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SubjectRe: divide error in select_task_rq_fair()
Date
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?

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
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