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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 EC4AEC433E0 for ; Wed, 3 Mar 2021 14:33:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A318264DF4 for ; Wed, 3 Mar 2021 14:33:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1381320AbhCCOan (ORCPT ); Wed, 3 Mar 2021 09:30:43 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37822 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1842965AbhCCKXK (ORCPT ); Wed, 3 Mar 2021 05:23:10 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DFD2EC08ECA4 for ; Wed, 3 Mar 2021 01:49:56 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=RRqVDNmqK8qWgfS2O35qxiJ4bts6fZ0niwzW8fuS1pE=; b=RZxNeBzv4lnmfhVYK840iREUeF W6lHMxTpkq+4pRUUJ3aOlWBf6ktD2mMo0UptaXOfuzkV/Z2RDN32ZJA/aMTTCsWkpM9cNEcS1sI3i /ns/0khSsg3vlB84N5dd0erf87ZjTK0rKw9AaixgMgUNKMioP7Ip7XAZHqXbm2km6XM7pE6Le3yEZ km+RvqDGMKiKRdiWjAGNuq3B6LQ0IohV2MDQeQPi0BUW9Q/jwOmgvmxqQEHFeumkhYZW06ZK6c7dN XC4YV16WVQmyIJKlaT5iIvEXcgg+5gVvYs/aatti7GwnBSbeJTyy9YvqZlCfmSaRABQLFC0XY/s1P 2Nf/5SFg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1lHO8G-0023te-UB; Wed, 03 Mar 2021 09:49:29 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 26027303205; Wed, 3 Mar 2021 10:49:27 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id D520E264F0E2E; Wed, 3 Mar 2021 10:49:27 +0100 (CET) Date: Wed, 3 Mar 2021 10:49:27 +0100 From: Peter Zijlstra To: Josh Don Cc: Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , linux-kernel , Clement Courbet , Oleg Rombakh Subject: Re: [PATCH] sched: Optimize __calc_delta. Message-ID: References: <20210226195239.3905966-1-joshdon@google.com> 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-kernel@vger.kernel.org On Tue, Mar 02, 2021 at 12:55:07PM -0800, Josh Don wrote: > On Fri, Feb 26, 2021 at 1:03 PM Peter Zijlstra wrote: > > > > On Fri, Feb 26, 2021 at 11:52:39AM -0800, Josh Don wrote: > > > From: Clement Courbet > > > > > > A significant portion of __calc_delta time is spent in the loop > > > shifting a u64 by 32 bits. Use a __builtin_clz instead of iterating. > > > > > > This is ~7x faster on benchmarks. > > > > Have you tried on hardware without such fancy instructions? > > Was not able to find any on hand unfortunately. Clement did rework the > patch to use fls() instead, and has benchmarks for the generic and asm > variations. All of which are faster than the loop. In my next reply, > I'll include the updated patch inline. Excellent; I have some vague memories where using fls ended up slower for some ARMs, but I can't seem to remember enough to even Google it :/