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=-5.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_MUTT 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 3DBEBC2BCA1 for ; Fri, 7 Jun 2019 23:39:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 187B2208C0 for ; Fri, 7 Jun 2019 23:39:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730636AbfFGXjO (ORCPT ); Fri, 7 Jun 2019 19:39:14 -0400 Received: from mga18.intel.com ([134.134.136.126]:29886 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729810AbfFGXjO (ORCPT ); Fri, 7 Jun 2019 19:39:14 -0400 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False Received: from orsmga007.jf.intel.com ([10.7.209.58]) by orsmga106.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 07 Jun 2019 16:39:13 -0700 X-ExtLoop1: 1 Received: from guptapadev.jf.intel.com (HELO guptapadev.amr) ([10.7.198.56]) by orsmga007.jf.intel.com with ESMTP; 07 Jun 2019 16:39:13 -0700 Date: Fri, 7 Jun 2019 16:36:56 -0700 From: Pawan Gupta To: Vineeth Remanan Pillai Cc: Nishanth Aravamudan , Julien Desfossez , Peter Zijlstra , Tim Chen , mingo@kernel.org, tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, subhra.mazumdar@oracle.com, fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com, Phil Auld , Aaron Lu , Aubrey Li , Valentin Schneider , Mel Gorman , Paolo Bonzini Subject: Re: [RFC PATCH v3 13/16] sched: Add core wide task selection and scheduling. Message-ID: <20190607233656.GB12952@guptapadev.amr> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.11.3 (2019-02-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 29, 2019 at 08:36:49PM +0000, Vineeth Remanan Pillai wrote: > From: Peter Zijlstra > > Instead of only selecting a local task, select a task for all SMT > siblings for every reschedule on the core (irrespective which logical > CPU does the reschedule). > > NOTE: there is still potential for siblings rivalry. > NOTE: this is far too complicated; but thus far I've failed to > simplify it further. Looks like there are still some race conditions while bringing cpu online/offline. I am seeing an easy to reproduce panic when turning SMT on/off in a loop with core scheduling ON. I dont see the panic with core scheduling OFF. Steps to reproduce: mkdir /sys/fs/cgroup/cpu/group1 mkdir /sys/fs/cgroup/cpu/group2 echo 1 > /sys/fs/cgroup/cpu/group1/cpu.tag echo 1 > /sys/fs/cgroup/cpu/group2/cpu.tag echo $$ > /sys/fs/cgroup/cpu/group1/tasks while [ 1 ]; do echo on > /sys/devices/system/cpu/smt/control echo off > /sys/devices/system/cpu/smt/control done Panic logs: [ 274.629437] BUG: unable to handle kernel NULL pointer dereference at 0000000000000024 [ 274.630366] #PF error: [normal kernel read fault] [ 274.630933] PGD 800000003e52c067 P4D 800000003e52c067 PUD 0 [ 274.631613] Oops: 0000 [#1] SMP PTI [ 274.632016] CPU: 0 PID: 1470 Comm: bash Tainted: G W 5.1.4+ #33 [ 274.632854] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20180724_192412-buildhw-07.phx2.fedoraproject.org-1.fc29 04/01/2014 [ 274.634248] RIP: 0010:__schedule+0x9d4/0x1350 [ 274.634699] Code: da 0f 83 21 04 00 00 48 8b 35 70 f3 ab 00 48 c7 c7 51 1c a8 81 e8 4c 4e 6b ff 49 8b 85 b8 0b 00 00 48 85 c0 0f 84 2f 09 00 01 [ 274.636648] RSP: 0018:ffffc900008f3ca8 EFLAGS: 00010046 [ 274.637197] RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000000040 [ 274.637941] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff82544890 [ 274.638691] RBP: ffffc900008f3d40 R08: 00000000000004c7 R09: 0000000000000030 [ 274.639449] R10: 0000000000000001 R11: ffffc900008f3b28 R12: ffff88803d2d0e80 [ 274.640172] R13: ffff88803eaa0a40 R14: ffff88803ea20a40 R15: ffff88803d2d0e80 [ 274.640915] FS: 0000000000000000(0000) GS:ffff88803ea00000(0063) knlGS:00000000f7f8b780 [ 274.641755] CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033 [ 274.642355] CR2: 0000000000000024 CR3: 000000003c01a005 CR4: 0000000000360ef0 [ 274.643135] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 [ 274.643995] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 [ 274.645023] Call Trace: [ 274.645336] schedule+0x28/0x70 [ 274.645621] native_cpu_up+0x271/0x6d0 [ 274.645959] ? cpus_read_trylock+0x40/0x40 [ 274.646324] bringup_cpu+0x2d/0xe0 [ 274.646631] cpuhp_invoke_callback+0x94/0x550 [ 274.647032] ? ring_buffer_record_is_set_on+0x10/0x10 [ 274.647478] _cpu_up+0xa9/0x140 [ 274.647763] store_smt_control+0x1cb/0x260 [ 274.648132] kernfs_fop_write+0x108/0x190 [ 274.648498] vfs_write+0xa5/0x1a0 [ 274.648794] ksys_write+0x57/0xd0 [ 274.649100] do_fast_syscall_32+0x92/0x220 [ 274.649468] entry_SYSENTER_compat+0x7c/0x8e NULL pointer exception is triggered when sibling is offline during core task pick in pick_next_task() leaving rq_i->core_pick = NULL and if sibling comes online before the "Reschedule siblings" block in the same function it causes panic in is_idle_task(rq_i->core_pick). Traces for the scenario: [ 274.599567] bash-1470 0d... 273921815us : __schedule: cpu(0) is online during core_pick [ 274.600339] bash-1470 0d... 273921816us : __schedule: cpu(1) is offline during core_pick [ 274.601106] bash-1470 0d... 273921816us : __schedule: picked: bash/1470 ffff88803cb9c000 [ 274.602106] bash-1470 0d... 273921816us : __schedule: cpu(0) is online.. during Reschedule siblings [ 274.603219] bash-1470 0d... 273921816us : __schedule: cpu(1) is online.. during Reschedule siblings [ 274.604333] -0 1d... 273921816us : start_secondary: cpu(1) is online now [ 274.605239] bash-1470 0d... 273922148us : __schedule: rq_i->core_pick on cpu(1) is NULL I am not able to reproduce the panic after the below change. Not sure if this is the right fix. Maybe we don't have to allow cpus to go online/offline while pick_next_task() is executing. -------------- 8< --------------- diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 90655c9ad937..b230b095772a 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3874,7 +3874,7 @@ next_class:; for_each_cpu(i, smt_mask) { struct rq *rq_i = cpu_rq(i); - if (cpu_is_offline(i)) + if (cpu_is_offline(i) || !rq_i->core_pick) continue; WARN_ON_ONCE(!rq_i->core_pick);