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=-12.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 42BF1C433ED for ; Tue, 6 Apr 2021 15:17:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0807661363 for ; Tue, 6 Apr 2021 15:17:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345569AbhDFPR1 (ORCPT ); Tue, 6 Apr 2021 11:17:27 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:44323 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1345563AbhDFPRZ (ORCPT ); Tue, 6 Apr 2021 11:17:25 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1617722237; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=soYidIvKsSOwqgZrinbzJOhQz2wKNaQ54JiSN4TC7cg=; b=URu1f0IoeTQGecrEQT6vRLw7bIOTb9Sp3mjs2b/nblPKxqO1XnlvJSbRTl5FJNW5mQSdO1 6GmM/Zvp2o1p/XpA48DkNE4yHFu1rHl8p732kraliOrWciCVEmUSb8EpoFd5HGvTB+deg7 mcuFykGtK8aBRncuq5wuD5FabrU/W0g= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-567-JR-AIwFJO2qci2j2pVIJMA-1; Tue, 06 Apr 2021 11:17:15 -0400 X-MC-Unique: JR-AIwFJO2qci2j2pVIJMA-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 8939B107ACCA; Tue, 6 Apr 2021 15:17:13 +0000 (UTC) Received: from llong.remote.csb (ovpn-117-144.rdu2.redhat.com [10.10.117.144]) by smtp.corp.redhat.com (Postfix) with ESMTP id C771B5D9D0; Tue, 6 Apr 2021 15:17:04 +0000 (UTC) Subject: Re: [PATCH v4] sched/debug: Use sched_debug_lock to serialize use of cgroup_path[] only To: Peter Zijlstra Cc: Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Bharata B Rao , Phil Auld , Daniel Thompson , linux-kernel@vger.kernel.org References: <20210405234203.23526-1-longman@redhat.com> From: Waiman Long Organization: Red Hat Message-ID: Date: Tue, 6 Apr 2021 11:17:04 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/6/21 5:15 AM, Peter Zijlstra wrote: > On Mon, Apr 05, 2021 at 07:42:03PM -0400, Waiman Long wrote: >> The handling of sysrq key can be activated by echoing the key to >> /proc/sysrq-trigger or via the magic key sequence typed into a terminal >> that is connected to the system in some way (serial, USB or other mean). >> In the former case, the handling is done in a user context. In the >> latter case, it is likely to be in an interrupt context. >> [ 7809.796281] >> [ 7809.796282] _raw_spin_lock_irqsave+0x32/0x40 >> [ 7809.796283] print_cpu+0x261/0x7c0 >> [ 7809.796283] sysrq_sched_debug_show+0x34/0x50 >> [ 7809.796284] sysrq_handle_showstate+0xc/0x20 >> [ 7809.796284] __handle_sysrq.cold.11+0x48/0xfb >> [ 7809.796285] write_sysrq_trigger+0x2b/0x30 >> [ 7809.796285] proc_reg_write+0x39/0x60 >> [ 7809.796286] vfs_write+0xa5/0x1a0 >> [ 7809.796286] ksys_write+0x4f/0xb0 >> [ 7809.796287] do_syscall_64+0x5b/0x1a0 >> [ 7809.796287] entry_SYSCALL_64_after_hwframe+0x65/0xca >> [ 7809.796288] RIP: 0033:0x7fabe4ceb648 >> >> The purpose of sched_debug_lock is to serialize the use of the global >> cgroup_path[] buffer in print_cpu(). The rests of the printk calls don't >> need serialization from sched_debug_lock. >> The print_cpu() function has two callers - sched_debug_show() and >> sysrq_sched_debug_show(). > So what idiot is doing sysrq and that proc file at the same time? Why is > it a problem now? We got a customer bug report on watchdog panic because a process somehow stay within the sched_debug_lock for too long. I don't know what exactly the customer was doing, but we can reproduce the panic just by having 2 parallel "echo t > /proc/sysrq-trigger" commands. This happens on certain selected systems. I was using some Dell systems for my testing. Of course, it is not sensible to have more than one sysrq happening at the same time. However, a parallel thread accessing /proc/sched_debug is certainly possible. That invokes similar code path. > >> @@ -470,16 +468,49 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group >> #endif >> >> #ifdef CONFIG_CGROUP_SCHED >> +static DEFINE_SPINLOCK(sched_debug_lock); >> static char group_path[PATH_MAX]; >> +static enum { >> + TOKEN_NONE, >> + TOKEN_ACQUIRED, >> + TOKEN_NA /* Not applicable */ >> +} console_token = TOKEN_ACQUIRED; >> +/* >> + * All the print_cpu() callers from sched_debug_show() will be allowed >> + * to contend for sched_debug_lock and use group_path[] as their SEQ_printf() >> + * calls will be much faster. However only one print_cpu() caller from >> + * sysrq_sched_debug_show() which outputs to the console will be allowed >> + * to use group_path[]. Another parallel console writer will have to use >> + * a shorter stack buffer instead. Since the console output will be garbled >> + * anyway, truncation of some cgroup paths shouldn't be a big issue. >> + */ >> +#define SEQ_printf_task_group_path(m, tg, fmt...) \ >> +{ \ >> + unsigned long flags; \ >> + int token = m ? TOKEN_NA \ >> + : xchg_acquire(&console_token, TOKEN_NONE); \ >> + \ >> + if (token == TOKEN_NONE) { \ >> + char buf[128]; \ >> + task_group_path(tg, buf, sizeof(buf)); \ >> + SEQ_printf(m, fmt, buf); \ >> + } else { \ >> + spin_lock_irqsave(&sched_debug_lock, flags); \ >> + task_group_path(tg, group_path, sizeof(group_path)); \ >> + SEQ_printf(m, fmt, group_path); \ >> + spin_unlock_irqrestore(&sched_debug_lock, flags); \ >> + if (token == TOKEN_ACQUIRED) \ >> + smp_store_release(&console_token, token); \ >> + } \ >> } > This is disgusting... you have an open-coded test-and-set lock like > thing *AND* a spinlock, what gives? > > > What's wrong with something simple like this? > > --- > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c > index 4b49cc2af5c4..2ac2977f3b96 100644 > --- a/kernel/sched/debug.c > +++ b/kernel/sched/debug.c > @@ -8,8 +8,6 @@ > */ > #include "sched.h" > > -static DEFINE_SPINLOCK(sched_debug_lock); > - > /* > * This allows printing both to /proc/sched_debug and > * to the console > @@ -470,6 +468,7 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group > #endif > > #ifdef CONFIG_CGROUP_SCHED > +static DEFINE_SPINLOCK(group_path_lock); > static char group_path[PATH_MAX]; > > static char *task_group_path(struct task_group *tg) > @@ -481,6 +480,22 @@ static char *task_group_path(struct task_group *tg) > > return group_path; > } > + > +#define SEQ_printf_task_group_path(m, tg) \ > +do { \ > + if (spin_trylock(&group_path_lock)) { \ > + task_group_path(tg, group_path, sizeof(group_path)); \ > + SEQ_printf(m, "%s", group_path); \ > + spin_unlock(&group_path_lock); \ > + } else { \ > + SEQ_printf(m, "looser!"); \ > + } > +} while (0) I have no problem with using this as a possible solution as long as others agree. Of course, I won't use the term "looser". I will be more polite:-) The current patch allows all /proc/sched_debug users and one sysrq user to use the full group_path[] buffer. I can certainly change it to allow 1 user to use the full size path and the rests running the risk of path truncation. Cheers, Longman