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authorNoah Goldstein <goldstein.w.n@gmail.com>2022-04-27 15:13:02 -0500
committerAndreas K. Hüttel <dilfridge@gentoo.org>2022-10-01 22:24:19 +0200
commit6e38004a932117b9264a928adef7ea75b500a781 (patch)
treee35e14aac686a684ca90d533c22511d5bb74cdc4
parentm68k: Enforce 4-byte alignment on internal locks (BZ #29537) (diff)
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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. The implementation generally uses 'HP_TIMING_NOW' iff it is inlined (avoid any potential syscall paths). Reviewed-by: H.J. Lu <hjl.tools@gmail.com> (cherry picked from commit 911c63a51c690dd1a97dfc587097277029baf00f) (cherry picked from commit 4f3c6dcd2048dcf2fb0143819e51135985effda1)
-rw-r--r--sysdeps/generic/fast-jitter.h42
1 files changed, 42 insertions, 0 deletions
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