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=-5.7 required=3.0 tests=BAYES_00,DKIM_ADSP_CUSTOM_MED, FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 CF969C4361B for ; Fri, 18 Dec 2020 18:07:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A424A23B54 for ; Fri, 18 Dec 2020 18:07:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728230AbgLRSHN (ORCPT ); Fri, 18 Dec 2020 13:07:13 -0500 Received: from smtp5.emailarray.com ([65.39.216.39]:32834 "EHLO smtp5.emailarray.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726269AbgLRSHM (ORCPT ); Fri, 18 Dec 2020 13:07:12 -0500 Received: (qmail 16827 invoked by uid 89); 18 Dec 2020 18:06:30 -0000 Received: from unknown (HELO localhost) (amxlbW9uQGZsdWdzdmFtcC5jb21AMTYzLjExNC4xMzIuNw==) (POLARISLOCAL) by smtp5.emailarray.com with SMTP; 18 Dec 2020 18:06:30 -0000 Date: Fri, 18 Dec 2020 10:06:28 -0800 From: Jonathan Lemon To: Yonghong Song Cc: netdev@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, bpf@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 1/1 v3 bpf-next] bpf: increment and use correct thread iterator Message-ID: <20201218180628.qz3qjb7y3sa4rbn3@bsd-mbp> References: <20201211171138.63819-1-jonathan.lemon@gmail.com> <20201211171138.63819-2-jonathan.lemon@gmail.com> <39fcac29-4c93-1c76-62ba-728618a25fe5@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <39fcac29-4c93-1c76-62ba-728618a25fe5@fb.com> Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Fri, Dec 18, 2020 at 08:53:22AM -0800, Yonghong Song wrote: > > > On 12/11/20 9:11 AM, Jonathan Lemon wrote: > > From: Jonathan Lemon > > > > On some systems, some variant of the following splat is > > repeatedly seen. The common factor in all traces seems > > to be the entry point to task_file_seq_next(). With the > > patch, all warnings go away. > > > > rcu: INFO: rcu_sched self-detected stall on CPU > > rcu: \x0926-....: (20992 ticks this GP) idle=d7e/1/0x4000000000000002 softirq=81556231/81556231 fqs=4876 > > \x09(t=21033 jiffies g=159148529 q=223125) > > NMI backtrace for cpu 26 > > CPU: 26 PID: 2015853 Comm: bpftool Kdump: loaded Not tainted 5.6.13-0_fbk4_3876_gd8d1f9bf80bb #1 > > Hardware name: Quanta Twin Lakes MP/Twin Lakes Passive MP, BIOS F09_3A12 10/08/2018 > > Call Trace: > > > > dump_stack+0x50/0x70 > > nmi_cpu_backtrace.cold.6+0x13/0x50 > > ? lapic_can_unplug_cpu.cold.30+0x40/0x40 > > nmi_trigger_cpumask_backtrace+0xba/0xca > > rcu_dump_cpu_stacks+0x99/0xc7 > > rcu_sched_clock_irq.cold.90+0x1b4/0x3aa > > ? tick_sched_do_timer+0x60/0x60 > > update_process_times+0x24/0x50 > > tick_sched_timer+0x37/0x70 > > __hrtimer_run_queues+0xfe/0x270 > > hrtimer_interrupt+0xf4/0x210 > > smp_apic_timer_interrupt+0x5e/0x120 > > apic_timer_interrupt+0xf/0x20 > > > > RIP: 0010:get_pid_task+0x38/0x80 > > Code: 89 f6 48 8d 44 f7 08 48 8b 00 48 85 c0 74 2b 48 83 c6 55 48 c1 e6 04 48 29 f0 74 19 48 8d 78 20 ba 01 00 00 00 f0 0f c1 50 20 <85> d2 74 27 78 11 83 c2 01 78 0c 48 83 c4 08 c3 31 c0 48 83 c4 08 > > RSP: 0018:ffffc9000d293dc8 EFLAGS: 00000202 ORIG_RAX: ffffffffffffff13 > > RAX: ffff888637c05600 RBX: ffffc9000d293e0c RCX: 0000000000000000 > > RDX: 0000000000000001 RSI: 0000000000000550 RDI: ffff888637c05620 > > RBP: ffffffff8284eb80 R08: ffff88831341d300 R09: ffff88822ffd8248 > > R10: ffff88822ffd82d0 R11: 00000000003a93c0 R12: 0000000000000001 > > R13: 00000000ffffffff R14: ffff88831341d300 R15: 0000000000000000 > > ? find_ge_pid+0x1b/0x20 > > task_seq_get_next+0x52/0xc0 > > task_file_seq_get_next+0x159/0x220 > > task_file_seq_next+0x4f/0xa0 > > bpf_seq_read+0x159/0x390 > > vfs_read+0x8a/0x140 > > ksys_read+0x59/0xd0 > > do_syscall_64+0x42/0x110 > > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > RIP: 0033:0x7f95ae73e76e > > Code: Bad RIP value. > > RSP: 002b:00007ffc02c1dbf8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 > > RAX: ffffffffffffffda RBX: 000000000170faa0 RCX: 00007f95ae73e76e > > RDX: 0000000000001000 RSI: 00007ffc02c1dc30 RDI: 0000000000000007 > > RBP: 00007ffc02c1ec70 R08: 0000000000000005 R09: 0000000000000006 > > R10: fffffffffffff20b R11: 0000000000000246 R12: 00000000019112a0 > > R13: 0000000000000000 R14: 0000000000000007 R15: 00000000004283c0 > > > > The attached patch does 3 things: > > > > 1) If unable to obtain the file structure for the current task, > > proceed to the next task number after the one returned from > > task_seq_get_next(), instead of the next task number from the > > original iterator. > > Looks like this fix is the real fix for the above warnings. > Basically, say we have > info->tid = 10 and returned curr_tid = 3000 and tid 3000 has no files. > the current logic will go through > - set curr_tid = 11 (info->tid++) and returned curr_tid = 3000 > - set curr_tid = 12 and returned curr_tid = 3000 > ... > - set curr_tid = 3000 and returned curr_tid = 3000 > - set curr_tid = 3001 and return curr_tid >= 3001 > > All the above works are redundant work, and it may cause issues > for non preemptable kernel. > > I suggest you factor out this change plus the following change > which suggested by Andrii early to a separate patch carried with > the below Fixes tag. > > diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c > index 0458a40edf10..56bcaef72e36 100644 > --- a/kernel/bpf/task_iter.c > +++ b/kernel/bpf/task_iter.c > @@ -158,6 +158,7 @@ task_file_seq_get_next(struct > bpf_iter_seq_task_file_info *info) > if (!curr_task) { > info->task = NULL; > info->files = NULL; > + info->tid = curr_tid + 1; > return NULL; > } Sure this isn't supposed to be 'curr_tid'? task_seq_get_next() stops when there are no more threads found. This increments the thread id past the search point, and would seem to introduce a potential off-by-one error. That is: curr_tid = 3000. call task_seq_get_next() --> return NULL, curr_tid = 3000. (so there is no tid >= 3000) set curr_tid = 3001. next restart (if there is one) skips a newly created 3000. -- Jonathan