[PATCH v2] sysdeps: Add 'get_fast_jitter' interace in fast-jitter.h

[Noah Goldstein]

On Sun, Sep 11, 2022 at 4:27 PM Sunil Pandey <skpgkp2@gmail.com> wrote:
>
> On Wed, Apr 27, 2022 at 1:47 PM H.J. Lu via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > On Wed, Apr 27, 2022 at 1:13 PM Noah Goldstein <goldstein.w.n@gmail.com> 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' iff it is
> > > inlined (avoid any potential syscall paths).
> > > ---
> > >  sysdeps/generic/fast-jitter.h | 42 +++++++++++++++++++++++++++++++++++
> > >  1 file changed, 42 insertions(+)
> > >  create mode 100644 sysdeps/generic/fast-jitter.h
> > >
> > > diff --git a/sysdeps/generic/fast-jitter.h b/sysdeps/generic/fast-jitter.h
> > > new file mode 100644
> > > index 0000000000..4dd53e3475
> > > --- /dev/null
> > > +++ b/sysdeps/generic/fast-jitter.h
> > > @@ -0,0 +1,42 @@
> > > +/* 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>
> > > +# include <hp-timing.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)
> > > +{
> > > +# if HP_TIMING_INLINE
> > > +  hp_timing_t jitter;
> > > +  HP_TIMING_NOW (jitter);
> > > +  return (uint32_t) jitter;
> > > +# else
> > > +  return 0;
> > > +# endif
> > > +}
> > > +
> > > +#endif
> > > --
> > > 2.25.1
> > >
> >
> > LGTM.    An arch can provide a better one if needed.
> >
> > Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
> >
> > Thanks.
> >
> > --
> > H.J.
>
> I would like to backport this patch to release branch 2.33, 2.34 and 2.35

Fine by me.
>
> Any comments/suggestions or objections on this.
>
> commit 911c63a51c690dd1a97dfc587097277029baf00f
> Author: Noah Goldstein <goldstein.w.n@gmail.com>
> Date:   Wed Apr 27 15:13:02 2022 -0500
>
>     sysdeps: Add 'get_fast_jitter' interace in fast-jitter.h
>
>     '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.