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

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Wed, Apr 27, 2022 at 1:05 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> 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?
>

Fixed in V2, alpha arch will be slower but otherwise should be the same.

> Thanks,

>
> --
> H.J.