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

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, Apr 27, 2022 at 10:48 AM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
>
>
> 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
> }

I'd like this version.  Noah, can you send v2?

Thanks,

-- 
H.J.