From 9a0133613e4412b2caaaf0d9dd81f213bcededf1 Mon Sep 17 00:00:00 2001 From: Michael Wang <wangyun@linux.vnet.ibm.com> Date: Tue, 8 Apr 2014 11:19:36 +0800 Subject: power, sched: stop updating inside arch_update_cpu_topology() when nothing to be update Since v1: Edited the comment according to Srivatsa's suggestion. 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 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. Along with the following wrong updating, it causes the sched-domain rebuild code to break and crash the system. 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 <benh@kernel.crashing.org> CC: Paul Mackerras <paulus@samba.org> CC: Nathan Fontenot <nfont@linux.vnet.ibm.com> CC: Stephen Rothwell <sfr@canb.auug.org.au> CC: Andrew Morton <akpm@linux-foundation.org> CC: Robert Jennings <rcj@linux.vnet.ibm.com> CC: Jesse Larrew <jlarrew@linux.vnet.ibm.com> CC: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com> CC: Alistair Popple <alistair@popple.id.au> Suggested-by: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com> Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com> Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org> --- arch/powerpc/mm/numa.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) (limited to 'arch/powerpc/mm') diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c index 30a42e24bf14..4ebbb9e99286 100644 --- a/arch/powerpc/mm/numa.c +++ b/arch/powerpc/mm/numa.c @@ -1591,6 +1591,20 @@ int arch_update_cpu_topology(void) cpu = cpu_last_thread_sibling(cpu); } + /* + * 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) since 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). + * + * And for the similar reason, we will skip all the following updating. + */ + if (!cpumask_weight(&updated_cpus)) + goto out; + stop_machine(update_cpu_topology, &updates[0], &updated_cpus); /* @@ -1612,6 +1626,7 @@ int arch_update_cpu_topology(void) changed = 1; } +out: kfree(updates); return changed; } -- cgit