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Message-ID: <1257770720.4108.195.camel@laptop>
Date: Mon, 09 Nov 2009 13:45:20 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>
Cc: mingo@...e.hu, linux-kernel@...r.kernel.org,
Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: Kernel oops in resched_task() with 2.6.31.5
On Mon, 2009-11-09 at 21:31 +0900, Kenji Kaneshige wrote:
> Hi,
>
> I frequently encounter the kernel oops attached below in resched_task()
> with 2.6.31.5. This kernel oops happens also with 2.6.32-rc5. I don't
> know about other kernel.
>
> Here is my analysis:
>
> The immediate cause of this kernel oops is that NULL was passed to
> resched_task() from resched_cpu(). From my investigation, this was
> caused as follows:
>
> - trigger_load_balance() caluculated cpu number of idle load balancer
> using find_new_ilb(), and find_new_ilb() returned *offline* CPU
> number (16 in my case). Note that I didn't do any CPU hotplug
> operation. On my system, present, online and offline under
> /sys/devices/system/cpu/ are
>
> [kanesige@...alhost ~]$ cat /sys/devices/system/cpu/present
> 0-15
> [kanesige@...alhost ~]$ cat /sys/devices/system/cpu/online
> 0-15
> [kanesige@...alhost ~]$ cat /sys/devices/system/cpu/offline
> 16-255
>
> And nr_cpu_ids is 256.
>
> - resched_cpu() calculated current task by cpu_curr() with offline CPU
> number.
>
> So this kernel oops seems to be caused by invalid CPU number returned
> from find_new_ilb(). I don't know the find_new_ilb() implementation,
> but I suspect the initialization of cpumasks used by find_new_ilb().
> The patch attached below seems to fix the problem (With this patch,
> the kernel oops doesn't happen). But I don't know if this is the
> correct fix.
Please send patches against -tip.
You might find that Rusty has already fixed a similar issue there in
commit: 49557e620339cb134127b5bfbcfecc06b77d0232.
Now, Rusty's patch does not clear the ilb mask, so maybe it doesn't
fully cover your issue, please test.
> ---
> kernel/sched.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> Index: linux-2.6.31.5/kernel/sched.c
> ===================================================================
> --- linux-2.6.31.5.orig/kernel/sched.c 2009-11-09 17:03:33.818457759 +0900
> +++ linux-2.6.31.5/kernel/sched.c 2009-11-09 18:02:39.619934041 +0900
> @@ -9386,8 +9386,8 @@
> alloc_cpumask_var(&nohz_cpu_mask, GFP_NOWAIT);
> #ifdef CONFIG_SMP
> #ifdef CONFIG_NO_HZ
> - alloc_cpumask_var(&nohz.cpu_mask, GFP_NOWAIT);
> - alloc_cpumask_var(&nohz.ilb_grp_nohz_mask, GFP_NOWAIT);
> + zalloc_cpumask_var(&nohz.cpu_mask, GFP_NOWAIT);
> + zalloc_cpumask_var(&nohz.ilb_grp_nohz_mask, GFP_NOWAIT);
> #endif
> alloc_cpumask_var(&cpu_isolated_map, GFP_NOWAIT);
> #endif /* SMP */
>
>
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