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=-6.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 DA584C2D0DB for ; Sat, 25 Jan 2020 14:52:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4C92D20801 for ; Sat, 25 Jan 2020 14:52:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=yandex-team.ru header.i=@yandex-team.ru header.b="d8cBPn++" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726327AbgAYOun (ORCPT ); Sat, 25 Jan 2020 09:50:43 -0500 Received: from forwardcorp1o.mail.yandex.net ([95.108.205.193]:43284 "EHLO forwardcorp1o.mail.yandex.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725710AbgAYOum (ORCPT ); Sat, 25 Jan 2020 09:50:42 -0500 Received: from mxbackcorp2j.mail.yandex.net (mxbackcorp2j.mail.yandex.net [IPv6:2a02:6b8:0:1619::119]) by forwardcorp1o.mail.yandex.net (Yandex) with ESMTP id A7FDA2E149A; Sat, 25 Jan 2020 17:50:39 +0300 (MSK) Received: from myt5-70c90f7d6d7d.qloud-c.yandex.net (myt5-70c90f7d6d7d.qloud-c.yandex.net [2a02:6b8:c12:3e2c:0:640:70c9:f7d]) by mxbackcorp2j.mail.yandex.net (mxbackcorp/Yandex) with ESMTP id h41kkN8J6N-oci4xH5e; Sat, 25 Jan 2020 17:50:39 +0300 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=yandex-team.ru; s=default; t=1579963839; bh=ocOla4qtEwcngqAdi0s960TFePGyqFArFuiwlheC4x0=; h=Message-ID:Date:To:From:Subject:Cc; b=d8cBPn++wCta2/Eroy636RY7aBSkcQavFjO4hTovVf+4WF1ciUs/6W5ylmkPqrRUP bIrZkcuxSBULF3KDprd76F1TDxWFoh+hSaN7VbvBXlSCuEV8dSEoAXERXkQNiyRbFC cSJL+/C67nz6854E/EUGSh5BxgTpktWnZPS4Zlfg= Authentication-Results: mxbackcorp2j.mail.yandex.net; dkim=pass header.i=@yandex-team.ru Received: from unknown (unknown [2a02:6b8:b080:6910::1:5]) by myt5-70c90f7d6d7d.qloud-c.yandex.net (smtpcorp/Yandex) with ESMTPSA id nzgQNjcdmu-ocWmLHdx; Sat, 25 Jan 2020 17:50:38 +0300 (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (Client certificate not present) Subject: [PATCH] sched/rt: optimize checking group rt scheduler constraints From: Konstantin Khlebnikov To: linux-kernel@vger.kernel.org Cc: Juri Lelli , Vincent Guittot , Peter Zijlstra , Steven Rostedt , Ben Segall , Ingo Molnar , Mel Gorman , Dietmar Eggemann Date: Sat, 25 Jan 2020 17:50:38 +0300 Message-ID: <157996383820.4651.11292439232549211693.stgit@buzz> User-Agent: StGit/0.17.1-dirty MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Group RT scheduler contains protection against setting zero runtime for cgroup with rt tasks. Right now function tg_set_rt_bandwidth() iterates over all cpu cgroups and calls tg_has_rt_tasks() for any cgroup which runtime is zero (not only for changed one). Default rt runtime is zero, thus tg_has_rt_tasks() will is called for almost at cpu cgroups. This protection already is slightly racy: runtime limit could be changed between cpu_cgroup_can_attach() and cpu_cgroup_attach() because changing cgroup attribute does not lock cgroup_mutex while attach does not lock rt_constraints_mutex. Changing task scheduler class also races with changing rt runtime: check in __sched_setscheduler() isn't protected. Function tg_has_rt_tasks() iterates over all threads in the system. This gives NR_CGROUPS * NR_TASKS operations under single tasklist_lock locked for read tg_set_rt_bandwidth(). Any concurrent attempt of locking tasklist_lock for write (for example fork) will stuck with disabled irqs. This patch makes two optimizations: 1) Remove locking tasklist_lock and iterate only tasks in cgroup 2) Call tg_has_rt_tasks() iff rt runtime changes from non-zero to zero All changed code is under CONFIG_RT_GROUP_SCHED. Testcase: # mkdir /sys/fs/cgroup/cpu/test{1..10000} # echo 0 | tee /sys/fs/cgroup/cpu/test*/cpu.rt_runtime_us At the same time without patch fork time will be >100ms: # perf trace -e clone --duration 100 stress-ng --fork 1 Also remote ping will show timings >100ms caused by irq latency. Signed-off-by: Konstantin Khlebnikov --- kernel/sched/rt.c | 24 +++++++++++------------- 1 file changed, 11 insertions(+), 13 deletions(-) diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index e591d40fd645..95d1d7be84ef 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -2396,10 +2396,11 @@ const struct sched_class rt_sched_class = { */ static DEFINE_MUTEX(rt_constraints_mutex); -/* Must be called with tasklist_lock held */ static inline int tg_has_rt_tasks(struct task_group *tg) { - struct task_struct *g, *p; + struct task_struct *task; + struct css_task_iter it; + int ret = 0; /* * Autogroups do not have RT tasks; see autogroup_create(). @@ -2407,12 +2408,12 @@ static inline int tg_has_rt_tasks(struct task_group *tg) if (task_group_is_autogroup(tg)) return 0; - for_each_process_thread(g, p) { - if (rt_task(p) && task_group(p) == tg) - return 1; - } + css_task_iter_start(&tg->css, 0, &it); + while (!ret && (task = css_task_iter_next(&it))) + ret |= rt_task(task); + css_task_iter_end(&it); - return 0; + return ret; } struct rt_schedulable_data { @@ -2443,9 +2444,10 @@ static int tg_rt_schedulable(struct task_group *tg, void *data) return -EINVAL; /* - * Ensure we don't starve existing RT tasks. + * Ensure we don't starve existing RT tasks if runtime turns zero. */ - if (rt_bandwidth_enabled() && !runtime && tg_has_rt_tasks(tg)) + if (rt_bandwidth_enabled() && !runtime && + tg->rt_bandwidth.rt_runtime && tg_has_rt_tasks(tg)) return -EBUSY; total = to_ratio(period, runtime); @@ -2511,7 +2513,6 @@ static int tg_set_rt_bandwidth(struct task_group *tg, return -EINVAL; mutex_lock(&rt_constraints_mutex); - read_lock(&tasklist_lock); err = __rt_schedulable(tg, rt_period, rt_runtime); if (err) goto unlock; @@ -2529,7 +2530,6 @@ static int tg_set_rt_bandwidth(struct task_group *tg, } raw_spin_unlock_irq(&tg->rt_bandwidth.rt_runtime_lock); unlock: - read_unlock(&tasklist_lock); mutex_unlock(&rt_constraints_mutex); return err; @@ -2588,9 +2588,7 @@ static int sched_rt_global_constraints(void) int ret = 0; mutex_lock(&rt_constraints_mutex); - read_lock(&tasklist_lock); ret = __rt_schedulable(NULL, 0, 0); - read_unlock(&tasklist_lock); mutex_unlock(&rt_constraints_mutex); return ret;