[src/trunk]: src/sys/kern pserialize(9): Micro-optimize pserialize_not_in_rea...

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

details:   https://anonhg.NetBSD.org/src/rev/3d1bb2e8a8a8
branches:  trunk
changeset: 374181:3d1bb2e8a8a8
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sun Apr 09 08:17:45 2023 +0000

description:
pserialize(9): Micro-optimize pserialize_not_in_read_section_p.

Load l_ncsw to test whether we have been preempted, rather than
loading and storing l_nopreempt (via function call) to prevent it.

diffstat:

 sys/kern/subr_pserialize.c |  27 +++++++++++++++++++++------
 1 files changed, 21 insertions(+), 6 deletions(-)

diffs (56 lines):

diff -r 15bf8d78cbfa -r 3d1bb2e8a8a8 sys/kern/subr_pserialize.c
--- a/sys/kern/subr_pserialize.c        Sun Apr 09 08:17:36 2023 +0000
+++ b/sys/kern/subr_pserialize.c        Sun Apr 09 08:17:45 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: subr_pserialize.c,v 1.19 2022/11/15 10:29:56 macallan Exp $    */
+/*     $NetBSD: subr_pserialize.c,v 1.20 2023/04/09 08:17:45 riastradh Exp $   */
 
 /*-
  * Copyright (c) 2010, 2011 The NetBSD Foundation, Inc.
@@ -31,14 +31,16 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: subr_pserialize.c,v 1.19 2022/11/15 10:29:56 macallan Exp $");
+__KERNEL_RCSID(0, "$NetBSD: subr_pserialize.c,v 1.20 2023/04/09 08:17:45 riastradh Exp $");
 
 #include <sys/param.h>
+
 #include <sys/atomic.h>
 #include <sys/cpu.h>
+#include <sys/evcnt.h>
 #include <sys/kernel.h>
-#include <sys/evcnt.h>
 #include <sys/kmem.h>
+#include <sys/lwp.h>
 #include <sys/mutex.h>
 #include <sys/pserialize.h>
 #include <sys/xcall.h>
@@ -172,11 +174,24 @@ pserialize_in_read_section(void)
 bool
 pserialize_not_in_read_section(void)
 {
+       struct lwp *l = curlwp;
+       uint64_t ncsw;
        bool notin;
 
-       kpreempt_disable();
-       notin = (curcpu()->ci_psz_read_depth == 0);
-       kpreempt_enable();
+       ncsw = l->l_ncsw;
+       __insn_barrier();
+       notin = __predict_true(l->l_cpu->ci_psz_read_depth == 0);
+       __insn_barrier();
+
+       /*
+        * If we had a context switch, we're definitely not in a
+        * pserialize read section because pserialize read sections
+        * block preemption.
+        */
+       if (__predict_false(ncsw != l->l_ncsw)) {
+               KDASSERT(notin);
+               notin = true;
+       }
 
        return notin;
 }