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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C2BA2EB64DA for ; Thu, 20 Jul 2023 09:58:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231134AbjGTJ6O (ORCPT ); Thu, 20 Jul 2023 05:58:14 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55174 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231401AbjGTJ6E (ORCPT ); Thu, 20 Jul 2023 05:58:04 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 98A80EE for ; Thu, 20 Jul 2023 02:58:03 -0700 (PDT) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 4A5D022B4E; Thu, 20 Jul 2023 09:58:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1689847082; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=sHzbbuCMHfXitdRlhu6CbOu8h6g1SgL0u67GCVg312E=; b=EH0tPb84wa+gVtdOZdXgNL5NQIPNb66mpUXoBRWwL6GOYaDmeJ73oNfl7Vnc1UGIKuDYCW 87auuZNyKIItvtaFxRyTxVFb9V2A0cDPsjNUH+dPBwogyQ/fyByenD1Gkhd0VeKEdK1SLq b2m93e0Sc4zkgddM/cqJkfjnAZLNqGk= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1689847082; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=sHzbbuCMHfXitdRlhu6CbOu8h6g1SgL0u67GCVg312E=; b=jS4/SYMM25FVLJF4c4U12FGyRKwC3mQUVQ0Cl7iLBI28ToDyEnpBbAJQAtXLSUXA72pVfp CfMX2sjLjxRp2LDQ== Received: from suse.de (unknown [10.163.43.106]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 7AFCF2C143; Thu, 20 Jul 2023 09:58:00 +0000 (UTC) Date: Thu, 20 Jul 2023 10:57:57 +0100 From: Mel Gorman To: Cyril Hrubis Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Daniel Bristot de Oliveira , Valentin Schneider , linux-kernel@vger.kernel.org, ltp@lists.linux.it, Jiri Bohac Subject: Re: [PATCH 1/2] sched/rt: Fix sysctl_sched_rr_timeslice intial value Message-ID: <20230720095757.syyntxzoqgiucxng@suse.de> References: <20230719103743.4775-1-chrubis@suse.cz> <20230719103743.4775-2-chrubis@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20230719103743.4775-2-chrubis@suse.cz> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 19, 2023 at 12:37:42PM +0200, Cyril Hrubis wrote: > Thre is 10% rounding error in the intial value of the > sysctl_sched_rr_timeslice with CONFIG_HZ_300=y. > > This was found with LTP test sched_rr_get_interval01: > > sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed > sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns > sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90 > sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed > sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns > sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90 > > What this test does is to compare the return value from the > sched_rr_get_interval() and the sched_rr_timeslice_ms sysctl file and > fails if they do not match. > > The prolem it found is the intial sysctl file value which was computed as: > > static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE; > > which works fine as long as MSEC_PER_SEC is multiple of HZ, however it > introduces 10% rounding error for CONFIG_HZ_300: > > (MSEC_PER_SEC / HZ) * (100 * HZ / 1000) > > (1000 / 300) * (100 * 300 / 1000) > > 3 * 30 = 90 > > This can be easily fixed by reversing the order of the multiplication > and division. After this fix we get: > > (MSEC_PER_SEC * (100 * HZ / 1000)) / HZ > > (1000 * (100 * 300 / 1000)) / 300 > > (1000 * 30) / 300 = 100 > > Signed-off-by: Cyril Hrubis > CC: Jiri Bohac Acked-by: Mel Gorman -- Mel Gorman SUSE Labs