Re: [RFC PATCH 19/19] riscv: Add __riscv_cpu_relax() to allow yielding in busy loops

["Christoph Müllner" <christoph.muellner@vrull.eu>]

[-- Attachment #1: Type: text/plain, Size: 5921 bytes --]

On Tue, Feb 7, 2023 at 1:23 AM Andrew Waterman <andrew@sifive.com> wrote:

> Note that all implementations must support `pause`, since it's a HINT
> instruction encoded within a base-ISA instruction that has no
> architecturally visible effect.  So it's not clear to me that there's
> any virtue in distinguishing implementations that claim to support
> Zihintpause from those that don't.
>

Will be considered in a v2.
Thanks!


>
>
> On Mon, Feb 6, 2023 at 4:17 PM Christoph Muellner
> <christoph.muellner@vrull.eu> wrote:
> >
> > From: Christoph Müllner <christoph.muellner@vrull.eu>
> >
> > The spinning loop of PTHREAD_MUTEX_ADAPTIVE_NP provides the hook
> > atomic_spin_nop() that can be used by architectures.
> >
> > On RISC-V we have two instructions that can be used here:
> > * WRS.STO from the Zawrs extension
> > * PAUSE from the Zihintpause extension
> >
> > Let's use these instructions and prefer WRS.STO over PAUSE
> > (based on availability of the corresponding ISA extension
> > at runtime).
> >
> > Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
> > ---
> >  sysdeps/riscv/multiarch/Makefile              |  5 +++
> >  sysdeps/riscv/multiarch/cpu_relax.c           | 36 +++++++++++++++++
> >  sysdeps/riscv/multiarch/cpu_relax_impl.S      | 40 +++++++++++++++++++
> >  .../unix/sysv/linux/riscv/atomic-machine.h    |  3 ++
> >  4 files changed, 84 insertions(+)
> >  create mode 100644 sysdeps/riscv/multiarch/cpu_relax.c
> >  create mode 100644 sysdeps/riscv/multiarch/cpu_relax_impl.S
> >
> > diff --git a/sysdeps/riscv/multiarch/Makefile
> b/sysdeps/riscv/multiarch/Makefile
> > index 9f22e31b99..b5b9fcf986 100644
> > --- a/sysdeps/riscv/multiarch/Makefile
> > +++ b/sysdeps/riscv/multiarch/Makefile
> > @@ -17,3 +17,8 @@ sysdep_routines += \
> >         strncmp_generic \
> >         strncmp_zbb
> >  endif
> > +
> > +# nscd uses atomic_spin_nop which in turn requires cpu_relax
> > +ifeq ($(subdir),nscd)
> > +routines += cpu_relax cpu_relax_impl
> > +endif
> > diff --git a/sysdeps/riscv/multiarch/cpu_relax.c
> b/sysdeps/riscv/multiarch/cpu_relax.c
> > new file mode 100644
> > index 0000000000..4e6825ca50
> > --- /dev/null
> > +++ b/sysdeps/riscv/multiarch/cpu_relax.c
> > @@ -0,0 +1,36 @@
> > +/* CPU strand yielding for busy loops.  RISC-V version.
> > +   Copyright (C) 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/>.  */
> > +
> > +#include <ldsodefs.h>
> > +#include <init-arch.h>
> > +
> > +void __cpu_relax (void);
> > +extern void __cpu_relax_zawrs (void);
> > +extern void __cpu_relax_zihintpause (void);
> > +
> > +static void
> > +__cpu_relax_generic (void)
> > +{
> > +}
> > +
> > +libc_ifunc (__cpu_relax,
> > +           HAVE_RV(zawrs)
> > +           ? __cpu_relax_zawrs
> > +           : HAVE_RV(zihintpause)
> > +             ? __cpu_relax_zihintpause
> > +             : __cpu_relax_generic);
> > diff --git a/sysdeps/riscv/multiarch/cpu_relax_impl.S
> b/sysdeps/riscv/multiarch/cpu_relax_impl.S
> > new file mode 100644
> > index 0000000000..5d349c351f
> > --- /dev/null
> > +++ b/sysdeps/riscv/multiarch/cpu_relax_impl.S
> > @@ -0,0 +1,40 @@
> > +/* Copyright (C) 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/>.  */
> > +
> > +#include <sysdep.h>
> > +#include <sys/asm.h>
> > +
> > +.option push
> > +.option arch,+zawrs
> > +
> > +ENTRY_ALIGN (__cpu_relax_zawrs, 4)
> > +       wrs.sto
> > +       ret
> > +END (__cpu_relax_zawrs)
> > +
> > +.option pop
> > +
> > +.option push
> > +.option arch,+zihintpause
> > +
> > +ENTRY_ALIGN (__cpu_relax_zihintpause, 4)
> > +       pause
> > +       ret
> > +END (__cpu_relax_zihintpause)
> > +
> > +.option pop
> > diff --git a/sysdeps/unix/sysv/linux/riscv/atomic-machine.h
> b/sysdeps/unix/sysv/linux/riscv/atomic-machine.h
> > index dbf70d8d57..88aa58ef95 100644
> > --- a/sysdeps/unix/sysv/linux/riscv/atomic-machine.h
> > +++ b/sysdeps/unix/sysv/linux/riscv/atomic-machine.h
> > @@ -178,4 +178,7 @@
> >  # error "ISAs that do not subsume the A extension are not supported"
> >  #endif /* !__riscv_atomic */
> >
> > +extern void __cpu_relax (void);
> > +#define atomic_spin_nop() __cpu_relax()
> > +
> >  #endif /* bits/atomic.h */
> > --
> > 2.39.1
> >
>