| From c97c452a2f2f81f466aaf1135d1120382e117142 Mon Sep 17 00:00:00 2001 |
| From: Johannes Weiner <hannes@cmpxchg.org> |
| Date: Tue, 3 Dec 2019 13:35:23 -0500 |
| Subject: [PATCH] sched/psi: Fix sampling error and rare div0 crashes with |
| cgroups and high uptime |
| |
| commit 3dfbe25c27eab7c90c8a7e97b4c354a9d24dd985 upstream. |
| |
| Jingfeng reports rare div0 crashes in psi on systems with some uptime: |
| |
| [58914.066423] divide error: 0000 [#1] SMP |
| [58914.070416] Modules linked in: ipmi_poweroff ipmi_watchdog toa overlay fuse tcp_diag inet_diag binfmt_misc aisqos(O) aisqos_hotfixes(O) |
| [58914.083158] CPU: 94 PID: 140364 Comm: kworker/94:2 Tainted: G W OE K 4.9.151-015.ali3000.alios7.x86_64 #1 |
| [58914.093722] Hardware name: Alibaba Alibaba Cloud ECS/Alibaba Cloud ECS, BIOS 3.23.34 02/14/2019 |
| [58914.102728] Workqueue: events psi_update_work |
| [58914.107258] task: ffff8879da83c280 task.stack: ffffc90059dcc000 |
| [58914.113336] RIP: 0010:[] [] psi_update_stats+0x1c1/0x330 |
| [58914.122183] RSP: 0018:ffffc90059dcfd60 EFLAGS: 00010246 |
| [58914.127650] RAX: 0000000000000000 RBX: ffff8858fe98be50 RCX: 000000007744d640 |
| [58914.134947] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00003594f700648e |
| [58914.142243] RBP: ffffc90059dcfdf8 R08: 0000359500000000 R09: 0000000000000000 |
| [58914.149538] R10: 0000000000000000 R11: 0000000000000000 R12: 0000359500000000 |
| [58914.156837] R13: 0000000000000000 R14: 0000000000000000 R15: ffff8858fe98bd78 |
| [58914.164136] FS: 0000000000000000(0000) GS:ffff887f7f380000(0000) knlGS:0000000000000000 |
| [58914.172529] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 |
| [58914.178467] CR2: 00007f2240452090 CR3: 0000005d5d258000 CR4: 00000000007606f0 |
| [58914.185765] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 |
| [58914.193061] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 |
| [58914.200360] PKRU: 55555554 |
| [58914.203221] Stack: |
| [58914.205383] ffff8858fe98bd48 00000000000002f0 0000002e81036d09 ffffc90059dcfde8 |
| [58914.213168] ffff8858fe98bec8 0000000000000000 0000000000000000 0000000000000000 |
| [58914.220951] 0000000000000000 0000000000000000 0000000000000000 0000000000000000 |
| [58914.228734] Call Trace: |
| [58914.231337] [] psi_update_work+0x22/0x60 |
| [58914.237067] [] process_one_work+0x189/0x420 |
| [58914.243063] [] worker_thread+0x4e/0x4b0 |
| [58914.248701] [] ? process_one_work+0x420/0x420 |
| [58914.254869] [] kthread+0xe6/0x100 |
| [58914.259994] [] ? kthread_park+0x60/0x60 |
| [58914.265640] [] ret_from_fork+0x39/0x50 |
| [58914.271193] Code: 41 29 c3 4d 39 dc 4d 0f 42 dc <49> f7 f1 48 8b 13 48 89 c7 48 c1 |
| [58914.279691] RIP [] psi_update_stats+0x1c1/0x330 |
| |
| The crashing instruction is trying to divide the observed stall time |
| by the sampling period. The period, stored in R8, is not 0, but we are |
| dividing by the lower 32 bits only, which are all 0 in this instance. |
| |
| We could switch to a 64-bit division, but the period shouldn't be that |
| big in the first place. It's the time between the last update and the |
| next scheduled one, and so should always be around 2s and comfortably |
| fit into 32 bits. |
| |
| The bug is in the initialization of new cgroups: we schedule the first |
| sampling event in a cgroup as an offset of sched_clock(), but fail to |
| initialize the last_update timestamp, and it defaults to 0. That |
| results in a bogusly large sampling period the first time we run the |
| sampling code, and consequently we underreport pressure for the first |
| 2s of a cgroup's life. But worse, if sched_clock() is sufficiently |
| advanced on the system, and the user gets unlucky, the period's lower |
| 32 bits can all be 0 and the sampling division will crash. |
| |
| Fix this by initializing the last update timestamp to the creation |
| time of the cgroup, thus correctly marking the start of the first |
| pressure sampling period in a new cgroup. |
| |
| Reported-by: Jingfeng Xie <xiejingfeng@linux.alibaba.com> |
| Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> |
| Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> |
| Cc: Suren Baghdasaryan <surenb@google.com> |
| Link: https://lkml.kernel.org/r/20191203183524.41378-2-hannes@cmpxchg.org |
| Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com> |
| |
| diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c |
| index 517e3719027e..970db4686dd4 100644 |
| --- a/kernel/sched/psi.c |
| +++ b/kernel/sched/psi.c |
| @@ -185,7 +185,8 @@ static void group_init(struct psi_group *group) |
| |
| for_each_possible_cpu(cpu) |
| seqcount_init(&per_cpu_ptr(group->pcpu, cpu)->seq); |
| - group->avg_next_update = sched_clock() + psi_period; |
| + group->avg_last_update = sched_clock(); |
| + group->avg_next_update = group->avg_last_update + psi_period; |
| INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work); |
| mutex_init(&group->avgs_lock); |
| /* Init trigger-related members */ |
| -- |
| 2.7.4 |
| |
