Hi Michael, On 04/02/2014 08:59 AM, Michael wang wrote: > During the testing, we encounter below WARN followed by Oops: > > WARNING: at kernel/sched/core.c:6218 > ... > NIP [c000000000101660] .build_sched_domains+0x11d0/0x1200 > LR [c000000000101358] .build_sched_domains+0xec8/0x1200 > PACATMSCRATCH [800000000000f032] > Call Trace: > [c00000001b103850] [c000000000101358] .build_sched_domains+0xec8/0x1200 > [c00000001b1039a0] [c00000000010aad4] > .partition_sched_domains+0x484/0x510 > [c00000001b103aa0] [c00000000016d0a8] .rebuild_sched_domains+0x68/0xa0 > [c00000001b103b30] [c00000000005cbf0] .topology_work_fn+0x10/0x30 > ... > Oops: Kernel access of bad area, sig: 11 [#1] > ... > NIP [c00000000045c000] .__bitmap_weight+0x60/0xf0 > LR [c00000000010132c] .build_sched_domains+0xe9c/0x1200 > PACATMSCRATCH [8000000000029032] > Call Trace: > [c00000001b1037a0] [c000000000288ff4] > .kmem_cache_alloc_node_trace+0x184/0x3a0 > [c00000001b103850] [c00000000010132c] .build_sched_domains+0xe9c/0x1200 > [c00000001b1039a0] [c00000000010aad4] > .partition_sched_domains+0x484/0x510 > [c00000001b103aa0] [c00000000016d0a8] .rebuild_sched_domains+0x68/0xa0 > [c00000001b103b30] [c00000000005cbf0] .topology_work_fn+0x10/0x30 > ... > > This was caused by that 'sd->groups == NULL' after building groups, which > was caused by the empty 'sd->span'. > > The cpu's domain contain nothing because the cpu was assigned to wrong > node inside arch_update_cpu_topology() by calling update_lookup_table() > with the uninitialized param, in the case when there is nothing to be > update. >
Can you reword the above paragraph to something like this: The cpu's domain contained nothing because the cpu was assigned to a wrong node, due to the following unfortunate sequence of events: 1. The hypervisor sent a topology update to the guest OS, to notify changes to the cpu-node mapping. However, the update was actually redundant - i.e., the "new" mapping was exactly the same as the old one. 2. Due to this, the 'updated_cpus' mask turned out to be empty after exiting the 'for-loop' in arch_update_cpu_topology(). 3. So we ended up calling stop-machine() with an empty cpumask list, which made stop-machine internally elect cpumask_first(cpu_online_mask), i.e., CPU0 as the cpu to run the payload (the update_cpu_topology() function). 4. This causes update_cpu_topology() to be run by CPU0. And since 'updates' is kzalloc()'ed inside arch_update_cpu_topology(), update_cpu_topology() finds update->cpu as well as update->new_nid to be 0. In other words, we end up assigning CPU0 (and eventually its siblings) to node 0, incorrectly. This causes the sched-domain rebuild code to break and crash the system. > Thus we should stop the updating in such cases, this patch will achieve > this and fix the issue. > We can reword this part as: Fix this by skipping the topology update in cases where we find that the topology has not actually changed in reality (ie., spurious updates). > CC: Benjamin Herrenschmidt <b...@kernel.crashing.org> > CC: Paul Mackerras <pau...@samba.org> > CC: Nathan Fontenot <nf...@linux.vnet.ibm.com> > CC: Stephen Rothwell <s...@canb.auug.org.au> > CC: Andrew Morton <a...@linux-foundation.org> > CC: Robert Jennings <r...@linux.vnet.ibm.com> > CC: Jesse Larrew <jlar...@linux.vnet.ibm.com> > CC: "Srivatsa S. Bhat" <srivatsa.b...@linux.vnet.ibm.com> > CC: Alistair Popple <alist...@popple.id.au> > Signed-off-by: Michael Wang <wang...@linux.vnet.ibm.com> > --- > arch/powerpc/mm/numa.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c > index 30a42e2..6757690 100644 > --- a/arch/powerpc/mm/numa.c > +++ b/arch/powerpc/mm/numa.c > @@ -1591,6 +1591,14 @@ int arch_update_cpu_topology(void) > cpu = cpu_last_thread_sibling(cpu); > } > > + /* > + * The 'cpu_associativity_changes_mask' could be cleared if > + * all the cpus it indicates won't change their node, in > + * which case the 'updated_cpus' will be empty. > + */ How about rewording the comment like this: In cases where we have nothing to update (because the updates list is too short or because the new topology is same as the old one), skip invoking update_cpu_topology() via stop-machine(). This is necessary (and not just a fast-path optimization) because stop-machine can end up electing a random CPU to run update_cpu_topology(), and thus trick us into setting up incorrect cpu-node mappings (since 'updates' is kzalloc()'ed). Regards, Srivatsa S. Bhat > + if (!cpumask_weight(&updated_cpus)) > + goto out; > + > stop_machine(update_cpu_topology, &updates[0], &updated_cpus); > > /* > @@ -1612,6 +1620,7 @@ int arch_update_cpu_topology(void) > changed = 1; > } > > +out: > kfree(updates); > return changed; > } > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
