[PATCH v1 1/2] linux: Implement sched_yield in C

[PATCH v1 1/2] linux: Implement sched_yield in C

[Noah Goldstein]

This is in preperation for a minor optimization before expected
context switches.
---
 sysdeps/unix/sysv/linux/sched_yield.c | 27 +++++++++++++++++++++++++++
 sysdeps/unix/sysv/linux/syscalls.list |  1 -
 2 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 sysdeps/unix/sysv/linux/sched_yield.c

diff --git a/sysdeps/unix/sysv/linux/sched_yield.c b/sysdeps/unix/sysv/linux/sched_yield.c
new file mode 100644
index 0000000000..154bf725b0
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/sched_yield.c
@@ -0,0 +1,27 @@
+/* Yield current process.  Linux specific syscall.
+   Copyright (C) 2023 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>
+
+int
+__sched_yield (void)
+{
+    return INLINE_SYSCALL_CALL (sched_yield);
+}
+libc_hidden_def (__sched_yield);
+weak_alias (__sched_yield, sched_yield)
diff --git a/sysdeps/unix/sysv/linux/syscalls.list b/sysdeps/unix/sysv/linux/syscalls.list
index 73e941ef89..5f571df937 100644
--- a/sysdeps/unix/sysv/linux/syscalls.list
+++ b/sysdeps/unix/sysv/linux/syscalls.list
@@ -59,7 +59,6 @@ sched_primax	-	sched_get_priority_max	i:i	__sched_get_priority_max	sched_get_pri
 sched_primin	-	sched_get_priority_min	i:i	__sched_get_priority_min	sched_get_priority_min
 sched_setp	-	sched_setparam	i:ip	__sched_setparam	sched_setparam
 sched_sets	-	sched_setscheduler	i:iip	__sched_setscheduler	sched_setscheduler
-sched_yield	-	sched_yield	i:	__sched_yield	sched_yield
 setfsgid	EXTRA	setfsgid	i:i	setfsgid
 setfsuid	EXTRA	setfsuid	i:i	setfsuid
 setpgid		-	setpgid		i:ii	__setpgid	setpgid
-- 
2.34.1

[PATCH v1 2/2] x86: Add `prepare_context_switch` to initialize register inuse states

[Noah Goldstein]

xsave/xrstor have optimization to skip saving/restoring register
classes if those register classes are in the init state
(inuse[bit]==0).

We can get:
    SSE state
    AVX state
    ZMM_HI256 state

to init state using `vzeroall`. Doing this before syscalls that will
cause a proper context switch can be beneficial in terms of the amount
of state the kernel needs to save/restore. This can save time and
memory.
---
 sysdeps/generic/prepare-context-switch.h  | 28 +++++++++++++
 sysdeps/unix/sysv/linux/clock_nanosleep.c |  2 +
 sysdeps/unix/sysv/linux/sched_yield.c     |  2 +
 sysdeps/x86/prepare-context-switch.h      | 50 +++++++++++++++++++++++
 4 files changed, 82 insertions(+)
 create mode 100644 sysdeps/generic/prepare-context-switch.h
 create mode 100644 sysdeps/x86/prepare-context-switch.h

diff --git a/sysdeps/generic/prepare-context-switch.h b/sysdeps/generic/prepare-context-switch.h
new file mode 100644
index 0000000000..6153847905
--- /dev/null
+++ b/sysdeps/generic/prepare-context-switch.h
@@ -0,0 +1,28 @@
+/* Prepare process for context switch. generic version
+   Copyright (C) 2023 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 _PREPARE_CONTEXT_SWITCH_H
+#define _PREPARE_CONTEXT_SWITCH_H
+
+static void
+prepare_context_switch (void)
+{
+  /* Empty.  */
+}
+
+#endif
diff --git a/sysdeps/unix/sysv/linux/clock_nanosleep.c b/sysdeps/unix/sysv/linux/clock_nanosleep.c
index ac2d810632..e674f0ac54 100644
--- a/sysdeps/unix/sysv/linux/clock_nanosleep.c
+++ b/sysdeps/unix/sysv/linux/clock_nanosleep.c
@@ -23,6 +23,7 @@
 #include "kernel-posix-cpu-timers.h"
 
 #include <shlib-compat.h>
+#include <prepare-context-switch.h>
 
 /* We can simply use the syscall.  The CPU clocks are not supported
    with this function.  */
@@ -44,6 +45,7 @@ __clock_nanosleep_time64 (clockid_t clock_id, int flags,
 #endif
 
   int r;
+  prepare_context_switch();
 #ifdef __ASSUME_TIME64_SYSCALLS
   r = INTERNAL_SYSCALL_CANCEL (clock_nanosleep_time64, clock_id, flags, req,
 			       rem);
diff --git a/sysdeps/unix/sysv/linux/sched_yield.c b/sysdeps/unix/sysv/linux/sched_yield.c
index 154bf725b0..d26c0f8a9f 100644
--- a/sysdeps/unix/sysv/linux/sched_yield.c
+++ b/sysdeps/unix/sysv/linux/sched_yield.c
@@ -17,10 +17,12 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <sysdep.h>
+#include <prepare-context-switch.h>
 
 int
 __sched_yield (void)
 {
+    prepare_context_switch();
     return INLINE_SYSCALL_CALL (sched_yield);
 }
 libc_hidden_def (__sched_yield);
diff --git a/sysdeps/x86/prepare-context-switch.h b/sysdeps/x86/prepare-context-switch.h
new file mode 100644
index 0000000000..bf33a7a1b3
--- /dev/null
+++ b/sysdeps/x86/prepare-context-switch.h
@@ -0,0 +1,50 @@
+/* Prepare process for context switch. x86 version
+   Copyright (C) 2023 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 _PREPARE_CONTEXT_SWITCH_H
+#define _PREPARE_CONTEXT_SWITCH_H
+
+#ifdef __AVX__
+static void
+prepare_context_switch (void)
+{
+  /* vzeroall before context switch will restore xsave/xrstor state of the
+     following to init state:
+      - SSE state
+      - AVX state
+      - ZMM_HI256 state
+     This saves a touch of overhead and memory in context switches.
+     This function can/should be used before an operation that will
+     cause a context switch in the current process (sched_yield,
+     *sleep, etc...).
+   */
+  __asm__ volatile ("vzeroall"
+		    :
+		    :
+		    : "zmm0", "zmm1", "zmm2", "zmm3", "zmm4", "zmm5", "zmm6",
+		      "zmm7", "zmm8", "zmm9", "zmm10", "zmm11", "zmm12",
+		      "zmm13", "zmm14", "zmm15");
+  /* TODO: Add xtilerelease for amx state.  */
+}
+
+#else
+# undef _PREPARE_CONTEXT_SWITCH_H
+# include <sysdeps/generic/prepare-context-switch.h>
+#endif
+
+#endif
-- 
2.34.1

Re: [PATCH v1 1/2] linux: Implement sched_yield in C

[H.J. Lu]

On Wed, Jun 7, 2023 at 12:46 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> This is in preperation for a minor optimization before expected
> context switches.
> ---
>  sysdeps/unix/sysv/linux/sched_yield.c | 27 +++++++++++++++++++++++++++
>  sysdeps/unix/sysv/linux/syscalls.list |  1 -
>  2 files changed, 27 insertions(+), 1 deletion(-)
>  create mode 100644 sysdeps/unix/sysv/linux/sched_yield.c
>
> diff --git a/sysdeps/unix/sysv/linux/sched_yield.c b/sysdeps/unix/sysv/linux/sched_yield.c
> new file mode 100644
> index 0000000000..154bf725b0
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/sched_yield.c
> @@ -0,0 +1,27 @@
> +/* Yield current process.  Linux specific syscall.
> +   Copyright (C) 2023 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>
> +
> +int
> +__sched_yield (void)
> +{
> +    return INLINE_SYSCALL_CALL (sched_yield);
> +}
> +libc_hidden_def (__sched_yield);
> +weak_alias (__sched_yield, sched_yield)
> diff --git a/sysdeps/unix/sysv/linux/syscalls.list b/sysdeps/unix/sysv/linux/syscalls.list
> index 73e941ef89..5f571df937 100644
> --- a/sysdeps/unix/sysv/linux/syscalls.list
> +++ b/sysdeps/unix/sysv/linux/syscalls.list
> @@ -59,7 +59,6 @@ sched_primax  -       sched_get_priority_max  i:i     __sched_get_priority_max        sched_get_pri
>  sched_primin   -       sched_get_priority_min  i:i     __sched_get_priority_min        sched_get_priority_min
>  sched_setp     -       sched_setparam  i:ip    __sched_setparam        sched_setparam
>  sched_sets     -       sched_setscheduler      i:iip   __sched_setscheduler    sched_setscheduler
> -sched_yield    -       sched_yield     i:      __sched_yield   sched_yield
>  setfsgid       EXTRA   setfsgid        i:i     setfsgid
>  setfsuid       EXTRA   setfsuid        i:i     setfsuid
>  setpgid                -       setpgid         i:ii    __setpgid       setpgid
> --
> 2.34.1


Is this really needed?   We can add x86-64 specific sched_yield.c.


-- 
H.J.

Re: [PATCH v1 2/2] x86: Add `prepare_context_switch` to initialize register inuse states

[H.J. Lu]

On Wed, Jun 7, 2023 at 12:46 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> xsave/xrstor have optimization to skip saving/restoring register
> classes if those register classes are in the init state
> (inuse[bit]==0).
>
> We can get:
>     SSE state
>     AVX state
>     ZMM_HI256 state
>
> to init state using `vzeroall`. Doing this before syscalls that will
> cause a proper context switch can be beneficial in terms of the amount
> of state the kernel needs to save/restore. This can save time and
> memory.
> ---
>  sysdeps/generic/prepare-context-switch.h  | 28 +++++++++++++
>  sysdeps/unix/sysv/linux/clock_nanosleep.c |  2 +
>  sysdeps/unix/sysv/linux/sched_yield.c     |  2 +
>  sysdeps/x86/prepare-context-switch.h      | 50 +++++++++++++++++++++++
>  4 files changed, 82 insertions(+)
>  create mode 100644 sysdeps/generic/prepare-context-switch.h
>  create mode 100644 sysdeps/x86/prepare-context-switch.h
>
> diff --git a/sysdeps/generic/prepare-context-switch.h b/sysdeps/generic/prepare-context-switch.h
> new file mode 100644
> index 0000000000..6153847905
> --- /dev/null
> +++ b/sysdeps/generic/prepare-context-switch.h
> @@ -0,0 +1,28 @@
> +/* Prepare process for context switch. generic version
> +   Copyright (C) 2023 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 _PREPARE_CONTEXT_SWITCH_H
> +#define _PREPARE_CONTEXT_SWITCH_H
> +
> +static void
> +prepare_context_switch (void)
> +{
> +  /* Empty.  */
> +}
> +
> +#endif
> diff --git a/sysdeps/unix/sysv/linux/clock_nanosleep.c b/sysdeps/unix/sysv/linux/clock_nanosleep.c
> index ac2d810632..e674f0ac54 100644
> --- a/sysdeps/unix/sysv/linux/clock_nanosleep.c
> +++ b/sysdeps/unix/sysv/linux/clock_nanosleep.c
> @@ -23,6 +23,7 @@
>  #include "kernel-posix-cpu-timers.h"
>
>  #include <shlib-compat.h>
> +#include <prepare-context-switch.h>
>
>  /* We can simply use the syscall.  The CPU clocks are not supported
>     with this function.  */
> @@ -44,6 +45,7 @@ __clock_nanosleep_time64 (clockid_t clock_id, int flags,
>  #endif
>
>    int r;
> +  prepare_context_switch();
>  #ifdef __ASSUME_TIME64_SYSCALLS
>    r = INTERNAL_SYSCALL_CANCEL (clock_nanosleep_time64, clock_id, flags, req,
>                                rem);
> diff --git a/sysdeps/unix/sysv/linux/sched_yield.c b/sysdeps/unix/sysv/linux/sched_yield.c
> index 154bf725b0..d26c0f8a9f 100644
> --- a/sysdeps/unix/sysv/linux/sched_yield.c
> +++ b/sysdeps/unix/sysv/linux/sched_yield.c
> @@ -17,10 +17,12 @@
>     <https://www.gnu.org/licenses/>.  */
>
>  #include <sysdep.h>
> +#include <prepare-context-switch.h>
>
>  int
>  __sched_yield (void)
>  {
> +    prepare_context_switch();
>      return INLINE_SYSCALL_CALL (sched_yield);
>  }
>  libc_hidden_def (__sched_yield);
> diff --git a/sysdeps/x86/prepare-context-switch.h b/sysdeps/x86/prepare-context-switch.h
> new file mode 100644
> index 0000000000..bf33a7a1b3
> --- /dev/null
> +++ b/sysdeps/x86/prepare-context-switch.h
> @@ -0,0 +1,50 @@
> +/* Prepare process for context switch. x86 version
> +   Copyright (C) 2023 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 _PREPARE_CONTEXT_SWITCH_H
> +#define _PREPARE_CONTEXT_SWITCH_H
> +
> +#ifdef __AVX__

Please use

   if (CPU_FEATURE_ACTIVE (AVX))

to detect it at run-time.

> +static void
> +prepare_context_switch (void)
> +{
> +  /* vzeroall before context switch will restore xsave/xrstor state of the
> +     following to init state:
> +      - SSE state
> +      - AVX state
> +      - ZMM_HI256 state
> +     This saves a touch of overhead and memory in context switches.
> +     This function can/should be used before an operation that will
> +     cause a context switch in the current process (sched_yield,
> +     *sleep, etc...).
> +   */
> +  __asm__ volatile ("vzeroall"

Can you use _mm256_zeroall?

> +                   :
> +                   :
> +                   : "zmm0", "zmm1", "zmm2", "zmm3", "zmm4", "zmm5", "zmm6",
> +                     "zmm7", "zmm8", "zmm9", "zmm10", "zmm11", "zmm12",
> +                     "zmm13", "zmm14", "zmm15");
> +  /* TODO: Add xtilerelease for amx state.  */
> +}
> +
> +#else
> +# undef _PREPARE_CONTEXT_SWITCH_H
> +# include <sysdeps/generic/prepare-context-switch.h>
> +#endif
> +
> +#endif
> --
> 2.34.1
>


-- 
H.J.

Re: [PATCH v1 2/2] x86: Add `prepare_context_switch` to initialize register inuse states

[Noah Goldstein]

On Wed, Jun 7, 2023 at 3:46 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Wed, Jun 7, 2023 at 12:46 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > xsave/xrstor have optimization to skip saving/restoring register
> > classes if those register classes are in the init state
> > (inuse[bit]==0).
> >
> > We can get:
> >     SSE state
> >     AVX state
> >     ZMM_HI256 state
> >
> > to init state using `vzeroall`. Doing this before syscalls that will
> > cause a proper context switch can be beneficial in terms of the amount
> > of state the kernel needs to save/restore. This can save time and
> > memory.
> > ---
> >  sysdeps/generic/prepare-context-switch.h  | 28 +++++++++++++
> >  sysdeps/unix/sysv/linux/clock_nanosleep.c |  2 +
> >  sysdeps/unix/sysv/linux/sched_yield.c     |  2 +
> >  sysdeps/x86/prepare-context-switch.h      | 50 +++++++++++++++++++++++
> >  4 files changed, 82 insertions(+)
> >  create mode 100644 sysdeps/generic/prepare-context-switch.h
> >  create mode 100644 sysdeps/x86/prepare-context-switch.h
> >
> > diff --git a/sysdeps/generic/prepare-context-switch.h b/sysdeps/generic/prepare-context-switch.h
> > new file mode 100644
> > index 0000000000..6153847905
> > --- /dev/null
> > +++ b/sysdeps/generic/prepare-context-switch.h
> > @@ -0,0 +1,28 @@
> > +/* Prepare process for context switch. generic version
> > +   Copyright (C) 2023 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 _PREPARE_CONTEXT_SWITCH_H
> > +#define _PREPARE_CONTEXT_SWITCH_H
> > +
> > +static void
> > +prepare_context_switch (void)
> > +{
> > +  /* Empty.  */
> > +}
> > +
> > +#endif
> > diff --git a/sysdeps/unix/sysv/linux/clock_nanosleep.c b/sysdeps/unix/sysv/linux/clock_nanosleep.c
> > index ac2d810632..e674f0ac54 100644
> > --- a/sysdeps/unix/sysv/linux/clock_nanosleep.c
> > +++ b/sysdeps/unix/sysv/linux/clock_nanosleep.c
> > @@ -23,6 +23,7 @@
> >  #include "kernel-posix-cpu-timers.h"
> >
> >  #include <shlib-compat.h>
> > +#include <prepare-context-switch.h>
> >
> >  /* We can simply use the syscall.  The CPU clocks are not supported
> >     with this function.  */
> > @@ -44,6 +45,7 @@ __clock_nanosleep_time64 (clockid_t clock_id, int flags,
> >  #endif
> >
> >    int r;
> > +  prepare_context_switch();
> >  #ifdef __ASSUME_TIME64_SYSCALLS
> >    r = INTERNAL_SYSCALL_CANCEL (clock_nanosleep_time64, clock_id, flags, req,
> >                                rem);
> > diff --git a/sysdeps/unix/sysv/linux/sched_yield.c b/sysdeps/unix/sysv/linux/sched_yield.c
> > index 154bf725b0..d26c0f8a9f 100644
> > --- a/sysdeps/unix/sysv/linux/sched_yield.c
> > +++ b/sysdeps/unix/sysv/linux/sched_yield.c
> > @@ -17,10 +17,12 @@
> >     <https://www.gnu.org/licenses/>.  */
> >
> >  #include <sysdep.h>
> > +#include <prepare-context-switch.h>
> >
> >  int
> >  __sched_yield (void)
> >  {
> > +    prepare_context_switch();
> >      return INLINE_SYSCALL_CALL (sched_yield);
> >  }
> >  libc_hidden_def (__sched_yield);
> > diff --git a/sysdeps/x86/prepare-context-switch.h b/sysdeps/x86/prepare-context-switch.h
> > new file mode 100644
> > index 0000000000..bf33a7a1b3
> > --- /dev/null
> > +++ b/sysdeps/x86/prepare-context-switch.h
> > @@ -0,0 +1,50 @@
> > +/* Prepare process for context switch. x86 version
> > +   Copyright (C) 2023 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 _PREPARE_CONTEXT_SWITCH_H
> > +#define _PREPARE_CONTEXT_SWITCH_H
> > +
> > +#ifdef __AVX__
>
> Please use
>
>    if (CPU_FEATURE_ACTIVE (AVX))
>
> to detect it at run-time.
>
Wanted to avoid overhead. Think if we want runtime check should ifunc
the functions
we want to put it in (just clock_nanosleep64 and sched_yield). WDYT?

> > +static void
> > +prepare_context_switch (void)
> > +{
> > +  /* vzeroall before context switch will restore xsave/xrstor state of the
> > +     following to init state:
> > +      - SSE state
> > +      - AVX state
> > +      - ZMM_HI256 state
> > +     This saves a touch of overhead and memory in context switches.
> > +     This function can/should be used before an operation that will
> > +     cause a context switch in the current process (sched_yield,
> > +     *sleep, etc...).
> > +   */
> > +  __asm__ volatile ("vzeroall"
>
> Can you use _mm256_zeroall?
>
> > +                   :
> > +                   :
> > +                   : "zmm0", "zmm1", "zmm2", "zmm3", "zmm4", "zmm5", "zmm6",
> > +                     "zmm7", "zmm8", "zmm9", "zmm10", "zmm11", "zmm12",
> > +                     "zmm13", "zmm14", "zmm15");
> > +  /* TODO: Add xtilerelease for amx state.  */
> > +}
> > +
> > +#else
> > +# undef _PREPARE_CONTEXT_SWITCH_H
> > +# include <sysdeps/generic/prepare-context-switch.h>
> > +#endif
> > +
> > +#endif
> > --
> > 2.34.1
> >
>
>
> --
> H.J.

Re: [PATCH v1 2/2] x86: Add `prepare_context_switch` to initialize register inuse states

[Noah Goldstein]

On Wed, Jun 7, 2023 at 4:59 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Wed, Jun 7, 2023 at 3:46 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Wed, Jun 7, 2023 at 12:46 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> > >
> > > xsave/xrstor have optimization to skip saving/restoring register
> > > classes if those register classes are in the init state
> > > (inuse[bit]==0).
> > >
> > > We can get:
> > >     SSE state
> > >     AVX state
> > >     ZMM_HI256 state
> > >
> > > to init state using `vzeroall`. Doing this before syscalls that will
> > > cause a proper context switch can be beneficial in terms of the amount
> > > of state the kernel needs to save/restore. This can save time and
> > > memory.
> > > ---
> > >  sysdeps/generic/prepare-context-switch.h  | 28 +++++++++++++
> > >  sysdeps/unix/sysv/linux/clock_nanosleep.c |  2 +
> > >  sysdeps/unix/sysv/linux/sched_yield.c     |  2 +
> > >  sysdeps/x86/prepare-context-switch.h      | 50 +++++++++++++++++++++++
> > >  4 files changed, 82 insertions(+)
> > >  create mode 100644 sysdeps/generic/prepare-context-switch.h
> > >  create mode 100644 sysdeps/x86/prepare-context-switch.h
> > >
> > > diff --git a/sysdeps/generic/prepare-context-switch.h b/sysdeps/generic/prepare-context-switch.h
> > > new file mode 100644
> > > index 0000000000..6153847905
> > > --- /dev/null
> > > +++ b/sysdeps/generic/prepare-context-switch.h
> > > @@ -0,0 +1,28 @@
> > > +/* Prepare process for context switch. generic version
> > > +   Copyright (C) 2023 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 _PREPARE_CONTEXT_SWITCH_H
> > > +#define _PREPARE_CONTEXT_SWITCH_H
> > > +
> > > +static void
> > > +prepare_context_switch (void)
> > > +{
> > > +  /* Empty.  */
> > > +}
> > > +
> > > +#endif
> > > diff --git a/sysdeps/unix/sysv/linux/clock_nanosleep.c b/sysdeps/unix/sysv/linux/clock_nanosleep.c
> > > index ac2d810632..e674f0ac54 100644
> > > --- a/sysdeps/unix/sysv/linux/clock_nanosleep.c
> > > +++ b/sysdeps/unix/sysv/linux/clock_nanosleep.c
> > > @@ -23,6 +23,7 @@
> > >  #include "kernel-posix-cpu-timers.h"
> > >
> > >  #include <shlib-compat.h>
> > > +#include <prepare-context-switch.h>
> > >
> > >  /* We can simply use the syscall.  The CPU clocks are not supported
> > >     with this function.  */
> > > @@ -44,6 +45,7 @@ __clock_nanosleep_time64 (clockid_t clock_id, int flags,
> > >  #endif
> > >
> > >    int r;
> > > +  prepare_context_switch();
> > >  #ifdef __ASSUME_TIME64_SYSCALLS
> > >    r = INTERNAL_SYSCALL_CANCEL (clock_nanosleep_time64, clock_id, flags, req,
> > >                                rem);
> > > diff --git a/sysdeps/unix/sysv/linux/sched_yield.c b/sysdeps/unix/sysv/linux/sched_yield.c
> > > index 154bf725b0..d26c0f8a9f 100644
> > > --- a/sysdeps/unix/sysv/linux/sched_yield.c
> > > +++ b/sysdeps/unix/sysv/linux/sched_yield.c
> > > @@ -17,10 +17,12 @@
> > >     <https://www.gnu.org/licenses/>.  */
> > >
> > >  #include <sysdep.h>
> > > +#include <prepare-context-switch.h>
> > >
> > >  int
> > >  __sched_yield (void)
> > >  {
> > > +    prepare_context_switch();
> > >      return INLINE_SYSCALL_CALL (sched_yield);
> > >  }
> > >  libc_hidden_def (__sched_yield);
> > > diff --git a/sysdeps/x86/prepare-context-switch.h b/sysdeps/x86/prepare-context-switch.h
> > > new file mode 100644
> > > index 0000000000..bf33a7a1b3
> > > --- /dev/null
> > > +++ b/sysdeps/x86/prepare-context-switch.h
> > > @@ -0,0 +1,50 @@
> > > +/* Prepare process for context switch. x86 version
> > > +   Copyright (C) 2023 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 _PREPARE_CONTEXT_SWITCH_H
> > > +#define _PREPARE_CONTEXT_SWITCH_H
> > > +
> > > +#ifdef __AVX__
> >
> > Please use
> >
> >    if (CPU_FEATURE_ACTIVE (AVX))
> >
> > to detect it at run-time.
> >
> Wanted to avoid overhead. Think if we want runtime check should ifunc
> the functions
> we want to put it in (just clock_nanosleep64 and sched_yield). WDYT?
>
> > > +static void
> > > +prepare_context_switch (void)
> > > +{
> > > +  /* vzeroall before context switch will restore xsave/xrstor state of the
> > > +     following to init state:
> > > +      - SSE state
> > > +      - AVX state
> > > +      - ZMM_HI256 state
> > > +     This saves a touch of overhead and memory in context switches.
> > > +     This function can/should be used before an operation that will
> > > +     cause a context switch in the current process (sched_yield,
> > > +     *sleep, etc...).
> > > +   */
> > > +  __asm__ volatile ("vzeroall"
> >
> > Can you use _mm256_zeroall?
> >
> > > +                   :
> > > +                   :
> > > +                   : "zmm0", "zmm1", "zmm2", "zmm3", "zmm4", "zmm5", "zmm6",
> > > +                     "zmm7", "zmm8", "zmm9", "zmm10", "zmm11", "zmm12",
> > > +                     "zmm13", "zmm14", "zmm15");
> > > +  /* TODO: Add xtilerelease for amx state.  */
> > > +}
> > > +
> > > +#else
> > > +# undef _PREPARE_CONTEXT_SWITCH_H
> > > +# include <sysdeps/generic/prepare-context-switch.h>
> > > +#endif
> > > +
> > > +#endif
> > > --
> > > 2.34.1
> > >
> >
> >
> > --
> > H.J.

Abandoning this patch in favor of the versions at:
"x86: Implement sched_yield syscall for x86 only."
and
"x86: Implement clock_nanosleep{_time64} syscall for x86 only."

Re: [PATCH v1 1/2] linux: Implement sched_yield in C

[Noah Goldstein]

On Wed, Jun 7, 2023 at 3:43 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Wed, Jun 7, 2023 at 12:46 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > This is in preperation for a minor optimization before expected
> > context switches.
> > ---
> >  sysdeps/unix/sysv/linux/sched_yield.c | 27 +++++++++++++++++++++++++++
> >  sysdeps/unix/sysv/linux/syscalls.list |  1 -
> >  2 files changed, 27 insertions(+), 1 deletion(-)
> >  create mode 100644 sysdeps/unix/sysv/linux/sched_yield.c
> >
> > diff --git a/sysdeps/unix/sysv/linux/sched_yield.c b/sysdeps/unix/sysv/linux/sched_yield.c
> > new file mode 100644
> > index 0000000000..154bf725b0
> > --- /dev/null
> > +++ b/sysdeps/unix/sysv/linux/sched_yield.c
> > @@ -0,0 +1,27 @@
> > +/* Yield current process.  Linux specific syscall.
> > +   Copyright (C) 2023 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>
> > +
> > +int
> > +__sched_yield (void)
> > +{
> > +    return INLINE_SYSCALL_CALL (sched_yield);
> > +}
> > +libc_hidden_def (__sched_yield);
> > +weak_alias (__sched_yield, sched_yield)
> > diff --git a/sysdeps/unix/sysv/linux/syscalls.list b/sysdeps/unix/sysv/linux/syscalls.list
> > index 73e941ef89..5f571df937 100644
> > --- a/sysdeps/unix/sysv/linux/syscalls.list
> > +++ b/sysdeps/unix/sysv/linux/syscalls.list
> > @@ -59,7 +59,6 @@ sched_primax  -       sched_get_priority_max  i:i     __sched_get_priority_max        sched_get_pri
> >  sched_primin   -       sched_get_priority_min  i:i     __sched_get_priority_min        sched_get_priority_min
> >  sched_setp     -       sched_setparam  i:ip    __sched_setparam        sched_setparam
> >  sched_sets     -       sched_setscheduler      i:iip   __sched_setscheduler    sched_setscheduler
> > -sched_yield    -       sched_yield     i:      __sched_yield   sched_yield
> >  setfsgid       EXTRA   setfsgid        i:i     setfsgid
> >  setfsuid       EXTRA   setfsuid        i:i     setfsuid
> >  setpgid                -       setpgid         i:ii    __setpgid       setpgid
> > --
> > 2.34.1
>
>
> Is this really needed?   We can add x86-64 specific sched_yield.c.
>
>
> --
> H.J.
abandoning this.