[src/trunk]: src/lib/libpthread libpthread: New pthread__smt_wait to put CPU ...

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/321877da4d8e
branches:  trunk
changeset: 375999:321877da4d8e
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Thu May 25 14:30:02 2023 +0000

description:
libpthread: New pthread__smt_wait to put CPU in low power for spin.

This is now distinct from pthread__smt_pause, which is for spin lock
backoff with no paired wakeup.

On Arm, there is a single-bit event register per CPU, and there are two
instructions to manage it:

- wfe, wait for event -- if event register is clear, enter low power
  mode and wait until event register is set; then exit low power mode
  and clear event register

- sev, signal event -- sets event register on all CPUs (other
  circumstances like interrupts also set the event register and cause
  wfe to wake)

These can be used to reduce the power consumption of spinning for a
lock, but only if they are actually paired -- if there's no sev, wfe
might hang indefinitely.  Currently only pthread_spin(3) actually
pairs them; the other lock primitives (internal lock, mutex, rwlock)
do not -- they have spin lock backoff loops, but no corresponding
wakeup to cancel a wfe.

It may be worthwhile to teach the other lock primitives to pair
wfe/sev, but that requires some performance measurement to verify
it's actually worthwhile.  So for now, we just make sure not to use
wfe when there's no sev, and keep everything else the same -- this
should fix severe performance degredation in libpthread on Arm
without hurting anything else.

No change in the generated code on amd64 and i386.  No change in the
generated code for pthread_spin.c on arm and aarch64 -- changes only
the generated code for pthread_lock.c, pthread_mutex.c, and
pthread_rwlock.c, as intended.

PR port-arm/57437

XXX pullup-10

diffstat:

 lib/libpthread/arch/aarch64/pthread_md.h |  4 ++--
 lib/libpthread/arch/arm/pthread_md.h     |  8 ++++----
 lib/libpthread/arch/i386/pthread_md.h    |  3 ++-
 lib/libpthread/arch/x86_64/pthread_md.h  |  3 ++-
 lib/libpthread/pthread_int.h             |  5 ++++-
 lib/libpthread/pthread_spin.c            |  6 +++---
 6 files changed, 17 insertions(+), 12 deletions(-)

diffs (124 lines):

diff -r a571680208f3 -r 321877da4d8e lib/libpthread/arch/aarch64/pthread_md.h
--- a/lib/libpthread/arch/aarch64/pthread_md.h  Thu May 25 14:29:45 2023 +0000
+++ b/lib/libpthread/arch/aarch64/pthread_md.h  Thu May 25 14:30:02 2023 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: pthread_md.h,v 1.1 2014/08/10 05:47:37 matt Exp $ */
+/* $NetBSD: pthread_md.h,v 1.2 2023/05/25 14:30:02 riastradh Exp $ */
 
 /*-
  * Copyright (c) 2014 The NetBSD Foundation, Inc.
@@ -42,7 +42,7 @@ pthread__sp(void)
        return ret;
 }
 
-#define pthread__smt_pause()   __asm __volatile("wfe") /* wfe */
+#define pthread__smt_wait()    __asm __volatile("wfe") /* wfe */
 #define pthread__smt_wake()    __asm __volatile("sev") /* sev */
 
 #define        pthread__uc_sp(ucp)     ((ucp)->uc_mcontext.__gregs[_REG_SP])
diff -r a571680208f3 -r 321877da4d8e lib/libpthread/arch/arm/pthread_md.h
--- a/lib/libpthread/arch/arm/pthread_md.h      Thu May 25 14:29:45 2023 +0000
+++ b/lib/libpthread/arch/arm/pthread_md.h      Thu May 25 14:30:02 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: pthread_md.h,v 1.12 2023/05/25 14:29:45 riastradh Exp $        */
+/*     $NetBSD: pthread_md.h,v 1.13 2023/05/25 14:30:02 riastradh Exp $        */
 
 /*
  * Copyright (c) 2001 Wasabi Systems, Inc.
@@ -50,13 +50,13 @@ pthread__sp(void)
 }
 
 #if defined(__thumb__) && defined(_ARM_ARCH_6)
-#define pthread__smt_pause()   __asm __volatile(".inst.n 0xbf20") /* wfe */
+#define pthread__smt_wait()    __asm __volatile(".inst.n 0xbf20") /* wfe */
 #define pthread__smt_wake()    __asm __volatile(".inst.n 0xbf40") /* sev */
 #elif !defined(__thumb__)
-#define pthread__smt_pause()   __asm __volatile(".inst 0xe320f002") /* wfe */
+#define pthread__smt_wait()    __asm __volatile(".inst 0xe320f002") /* wfe */
 #define pthread__smt_wake()    __asm __volatile(".inst 0xe320f004") /* sev */
 #else
-#define pthread__smt_pause()   __nothing
+#define pthread__smt_wait()    __nothing
 #define pthread__smt_wake()    __nothing
 #endif
 
diff -r a571680208f3 -r 321877da4d8e lib/libpthread/arch/i386/pthread_md.h
--- a/lib/libpthread/arch/i386/pthread_md.h     Thu May 25 14:29:45 2023 +0000
+++ b/lib/libpthread/arch/i386/pthread_md.h     Thu May 25 14:30:02 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: pthread_md.h,v 1.20 2012/03/02 23:19:47 joerg Exp $    */
+/*     $NetBSD: pthread_md.h,v 1.21 2023/05/25 14:30:02 riastradh Exp $        */
 
 /*-
  * Copyright (c) 2001, 2007, 2008 The NetBSD Foundation, Inc.
@@ -61,6 +61,7 @@ static inline void
 #define        _INITCONTEXT_U_MD(ucp)  _initcontext_u_md(ucp);
 
 #define        pthread__smt_pause()    __asm __volatile("rep; nop" ::: "memory")
+#define        pthread__smt_wait()     __asm __volatile("rep; nop" ::: "memory")
 
 /* Don't need additional memory barriers. */
 #define        PTHREAD__ATOMIC_IS_MEMBAR
diff -r a571680208f3 -r 321877da4d8e lib/libpthread/arch/x86_64/pthread_md.h
--- a/lib/libpthread/arch/x86_64/pthread_md.h   Thu May 25 14:29:45 2023 +0000
+++ b/lib/libpthread/arch/x86_64/pthread_md.h   Thu May 25 14:30:02 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: pthread_md.h,v 1.12 2011/01/25 19:12:06 christos Exp $ */
+/*     $NetBSD: pthread_md.h,v 1.13 2023/05/25 14:30:03 riastradh Exp $        */
 
 /*-
  * Copyright (c) 2001, 2007, 2008 The NetBSD Foundation, Inc.
@@ -64,6 +64,7 @@ pthread__sp(void)
        (ucp)->uc_mcontext.__gregs[_REG_RFL] = 0x202;
 
 #define        pthread__smt_pause()    __asm __volatile("rep; nop" ::: "memory")
+#define        pthread__smt_wait()     __asm __volatile("rep; nop" ::: "memory")
 
 /* Don't need additional memory barriers. */
 #define        PTHREAD__ATOMIC_IS_MEMBAR
diff -r a571680208f3 -r 321877da4d8e lib/libpthread/pthread_int.h
--- a/lib/libpthread/pthread_int.h      Thu May 25 14:29:45 2023 +0000
+++ b/lib/libpthread/pthread_int.h      Thu May 25 14:30:02 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: pthread_int.h,v 1.110 2023/05/25 14:29:45 riastradh Exp $      */
+/*     $NetBSD: pthread_int.h,v 1.111 2023/05/25 14:30:03 riastradh Exp $      */
 
 /*-
  * Copyright (c) 2001, 2002, 2003, 2006, 2007, 2008, 2020
@@ -303,6 +303,9 @@ int pthread__add_specific(pthread_t, pth
 #ifndef pthread__smt_pause
 #define        pthread__smt_pause()    __nothing
 #endif
+#ifndef pthread__smt_wait
+#define        pthread__smt_wait()     __nothing
+#endif
 #ifndef pthread__smt_wake
 #define        pthread__smt_wake()     __nothing
 #endif
diff -r a571680208f3 -r 321877da4d8e lib/libpthread/pthread_spin.c
--- a/lib/libpthread/pthread_spin.c     Thu May 25 14:29:45 2023 +0000
+++ b/lib/libpthread/pthread_spin.c     Thu May 25 14:30:02 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: pthread_spin.c,v 1.10 2022/04/10 10:38:33 riastradh Exp $      */
+/*     $NetBSD: pthread_spin.c,v 1.11 2023/05/25 14:30:03 riastradh Exp $      */
 
 /*-
  * Copyright (c) 2001, 2006, 2007 The NetBSD Foundation, Inc.
@@ -34,7 +34,7 @@
  */
 
 #include <sys/cdefs.h>
-__RCSID("$NetBSD: pthread_spin.c,v 1.10 2022/04/10 10:38:33 riastradh Exp $");
+__RCSID("$NetBSD: pthread_spin.c,v 1.11 2023/05/25 14:30:03 riastradh Exp $");
 
 /* Need to use libc-private names for atomic operations. */
 #include "../../common/lib/libc/atomic/atomic_op_namespace.h"
@@ -98,7 +98,7 @@ pthread_spin_lock(pthread_spinlock_t *lo
 
        self = pthread__self();
        while (pthread__spintrylock(self, &lock->pts_spin) == 0) {
-               pthread__smt_pause();
+               pthread__smt_wait();
        }
 
        return 0;