From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-12.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F1D0EC48BC2 for ; Sun, 27 Jun 2021 16:47:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CBB90619F1 for ; Sun, 27 Jun 2021 16:47:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231139AbhF0Qtx (ORCPT ); Sun, 27 Jun 2021 12:49:53 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33392 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229820AbhF0Qtx (ORCPT ); Sun, 27 Jun 2021 12:49:53 -0400 Received: from mail-qt1-x832.google.com (mail-qt1-x832.google.com [IPv6:2607:f8b0:4864:20::832]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1BA11C061574; Sun, 27 Jun 2021 09:47:28 -0700 (PDT) Received: by mail-qt1-x832.google.com with SMTP id d5so11718325qtd.5; Sun, 27 Jun 2021 09:47:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=IGinKwiNSfxrJd+RUlITcVgWE2FaXzN4ff0xutRXDaU=; b=DeEb2ecHoc+9I/oFeMO1TfS4y4+YwjybIHliznobdI/nxpUlDavt/JLpost/dY8G3R 8cJOtmQqUlYDxeTfYaNQ1/1g1TYrdNZ/A32GllgnS0hha2/HadzrYR0ZR/tRkdZpbxH8 IzXE19/om+0BCtDEh+5G/TinS7EBlwCBx9XnhCN27I7baiLvgEtBOZ5RxX86Pt8qGGrL ti6MAYW24ozYHGBBcO1pRe0GOzYwFWDEmrfrVRXyBEeul012dxIt8kEK88EQW2k3Go1b IUG+ysf0WhcqHZpAScjamKwVwTjwtbeNNXEkL9mP8x8n8wShaIzm/RqO9jBhmSlRJ8kA 7pag== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=IGinKwiNSfxrJd+RUlITcVgWE2FaXzN4ff0xutRXDaU=; b=e3LpLYkSR+04tXSLRo8Bpz1Yn61xqdA83l4ZT0YEwJhSm/q8FfM9A7b0x/4SW3Ld4X DBXA2Lg8lrG/veUwANwu+zlMJzC8hBklgCb0Lzik0nMBM6avT9iYZIWtPJWtKjFt040n Hllusz+IzsxzFXu79Ti5/VOPMDfsBZcVhM4+a3BnYeETFuYuJx07lTC/S0B4kfipTahl mC10cELeMTDGwK2Pu4ELDaV0k65Oa6nvJ/Ch+fb3JLShsvPZLhYsBGhpWsgkgv00ntOz T9+KwDdnvOQS5pWg5osfP7esASDaUVIPLl4KaxoYFhPn7RQCEGiChVW+sYendkRp4huF aDgg== X-Gm-Message-State: AOAM531bUtiCWbCERNhrdnHiNJUFxHQt8lw9cRAoll+BqsWKtdx/cK5A dq0xeGk1EcAdrHdl0Baw17k= X-Google-Smtp-Source: ABdhPJzXp6YlfGmAUiYHzOtD1PJTSLr3XCovAHLZvis1ouC9/p4U2/5jfgoQakO2n+2S85GDybfXYQ== X-Received: by 2002:a05:622a:11cd:: with SMTP id n13mr18281355qtk.233.1624812447049; Sun, 27 Jun 2021 09:47:27 -0700 (PDT) Received: from localhost ([207.98.216.60]) by smtp.gmail.com with ESMTPSA id 202sm7563423qki.83.2021.06.27.09.47.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 27 Jun 2021 09:47:26 -0700 (PDT) Date: Sun, 27 Jun 2021 09:47:25 -0700 From: Yury Norov To: Marc Zyngier Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Lucas Stach , Russell King , Christian Gmeiner , David Airlie , Daniel Vetter , Jean Delvare , Guenter Roeck , Andy Shevchenko , Rasmus Villemoes , David Woodhouse , Andrew Morton , Wei Yang , Geert Uytterhoeven , Alexey Klimov , x86@kernel.org, linux-kernel@vger.kernel.org, etnaviv@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-hwmon@vger.kernel.org Subject: Re: [PATCH 2/3] find: micro-optimize for_each_{set,clear}_bit() Message-ID: References: <20210618195735.55933-1-yury.norov@gmail.com> <20210618195735.55933-3-yury.norov@gmail.com> <87bl81ddqo.wl-maz@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-hwmon@vger.kernel.org On Sat, Jun 19, 2021 at 10:28:07AM -0700, Yury Norov wrote: > On Sat, Jun 19, 2021 at 05:24:15PM +0100, Marc Zyngier wrote: > > On Fri, 18 Jun 2021 20:57:34 +0100, > > Yury Norov wrote: > > > > > > The macros iterate thru all set/clear bits in a bitmap. They search a > > > first bit using find_first_bit(), and the rest bits using find_next_bit(). > > > > > > Since find_next_bit() is called shortly after find_first_bit(), we can > > > save few lines of I-cache by not using find_first_bit(). > > > > Really? > > > > > > > > Signed-off-by: Yury Norov > > > --- > > > include/linux/find.h | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/include/linux/find.h b/include/linux/find.h > > > index 4500e8ab93e2..ae9ed52b52b8 100644 > > > --- a/include/linux/find.h > > > +++ b/include/linux/find.h > > > @@ -280,7 +280,7 @@ unsigned long find_next_bit_le(const void *addr, unsigned > > > #endif > > > > > > #define for_each_set_bit(bit, addr, size) \ > > > - for ((bit) = find_first_bit((addr), (size)); \ > > > + for ((bit) = find_next_bit((addr), (size), 0); \ > > > > On which architecture do you observe a gain? Only 32bit ARM and m68k > > implement their own version of find_first_bit(), and everyone else > > uses the canonical implementation: > > And those who enable GENERIC_FIND_FIRST_BIT - x86, arm64, arc, mips > and s390. > > > #ifndef find_first_bit > > #define find_first_bit(addr, size) find_next_bit((addr), (size), 0) > > #endif > > > > These architectures explicitly have different implementations for > > find_first_bit() and find_next_bit() because they can do better > > (whether that is true or not is another debate). I don't think you > > should remove this optimisation until it has been measured on these > > two architectures. > > This patch is based on a series that enables separate implementation > of find_first_bit() for all architectures; according to my tests, > find_first* is ~ twice faster than find_next* on arm64 and x86. > > https://lore.kernel.org/lkml/20210612123639.329047-1-yury.norov@gmail.com/T/#t > > After applying the series, I noticed that my small kernel module that > calls for_each_set_bit() is now using find_first_bit() to just find > one bit, and find_next_bit() for all others. I think it's better to > always use find_next_bit() in this case to minimize the chance of > cache miss. But if it's not that obvious, I'll try to write some test. This test measures the difference between for_each_set_bit() and for_each_set_bit_from(). diff --git a/lib/find_bit_benchmark.c b/lib/find_bit_benchmark.c index 5637c5711db9..1f37e99090b0 100644 --- a/lib/find_bit_benchmark.c +++ b/lib/find_bit_benchmark.c @@ -111,6 +111,59 @@ static int __init test_find_next_and_bit(const void *bitmap, return 0; } +#ifdef CONFIG_X86_64 +#define flush_cache_all() wbinvd() +#endif + +static int __init test_for_each_set_bit(int flags) +{ +#ifdef flush_cache_all + DECLARE_BITMAP(bm, BITS_PER_LONG * 2); + unsigned long i, cnt = 0; + ktime_t time; + + bm[0] = 1; bm[1] = 0; + + time = ktime_get(); + while (cnt < 1000) { + if (flags) + flush_cache_all(); + for_each_set_bit(i, bm, BITS_PER_LONG * 2) + cnt++; + } + + time = ktime_get() - time; + + pr_err("for_each_set_bit: %18llu ns, %6ld iterations\n", time, cnt); +#endif + return 0; +} + +static int __init test_for_each_set_bit_from(int flags) +{ +#ifdef flush_cache_all + DECLARE_BITMAP(bm, BITS_PER_LONG * 2); + unsigned long i, cnt = 0; + ktime_t time; + + bm[0] = 1; bm[1] = 0; + + time = ktime_get(); + while (cnt < 1000) { + if (flags) + flush_cache_all(); + i = 0; + for_each_set_bit_from(i, bm, BITS_PER_LONG * 2) + cnt++; + } + + time = ktime_get() - time; + + pr_err("for_each_set_bit_from:%16llu ns, %6ld iterations\n", time, cnt); +#endif + return 0; +} + static int __init find_bit_test(void) { unsigned long nbits = BITMAP_LEN / SPARSE; @@ -147,6 +200,16 @@ static int __init find_bit_test(void) test_find_first_bit(bitmap, BITMAP_LEN); test_find_next_and_bit(bitmap, bitmap2, BITMAP_LEN); + pr_err("\nStart testing for_each_bit()\n"); + + test_for_each_set_bit(0); + test_for_each_set_bit_from(0); + + pr_err("\nStart testing for_each_bit() with cash flushing\n"); + + test_for_each_set_bit(1); + test_for_each_set_bit_from(1); + /* * Everything is OK. Return error just to let user run benchmark * again without annoying rmmod. Here on each iteration: - for_each_set_bit() calls find_first_bit() once, and find_next_bit() once. - for_each_set_bit_from() calls find_next_bit() twice. On my AMD Ryzen 7 4700U, the result is like this: Start testing for_each_bit() for_each_set_bit: 15296 ns, 1000 iterations for_each_set_bit_from: 15225 ns, 1000 iterations Start testing for_each_bit() with cash flushing for_each_set_bit: 547626 ns, 1000 iterations for_each_set_bit_from: 497899 ns, 1000 iterations for_each_set_bit_from() is ~10% faster than for_each_set_bit() in case of cold caches, and no significant difference was observed if flush_cache_all() is not called. So, it looks reasonable to switch for_each_set_bit() to use find_next_bit() only. Thanks, Yury