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Date:   Fri, 08 Nov 2019 17:49:01 +0800
Subject: Re: [PATCH] psi:fix divide by zero in psi_update_stats
From:   Jingfeng Xie <xiejingfeng@linux.alibaba.com>
To:     Peter Zijlstra <peterz@infradead.org>
CC:     Johannes Weiner <hannes@cmpxchg.org>,
        Ingo Molnar <mingo@redhat.com>, <linux-kernel@vger.kernel.org>,
        Joseph Qi <joseph.qi@linux.alibaba.com>,
        Xunlei Pang <xlpang@linux.alibaba.com>
Message-ID: <4BB2BD4E-96A9-42C5-9EEC-115CF69A0C1D@linux.alibaba.com>
Thread-Topic: [PATCH] psi:fix divide by zero in psi_update_stats
References: <C377A5F1-F86F-4A27-966F-0285EC6EA934@linux.alibaba.com>
 <20191108093136.GI4114@hirez.programming.kicks-ass.net>
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It happens multiple times on our online machines, the crash call trace is like below:
[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
[58914.286053] RSP

With full kdump vmcore analysis,  The R8 is period in psi_update_stats which results in the zero division error.


﻿On 2019/11/8 PM 5:31，“Peter Zijlstra”<peterz@infradead.org> wrote:

    On Fri, Nov 08, 2019 at 03:33:24PM +0800, tim wrote:
    > In psi_update_stats, it is possible that period has value like
    > 0xXXXXXXXX00000000 where the lower 32 bit is 0, then it calls div_u64 which
    
    How can this happen? Is that a valid case or should we be avoiding that?