From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-x629.google.com (mail-pl1-x629.google.com [IPv6:2607:f8b0:4864:20::629]) by sourceware.org (Postfix) with ESMTPS id 5E9623858D28 for ; Wed, 27 Apr 2022 20:46:58 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 5E9623858D28 Received: by mail-pl1-x629.google.com with SMTP id n8so2614390plh.1 for ; Wed, 27 Apr 2022 13:46:58 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=Mccd0lxJeAJ863pIUkAWV6fny3TCM6eZOg/u/2Ra9XM=; b=oXBYTL/PM6CPO5NzGR/myAzwJHZ6zXy6wH75oS1CAg4LNWZSww3AiSTCkde/NDRQW+ C5TRghbDL2p0jSZy6jxgAC/jQ72N0Z5Pc9zJK1IVvnrb1nupoulV2qG+3jT54BS/jBa8 Bvj6itkGKz9URvUsHNWEiUPlEID4mf3T85u1bdyi3B9k+9h8uUpAngfsdZZl2FRqoudd AY4X7JU8i8mjtyd458ty9s0H5RJT23B1bqwB3mmX40OW3GGzyzgTAUjV6xim/K3UMTq/ bna5XNfYwkH0XzqUV7uSQuHwbFuHEzLK/MjkxSQeqVoTpI/0YyYDfDBTLnTxmlY+Nbhf VmFQ== X-Gm-Message-State: AOAM531iUA4jwfPT6egKx/50R6FjbQ9Ck8DaHGFSPC+WvhKGk6Pgq0Lz E9JG/Uho0xMQWEKgiuouiwbjD/D0/HYDqFMj1GiDMebXyRM= X-Google-Smtp-Source: ABdhPJzu0D9k9V+7eLeCPdF4Ubupmb+us8L8qJlOAZQicX8eZRQtZLj5S/VaasmDS10OJQ6f/SBWRxlGVzhzr/Z42hc= X-Received: by 2002:a17:90a:ba15:b0:1cb:be7d:bbca with SMTP id s21-20020a17090aba1500b001cbbe7dbbcamr45504073pjr.143.1651092417481; Wed, 27 Apr 2022 13:46:57 -0700 (PDT) MIME-Version: 1.0 References: <20220404213942.652799-1-goldstein.w.n@gmail.com> <20220427201302.2525203-1-goldstein.w.n@gmail.com> In-Reply-To: <20220427201302.2525203-1-goldstein.w.n@gmail.com> From: "H.J. Lu" Date: Wed, 27 Apr 2022 13:46:21 -0700 Message-ID: Subject: Re: [PATCH v2] sysdeps: Add 'get_fast_jitter' interace in fast-jitter.h To: Noah Goldstein Cc: GNU C Library , "Carlos O'Donell" Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-3026.3 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, GIT_PATCH_0, KAM_SHORT, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 27 Apr 2022 20:47:00 -0000 On Wed, Apr 27, 2022 at 1:13 PM Noah Goldstein wrote: > > '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). > --- > sysdeps/generic/fast-jitter.h | 42 +++++++++++++++++++++++++++++++++++ > 1 file changed, 42 insertions(+) > create mode 100644 sysdeps/generic/fast-jitter.h > > 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 > + . */ > + > +#ifndef _FAST_JITTER_H > +# define _FAST_JITTER_H > + > +# include > +# include > + > +/* 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 > -- > 2.25.1 > LGTM. An arch can provide a better one if needed. Reviewed-by: H.J. Lu Thanks. -- H.J.