Re: [Regression] sched: division by zero in find_busiest_group()

[Peter Zijlstra]

On Wed, Dec 18, 2013 at 04:28:35AM +0000, Hedi Berriche wrote:
> On Mon, Dec 09, 2013 at 18:10 Hedi Berriche wrote:
> | Folks,
> | 
> | The following panic occurs *early* at boot time on high *enough* CPU count
> | machines:
> | 
> | divide error: 0000 [#1] SMP 
> | Modules linked in:
> | CPU: 22 PID: 1146 Comm: kworker/22:0 Not tainted 3.13.0-rc2-00122-gdea4f48 #8
> | Hardware name: Intel Corp. Stoutland Platform, BIOS 2.20 UEFI2.10 PI1.0 X64 2013-09-20
> | task: ffff8827d49f31c0 ti: ffff8827d4a18000 task.ti: ffff8827d4a18000
> | RIP: 0010:[<ffffffff810a345b>]  [<ffffffff810a345b>] find_busiest_group+0x26b/0x890
> | RSP: 0000:ffff8827d4a19b68  EFLAGS: 00010006
> | RAX: 0000000000007fff RBX: 0000000000008000 RCX: 0000000000000200
> | RDX: 0000000000000000 RSI: 0000000000008000 RDI: 0000000000000020
> | RBP: ffff8827d4a19cc0 R08: 0000000000000000 R09: 0000000000000000
> | R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> | R13: ffff8827d4a19d28 R14: ffff8827d4a19b98 R15: 0000000000000000
> | FS:  0000000000000000(0000) GS:ffff8827dfd80000(0000) knlGS:0000000000000000
> | CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> | CR2: 00000000000000b8 CR3: 00000000018da000 CR4: 00000000000007e0
> | Stack:
> | ffff8827d4b35800 0000000000000000 0000000000014600 0000000000014600
> | 0000000000000000 ffff8827d4b35818 0000000000000000 0000000000000000
> | 0000000000000000 0000000000000000 0000000000008000 0000000000000000
> | Call Trace:
> | [<ffffffff810a3be6>] load_balance+0x166/0x7f0
> | [<ffffffff810a477e>] idle_balance+0x10e/0x1b0
> | [<ffffffff815d83d3>] __schedule+0x723/0x780
> | [<ffffffff815d8459>] schedule+0x29/0x70
> | [<ffffffff810818b9>] worker_thread+0x1c9/0x400
> | [<ffffffff810816f0>] ? rescuer_thread+0x3e0/0x3e0
> | [<ffffffff81088562>] kthread+0xd2/0xf0
> | [<ffffffff81088490>] ? kthread_create_on_node+0x180/0x180
> | [<ffffffff815e437c>] ret_from_fork+0x7c/0xb0
> | [<ffffffff81088490>] ? kthread_create_on_node+0x180/0x180
> 
> Hmm...had time to dig into this a bit deeper and looking at
> build_overlap_sched_groups(), specifically this bit of code:
> 
> kernel/sched/core.c:
> 
> 5066 static int
> 5067 build_overlap_sched_groups(struct sched_domain *sd, int cpu)
> 5068 {
> ...
> 5109                 /*
> 5110                  * Initialize sgp->power such that even if we mess up the
> 5111                  * domains and no possible iteration will get us here, we won't
> 5112                  * die on a /0 trap.
> 5113                  */
> 5114                 sg->sgp->power = SCHED_POWER_SCALE * cpumask_weight(sg_span);
> 
> I'm wondering whether the same precaution should be used when it comes to sg->sgp->power_orig.

http://marc.info/?l=linux-kernel&m=138684195315258
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