[PATCH v1] sysdeps: Add 'get_fast_jitter' interace in fast-jitter.h

[Adhemerval Zanella]

On 04/04/2022 18:39, Noah Goldstein via Libc-alpha wrote:
> 'get_fast_jitter' is meant to be used purely for performance
> purposes. In all cases it's used it should be acceptable to get no
> randomness (see default case). An example use case is in setting
> jitter for retries between threads at a lock. There is a
> performance benefit to having jitter, but only if the jitter can
> be generated very quickly and ultimately there is no serious issue
> if no jitter is generated.
> 
> The implementation generally uses 'HP_TIMING_NOW' or another cycle
> counter when the architecture supports it. The default case just
> returns zero.

I think it should ok, although I think it would be better to be sent
along with a actual usage (as the pthread mutex optimization).

> ---
>  sysdeps/aarch64/fast-jitter.h | 33 +++++++++++++++++++++++++++++++++
>  sysdeps/alpha/fast-jitter.h   | 34 ++++++++++++++++++++++++++++++++++
>  sysdeps/generic/fast-jitter.h | 35 +++++++++++++++++++++++++++++++++++
>  sysdeps/s390/fast-jitter.h    | 33 +++++++++++++++++++++++++++++++++
>  sysdeps/sparc/fast-jitter.h   | 33 +++++++++++++++++++++++++++++++++
>  sysdeps/x86_64/fast-jitter.h  | 30 ++++++++++++++++++++++++++++++
>  6 files changed, 198 insertions(+)
>  create mode 100644 sysdeps/aarch64/fast-jitter.h
>  create mode 100644 sysdeps/alpha/fast-jitter.h
>  create mode 100644 sysdeps/generic/fast-jitter.h
>  create mode 100644 sysdeps/s390/fast-jitter.h
>  create mode 100644 sysdeps/sparc/fast-jitter.h
>  create mode 100644 sysdeps/x86_64/fast-jitter.h
> 
> diff --git a/sysdeps/aarch64/fast-jitter.h b/sysdeps/aarch64/fast-jitter.h
> new file mode 100644
> index 0000000000..fb09d2632c
> --- /dev/null
> +++ b/sysdeps/aarch64/fast-jitter.h
> @@ -0,0 +1,33 @@
> +/* Fast jitter on arm with HP_TIMING_NOW
> +   Copyright (C) 2022-2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#ifndef _FAST_JITTER_H
> +# define _FAST_JITTER_H
> +
> +#include <stdint.h>
> +#include <hp-timing.h>
> +
> +static inline uint32_t
> +get_fast_jitter (void)
> +{
> +  hp_timing_t jitter;
> +  HP_TIMING_NOW (jitter);
> +  return (uint32_t) jitter;
> +}
> +
> +#endif
> diff --git a/sysdeps/alpha/fast-jitter.h b/sysdeps/alpha/fast-jitter.h
> new file mode 100644
> index 0000000000..0a24290eaa
> --- /dev/null
> +++ b/sysdeps/alpha/fast-jitter.h
> @@ -0,0 +1,34 @@
> +/* Fast jitter on alpha with rpcc
> +   Copyright (C) 2022-2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#ifndef _FAST_JITTER_H
> +# define _FAST_JITTER_H
> +
> +#include <stdint.h>
> +
> +static inline uint32_t
> +get_fast_jitter (void)
> +{
> +  unsigned long int x_;
> +  asm volatile ("rpcc %0" : "=r"(x_));
> +  /* Don't both with process offset.  We only need jitter not
> +     monotonic cycle count.  */
> +  return (uint32_t) x;
> +}
> +
> +#endif
> diff --git a/sysdeps/generic/fast-jitter.h b/sysdeps/generic/fast-jitter.h
> new file mode 100644
> index 0000000000..63bcf3ccd7
> --- /dev/null
> +++ b/sysdeps/generic/fast-jitter.h
> @@ -0,0 +1,35 @@
> +/* Fallback for fast jitter just return 0.
> +   Copyright (C) 2019-2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#ifndef _FAST_JITTER_H
> +# define _FAST_JITTER_H
> +
> +#include <stdint.h>
> +
> +/* Baseline just return 0.  We could create jitter using a clock or
> +   'random_bits' but that may imply a syscall and the goal of
> +   'get_fast_jitter' is minimal overhead "randomness" when such
> +   randomness helps performance.  Adding high overhead the function
> +   defeats the purpose.  */
> +static inline uint32_t
> +get_fast_jitter (void)
> +{
> +  return 0;
> +}
> +
> +#endif
> diff --git a/sysdeps/s390/fast-jitter.h b/sysdeps/s390/fast-jitter.h
> new file mode 100644
> index 0000000000..eb905b1413
> --- /dev/null
> +++ b/sysdeps/s390/fast-jitter.h
> @@ -0,0 +1,33 @@
> +/* Fast jitter on s390 with HP_TIMING_NOW
> +   Copyright (C) 2022-2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#ifndef _FAST_JITTER_H
> +# define _FAST_JITTER_H
> +
> +#include <stdint.h>
> +#include <hp-timing.h>
> +
> +static inline uint32_t
> +get_fast_jitter (void)
> +{
> +  hp_timing_t jitter;
> +  HP_TIMING_NOW (jitter);
> +  return (uint32_t) jitter;
> +}
> +
> +#endif
> diff --git a/sysdeps/sparc/fast-jitter.h b/sysdeps/sparc/fast-jitter.h
> new file mode 100644
> index 0000000000..f4413754c5
> --- /dev/null
> +++ b/sysdeps/sparc/fast-jitter.h
> @@ -0,0 +1,33 @@
> +/* Fast jitter on sparc with HP_TIMING_NOW
> +   Copyright (C) 2022-2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#ifndef _FAST_JITTER_H
> +# define _FAST_JITTER_H
> +
> +#include <stdint.h>
> +#include <hp-timing.h>
> +
> +static inline uint32_t
> +get_fast_jitter (void)
> +{
> +  hp_timing_t jitter;
> +  HP_TIMING_NOW (jitter);
> +  return (uint32_t) jitter;
> +}
> +
> +#endif
> diff --git a/sysdeps/x86_64/fast-jitter.h b/sysdeps/x86_64/fast-jitter.h
> new file mode 100644
> index 0000000000..1140f1c295
> --- /dev/null
> +++ b/sysdeps/x86_64/fast-jitter.h
> @@ -0,0 +1,30 @@
> +/* X86 use rdtsc to quickly generate pseudo-random for jitter
> +   Copyright (C) 2022-2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#ifndef _FAST_JITTER_H
> +# define _FAST_JITTER_H
> +
> +#include <stdint.h>
> +
> +static inline uint32_t
> +get_fast_jitter (void)
> +{
> +  return __builtin_ia32_rdtsc ();

Why can we use HP_TIMING_NOW for x86 as well?  So you then you can use
only a generic implementation like:

static inline uint32_t
get_fast_jitter (void)
{
#if HP_TIMING_INLINE
  hp_timing_t jitter;
  HP_TIMING_NOW (jitter);
  return (uint32_t) jitter;
#else
  return 0;
#endif
}