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=-6.0 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT autolearn=unavailable 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 A37EEC43381 for ; Tue, 26 Mar 2019 06:47:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 72EA82070B for ; Tue, 26 Mar 2019 06:47:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1553582851; bh=vACHm2q4bj398dRKP7fdfEkhhwgZMfEvNhIZQktEMuM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=YzK90WhxdpVSc5KlxOJXFIsFGkbQwXwaJgc4IFqBvIEJPQqQ8GmwOc4qw5LDG/urm YpZwt4szsqRruxges4UMRufJ82KG9GX000t3O56efbhwyVTqAKVP0aEUlJhI5ZFcIM o/ngZn4g2DeK93ucBOv25mZapHrldiPb3OrmdY9E= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731215AbfCZGb7 (ORCPT ); Tue, 26 Mar 2019 02:31:59 -0400 Received: from mail.kernel.org ([198.145.29.99]:41386 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731275AbfCZGb7 (ORCPT ); Tue, 26 Mar 2019 02:31:59 -0400 Received: from localhost (unknown [104.132.152.111]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 5653020863; Tue, 26 Mar 2019 06:31:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1553581918; bh=vACHm2q4bj398dRKP7fdfEkhhwgZMfEvNhIZQktEMuM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=pFbEfllnymFfhhB6+bjmHw+Pa9qbJgpGZdvwm+Jnq9XMmW2nAEYzAD4FlxQVpwltN VgCkp7ewzBfwaq0479Qy9mQFHCgB51FRiCaLOzIB6wyNbkVGoCizkeH0Xt3Fbvr/3x UNdQFJiElKjLgHu9gdBoFez04fPwjGS7k/tcv/c4= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, "Peter Zijlstra (Intel)" , Anshul Garg , Linus Torvalds , Davidlohr Bueso , Thomas Gleixner , Ingo Molnar , Will Deacon , Joe Perches , David Miller , Matthew Wilcox , Kees Cook , Michael Davidson , Andrew Morton , Arnd Bergmann Subject: [PATCH 4.9 24/30] lib/int_sqrt: optimize small argument Date: Tue, 26 Mar 2019 15:30:03 +0900 Message-Id: <20190326042608.381928654@linuxfoundation.org> X-Mailer: git-send-email 2.21.0 In-Reply-To: <20190326042607.558087893@linuxfoundation.org> References: <20190326042607.558087893@linuxfoundation.org> User-Agent: quilt/0.65 X-stable: review X-Patchwork-Hint: ignore MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org 4.9-stable review patch. If anyone has any objections, please let me know. ------------------ From: Peter Zijlstra commit 3f3295709edea6268ff1609855f498035286af73 upstream. The current int_sqrt() computation is sub-optimal for the case of small @x. Which is the interesting case when we're going to do cumulative distribution functions on idle times, which we assume to be a random variable, where the target residency of the deepest idle state gives an upper bound on the variable (5e6ns on recent Intel chips). In the case of small @x, the compute loop: while (m != 0) { b = y + m; y >>= 1; if (x >= b) { x -= b; y += m; } m >>= 2; } can be reduced to: while (m > x) m >>= 2; Because y==0, b==m and until x>=m y will remain 0. And while this is computationally equivalent, it runs much faster because there's less code, in particular less branches. cycles: branches: branch-misses: OLD: hot: 45.109444 +- 0.044117 44.333392 +- 0.002254 0.018723 +- 0.000593 cold: 187.737379 +- 0.156678 44.333407 +- 0.002254 6.272844 +- 0.004305 PRE: hot: 67.937492 +- 0.064124 66.999535 +- 0.000488 0.066720 +- 0.001113 cold: 232.004379 +- 0.332811 66.999527 +- 0.000488 6.914634 +- 0.006568 POST: hot: 43.633557 +- 0.034373 45.333132 +- 0.002277 0.023529 +- 0.000681 cold: 207.438411 +- 0.125840 45.333132 +- 0.002277 6.976486 +- 0.004219 Averages computed over all values <128k using a LFSR to generate order. Cold numbers have a LFSR based branch trace buffer 'confuser' ran between each int_sqrt() invocation. Link: http://lkml.kernel.org/r/20171020164644.876503355@infradead.org Fixes: 30493cc9dddb ("lib/int_sqrt.c: optimize square root algorithm") Signed-off-by: Peter Zijlstra (Intel) Suggested-by: Anshul Garg Acked-by: Linus Torvalds Cc: Davidlohr Bueso Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Will Deacon Cc: Joe Perches Cc: David Miller Cc: Matthew Wilcox Cc: Kees Cook Cc: Michael Davidson Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Arnd Bergmann Signed-off-by: Greg Kroah-Hartman --- lib/int_sqrt.c | 3 +++ 1 file changed, 3 insertions(+) --- a/lib/int_sqrt.c +++ b/lib/int_sqrt.c @@ -22,6 +22,9 @@ unsigned long int_sqrt(unsigned long x) return x; m = 1UL << (BITS_PER_LONG - 2); + while (m > x) + m >>= 2; + while (m != 0) { b = y + m; y >>= 1;