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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B1480C43217 for ; Wed, 27 Oct 2021 01:31:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9654161040 for ; Wed, 27 Oct 2021 01:31:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237528AbhJ0Bd7 (ORCPT ); Tue, 26 Oct 2021 21:33:59 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37004 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237353AbhJ0Bd5 (ORCPT ); Tue, 26 Oct 2021 21:33:57 -0400 Received: from mail-pj1-x1032.google.com (mail-pj1-x1032.google.com [IPv6:2607:f8b0:4864:20::1032]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2C784C061745 for ; Tue, 26 Oct 2021 18:31:33 -0700 (PDT) Received: by mail-pj1-x1032.google.com with SMTP id v1-20020a17090a088100b001a21156830bso3799821pjc.1 for ; Tue, 26 Oct 2021 18:31:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=hx2CTswgytxvss7kxEurcgmP5KcmJvBjnBSk15OJ7pE=; b=EXGwIRd4lsCuLvvRdiNRqffudQ8CM9ZJvYGUorXhwpiBZil4EbFzcNtL9Xvx3pz9M4 WSiggrp/PXpWi6Mld8Kqw/lAltNh9dryQmEgfnyRc04qeulm74CT9Sr5zohzDNhZ/7CY 74IVE6QNDELB8g1ZyOmVNofiRYPkOVKZhtaD/QFZuHnVHZupzbdN1S8WRVq8+dKd5CON +Uru4oNtlzHo5AwIwXu0yt6FD0nwqZxB6SV+ynhIj5hflchXNbyMiJTwFUOUNgM99qLF WJrVPzSe01sqhcnxIjKLmdq+dKTy1+LZCVrtbizb2fTdwM9bK4MsL9bVmIiI/n6sW65C IE8A== 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=hx2CTswgytxvss7kxEurcgmP5KcmJvBjnBSk15OJ7pE=; b=qUzY9FH2eHvJ9DmBimT8b6EvJexZ4Gh4fPzsp8N5qkwx7xB82Y5ASiya8d1WVyab3V PwTXed4JnTbD25K0M1IWos8UQLaZIGjeAG+anVVj+JDo8rU01EKVhWdfCbSlBC8cNNXp dqkxXhlAiyQyuq0syvOuiQ6AxWI2gWQ5BZv8zeal847j60ISgGYdIirHz6o0hnDsMIp2 xIUP1G7e2qYXnpbLzGSKDvCGr/GJYaIsfAi+9k6uGLHSWrn5wrl5kFpcQlmTPRJ+8LJh DBtw75TQHW5s0pQ7DotKvijxWM75JTAkSAXGUMxfX31Yyg1do9sM9HsYhIRZui/McPx5 fgkQ== X-Gm-Message-State: AOAM532z0eHcwK0df8Cy+II7Qj/r7+XYQLiGsWlMYC5Tn5h72lbb3Lrx /dW0oIJUGcneXh+3po707r5EOb31vi2Gv/NOIJIOKQ== X-Google-Smtp-Source: ABdhPJyLneM3xQMO3ldQoIW2aan64ir8E90WZfnjCWKqMjRrHioWLmsJ5Yy3Mi1n0G/AD6jIbmJK9F8yKMq3xQEmT3A= X-Received: by 2002:a17:90a:b288:: with SMTP id c8mr2480230pjr.67.1635298292469; Tue, 26 Oct 2021 18:31:32 -0700 (PDT) MIME-Version: 1.0 References: <20211025200852.3002369-1-kaleshsingh@google.com> <20211025200852.3002369-7-kaleshsingh@google.com> <20211026151451.7f3e09a4@gandalf.local.home> <20211026201846.08990d1d@rorschach.local.home> <20211026211511.403d76ca@rorschach.local.home> In-Reply-To: <20211026211511.403d76ca@rorschach.local.home> From: Kalesh Singh Date: Tue, 26 Oct 2021 18:31:21 -0700 Message-ID: Subject: Re: [PATCH v4 6/8] tracing/histogram: Optimize division by a power of 2 To: Steven Rostedt Cc: surenb@google.com, hridya@google.com, namhyung@kernel.org, kernel-team@android.com, Jonathan Corbet , Ingo Molnar , Shuah Khan , Masami Hiramatsu , Tom Zanussi , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 26, 2021 at 6:15 PM Steven Rostedt wrote: > > On Tue, 26 Oct 2021 18:09:22 -0700 > Kalesh Singh wrote: > > > > delta = mult * div / 2^20 > > > > > > That is if mult is a power of two, then there would be no rounding > > > errors, and the delta is zero, making the max infinite: > > That should have been (as shown in the algorithm) > > delta = mult * div - 2 ^ 20 > > As mult is 2^20 / div; and the above should end up zero if there's no > rounding issues, as it would be: > > delta = (2^20 / div) * div - 2^20 Good catch. We're checking if we get back the exact value. And IIUC max_div is an arbitrary value we decide on that's <= 2^shift? Is there a rule of thumb for choosing this? Thanks, Kalesh > > -- Steve