8000bc44 <+82>: ld s5,56(sp)
   0xffffffff8000bc46 <+84>: ld s6,48(sp)
   0xffffffff8000bc48 <+86>: ld s7,40(sp)
   0xffffffff8000bc4a <+88>: ld s8,32(sp)
   0xffffffff8000bc4c <+90>: ld s9,24(sp)
   0xffffffff8000bc4e <+92>: mv a0,s4
   0xffffffff8000bc50 <+94>: mv a1,s3
   0xffffffff8000bc52 <+96>: ld s4,64(sp)
   0xffffffff8000bc54 <+98>: ld s3,72(sp)
   0xffffffff8000bc56 <+100>: add sp,sp,112
   0xffffffff8000bc58 <+102>: ret

Not sure this is really problematic though, let me know what you think.

FYI, I'm about to send a patch to reorder the arguments of sbi_ecall()
to avoid the mv-dance before the ecall.

Thanks,

Alex