linux-kernel - Re: [PATCH v2 2/2] perf bench: Add support for 32-bit systems with 64-bit time

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Date:   Fri, 10 Sep 2021 09:17:42 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Alistair Francis <alistair.francis@...nsource.wdc.com>
Cc:     linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-perf-users@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alistair Francis <alistair23@...il.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Darren Hart <dvhart@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Atish Patra <atish.patra@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Alistair Francis <alistair.francis@....com>
Subject: Re: [PATCH v2 2/2] perf bench: Add support for 32-bit systems with
 64-bit time_t

On Fri, Sep 10, 2021 at 6:31 AM Alistair Francis
<alistair.francis@...nsource.wdc.com> wrote:
>
> From: Alistair Francis <alistair.francis@....com>
>
> Some 32-bit architectures (such are 32-bit RISC-V) only have a 64-bit
> time_t and as such don't have the SYS_futex syscall. This patch will
> allow us to use the SYS_futex_time64 syscall on those platforms.
>
> Signed-off-by: Alistair Francis <alistair.francis@....com>
> ---
>  tools/perf/bench/futex.h | 43 +++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 42 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/bench/futex.h b/tools/perf/bench/futex.h
> index f80a4759ee79b..09c5596726c60 100644
> --- a/tools/perf/bench/futex.h
> +++ b/tools/perf/bench/futex.h
> @@ -27,6 +27,12 @@ struct bench_futex_parameters {
>         unsigned int nrequeue;
>  };
>
> +/* A version of 'struct timespec' with 32-bit time_t and nanoseconds.  */
> +struct __timespec32 {
> +       __kernel_long_t tv_sec;
> +       __kernel_long_t tv_nsec;
> +};

Why not use the existing __kernel_old_timespec structure definition
from the header?

> +#if defined(SYS_futex)
> +       if (sizeof(*timeout) == sizeof(struct __timespec32))
> +               return syscall(SYS_futex, uaddr, op | opflags, val, timeout, uaddr2, val3);
> +
> +       if (timeout && timeout->tv_sec == (long)timeout->tv_sec) {
> +               struct __timespec32 ts32;
> +
> +               ts32.tv_sec = (__kernel_long_t) timeout->tv_sec;
> +               ts32.tv_nsec = (__kernel_long_t) timeout->tv_nsec;
> +
> +               return syscall(SYS_futex, uaddr, op | opflags, val, ts32, uaddr2, val3);
> +       } else if (!timeout) {
> +               return syscall(SYS_futex, uaddr, op | opflags, val, NULL, uaddr2, val3);
> +       }
> +#endif
> +
> +       errno = ENOSYS;
> +       return -1;

Similarly, I think just using the kernel's constants like __NR_futex instead of
SYS_futex would be more appropriate here, this way you know if futex_time64
is available and don't have to do the emulation for running the binary
on pre-5.1
kernels. Perf already uses __NR_perf_event_open, __NR_getcpu, and
__NR_setns instead of the SYS_* versions.

         Arnd