From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yb1-xb30.google.com (mail-yb1-xb30.google.com [IPv6:2607:f8b0:4864:20::b30]) by sourceware.org (Postfix) with ESMTPS id EC6F03858D32; Sun, 11 Sep 2022 20:27:48 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org EC6F03858D32 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gmail.com Received: by mail-yb1-xb30.google.com with SMTP id 202so9951962ybe.13; Sun, 11 Sep 2022 13:27:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date; bh=ah+qDct8j01hc7b2wvsSpRo4fgwKS6MUg2rg5jzqCSU=; b=FBQiKZQIpdukYWxcZ1C+/6K+synLKT09hgYoJKHcHJ2TrASdoTaB9vRIdWu0J9mKAN EF12CertC5gxi7EzP3Cax5qdRjxe6xiRmclbA1AjHuQ74uWXJ7CXa5DmB2IfYt0YxrKN DaU9D5yBT2wZoq10zoVCH5JpOstLNQ7jXL2UcVdkmRa8sgca1F86+OnIIXwpq3AsPkPI i7FA/1x2R1phF3Nr4yzrJu5PHFhErWTrxBHrID3knowImU77XYpJNw8bwTPDaxDOww1T Ewkj/7idJlp6/MmKCSlt2hfm42i9r0h1mmJrCVsyQpB9AV/X+1Ps+2Uhw6ZSEQz06LWu UhFQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date; bh=ah+qDct8j01hc7b2wvsSpRo4fgwKS6MUg2rg5jzqCSU=; b=MmaAKLacHZcdNNaxFrC1zwLD7eEF58lhtu2QFcVTspb5PeB+3RHPliKetPij2s/koB hHbyld+mFq+kUHJvAX/yvhCGXqYwSpONZn1gTU5mnfEygxuVAV32I8WDqU96KAUghV+j 3tWP+xWr/eQ6svl4vHLUGkrTxlVS4i/zLFuFTSEpiqa3ew0RzrY4sP3tHL7w5HoFRzG/ fArJl42U4biXPRh9wZ1hlXwjqV4uqiASzfLByigRG5xtU2Nx8vnMPffsNVuHEBGvKjJk 0zgRdlQj0X/WdZYbHg5qAFP7lJEW9WLukJ+8TCZda0RtFiZJ0HBV2KeQB4CPq77fV2bF VpaQ== X-Gm-Message-State: ACgBeo2B4KcNZmU6FKtfoZ6WUpMKDvnlNcZwcUFG4LtmHIGsA5wXqgqH asHI/UAOCJVaanrxU3RSOO1XMZJrZ2U4sWomHd4= X-Google-Smtp-Source: AA6agR4lNYxPzq8WoKHMVX/mJ0JDSZaUl4ZZfezi2+udXBz9WWRRwJ0U/po/z8KBu1twHvDErI7szT46ngttSCqnOTk= X-Received: by 2002:a25:424f:0:b0:6ac:9f04:5cf with SMTP id p76-20020a25424f000000b006ac9f0405cfmr18235837yba.60.1662928068358; Sun, 11 Sep 2022 13:27:48 -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: From: Sunil Pandey Date: Sun, 11 Sep 2022 13:27:12 -0700 Message-ID: Subject: Re: [PATCH v2] sysdeps: Add 'get_fast_jitter' interace in fast-jitter.h To: "H.J. Lu" , Libc-stable Mailing List , Florian Weimer Cc: Noah Goldstein , GNU C Library Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-6.6 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_ENVFROM_END_DIGIT,FREEMAIL_FROM,GIT_PATCH_0,HK_RANDOM_ENVFROM,HK_RANDOM_FROM,KAM_SHORT,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,TXREP,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On Wed, Apr 27, 2022 at 1:47 PM H.J. Lu via Libc-alpha wrote: > > 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. I would like to backport this patch to release branch 2.33, 2.34 and 2.35 Any comments/suggestions or objections on this. commit 911c63a51c690dd1a97dfc587097277029baf00f Author: Noah Goldstein Date: Wed Apr 27 15:13:02 2022 -0500 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.