From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EDFA81170E for ; Mon, 11 Sep 2023 13:53:02 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6F6A9C433C7; Mon, 11 Sep 2023 13:53:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1694440382; bh=h0ZK7sZAUV7zGqWGCiXihXvv9PRj+ZUj5xt0E5rfOrc=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=b0npOG4iS49RQcjmFFHJjHdWYVgJ4QztS/YabyJ18ibcDrw4qPw+MHz7hobyp7SwY 6iK1baQEr8G384VuGu7iS33/O9gor62LM2xFwHqkSePk8o1NO/fvoUf/ZQJfyscNce rqOgksgPKafg7Sp0hMx0B68kZhfAuTEQT+aGdWLo= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Cyril Hrubis , "Peter Zijlstra (Intel)" , Petr Vorel , Mel Gorman , Sasha Levin Subject: [PATCH 6.5 041/739] sched/rt: Fix sysctl_sched_rr_timeslice intial value Date: Mon, 11 Sep 2023 15:37:20 +0200 Message-ID: <20230911134652.254458257@linuxfoundation.org> X-Mailer: git-send-email 2.42.0 In-Reply-To: <20230911134650.921299741@linuxfoundation.org> References: <20230911134650.921299741@linuxfoundation.org> User-Agent: quilt/0.67 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.5-stable review patch. If anyone has any objections, please let me know. ------------------ From: Cyril Hrubis [ Upstream commit c7fcb99877f9f542c918509b2801065adcaf46fa ] There is a 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 problem 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 Fixes: 975e155ed873 ("sched/rt: Show the 'sched_rr_timeslice' SCHED_RR timeslice tuning knob in milliseconds") Signed-off-by: Cyril Hrubis Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Petr Vorel Acked-by: Mel Gorman Tested-by: Petr Vorel Link: https://lore.kernel.org/r/20230802151906.25258-2-chrubis@suse.cz Signed-off-by: Sasha Levin --- kernel/sched/rt.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 00e0e50741153..185d3d749f6b6 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -25,7 +25,7 @@ unsigned int sysctl_sched_rt_period = 1000000; int sysctl_sched_rt_runtime = 950000; #ifdef CONFIG_SYSCTL -static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE; +static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC * RR_TIMESLICE) / HZ; static int sched_rt_handler(struct ctl_table *table, int write, void *buffer, size_t *lenp, loff_t *ppos); static int sched_rr_handler(struct ctl_table *table, int write, void *buffer, -- 2.40.1