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SubjectRe: divide error in select_task_rq_fair()
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On Fri, 2010-11-12 at 11:52 +0530, Nikanth Karthikesan wrote:
> On Thursday 11 November 2010 23:58:04 Myron Stowe wrote:
> > On Fri, 2010-11-05 at 07:17 +0100, Eric Dumazet wrote:
> > > Le jeudi 04 novembre 2010 à 20:00 -0600, Bjorn Helgaas a écrit :
> > > > Is that going to help you debug the problem? The solution is not going
> > > > to be something like "set NR_CPUS=x". If NR_CPUS is too small, the
> > > > machine should still *boot*, even if we can't use all the CPUs in the
> > > > box.
> > >
> > > Yes, it will help to understand the layout of cpu / domains and make
> > > appropriate changes.
> > >
> > > Alternative is you send me such a machine :=)
> >
> > I opened a BZ on this issue as it seems to be a regression -
> > https://bugzilla.kernel.org/show_bug.cgi?id=22662
> >
> > I also, as indicated in the BZ, bisected the kernel which gave the
> > following results and reverting 50f2d7f682f9c0ed58191d0982fe77888d59d162
> > did re-enable booting on the box in question (an HP dl980g7). Let me
> > know what further info you need or patches to test for debugging this.
> >
> > Thanks,
> >
> > commit 50f2d7f682f9c0ed58191d0982fe77888d59d162
> > Author: Nikanth Karthikesan <knikanth@suse.de>
> > Date: Thu Sep 30 17:34:10 2010 +0530
> >
> > x86, numa: Assign CPUs to nodes in round-robin manner on fake NUMA
> >
> > commit d9c2d5ac6af87b4491bff107113aaf16f6c2b2d9 "x86, numa: Use
> > near(er) online node instead of roundrobin for NUMA" changed NUMA
> > initialization on Intel to choose the nearest online node or first node.
> > Fake NUMA would be better of with round-robin initialization, instead of
> > the all CPUS on first node. Change the choice of first node, back to
> > round-robin.
> >
> > For testing NUMA kernel behaviour without cpusets and NUMA aware
> > applications, it would be better to have cpus in different nodes,
> > rather than all in a single node. With cpusets migration of tasks
> > scenarios cannot not be tested.
> >
> > I guess having it round-robin shouldn't affect the use cases for all
> > cpus on the first node.
> >
> > The code comments in arch/x86/mm/numa_64.c:759 indicate that this used
> > to be the case, which was changed by commit d9c2d5ac6. It changed from
> > roundrobin to nearer or first node. And I couldn't find any reason for
> > this change in its changelog.
> >
> > Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
> > Cc: David Rientjes <rientjes@google.com>
> > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> >
> > > Thanks
> >
>
> Can you try with this patch?

I got the same divide error with this latest patch (see attachment). If
I revert commit 50f2d7f682f9, the platform boots successfully.

Thanks,

Myron

>
> Thanks
> Nikanth
>
> Fallback to first node, if the node is not online.
>
> Fixes regression of commit 50f2d7f682f9c0ed58191d0982fe77888d59d162
> x86, numa: Assign CPUs to nodes in round-robin manner on fake NUMA
>
> When some of the NUMA nodes are disabled, and the CPUs are assigned
> in round-robin fashion, CPUs might be assigned to disabled nodes
> resulting in the crash. While using round-robin assignment, check if the
> node is online. If the node is not online, use the first online node.
>
> Reported-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
> Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
>
> ---
>
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index d16c2c5..f31237c 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -287,6 +287,8 @@ static void __cpuinit srat_detect_node(struct cpuinfo_x86 *c)
> if (node == NUMA_NO_NODE || !node_online(node)) {
> /* reuse the value from init_cpu_to_node() */
> node = cpu_to_node(cpu);
> + if (!node_online(node))
> + node = first_node(node_online_map);
> }
> numa_set_node(cpu, node);
> #endif
>


--
Myron Stowe Linux Kernel Developer
Fort Collins, CO Office of Corporate Strategy and Technology
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