On Fri, Oct 9, 2015 at 8:54 AM, Jarno Rajahalme <jrajahalme@xxxxxxxxxx> wrote: > > On Oct 8, 2015, at 4:03 PM, Jesse Gross <jesse@xxxxxxxxxx> wrote: > > On Wed, Oct 7, 2015 at 10:47 AM, Jarno Rajahalme <jrajahalme@xxxxxxxxxx> > wrote: > > > On Oct 6, 2015, at 6:01 PM, Jesse Gross <jesse@xxxxxxxxxx> wrote: > > On Mon, Oct 5, 2015 at 1:25 PM, Alexander Duyck > <alexander.duyck@xxxxxxxxx> wrote: > > On 10/05/2015 06:59 AM, Vlastimil Babka wrote: > > > On 10/02/2015 12:18 PM, Konstantin Khlebnikov wrote: > > > When openvswitch tries allocate memory from offline numa node 0: > stats = kmem_cache_alloc_node(flow_stats_cache, GFP_KERNEL | __GFP_ZERO, > 0) > It catches VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES || !node_online(nid)) > [ replaced with VM_WARN_ON(!node_online(nid)) recently ] in linux/gfp.h > This patch disables numa affinity in this case. > > Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> > > > > ... > > diff --git a/net/openvswitch/flow_table.c b/net/openvswitch/flow_table.c > index f2ea83ba4763..c7f74aab34b9 100644 > --- a/net/openvswitch/flow_table.c > +++ b/net/openvswitch/flow_table.c > @@ -93,7 +93,8 @@ struct sw_flow *ovs_flow_alloc(void) > > /* Initialize the default stat node. */ > stats = kmem_cache_alloc_node(flow_stats_cache, > - GFP_KERNEL | __GFP_ZERO, 0); > + GFP_KERNEL | __GFP_ZERO, > + node_online(0) ? 0 : NUMA_NO_NODE); > > > > Stupid question: can node 0 become offline between this check, and the > VM_WARN_ON? :) BTW what kind of system has node 0 offline? > > > > Another question to ask would be is it possible for node 0 to be online, but > be a memoryless node? > > I would say you are better off just making this call kmem_cache_alloc. I > don't see anything that indicates the memory has to come from node 0, so > adding the extra overhead doesn't provide any value. > > > I agree that this at least makes me wonder, though I actually have > concerns in the opposite direction - I see assumptions about this > being on node 0 in net/openvswitch/flow.c. > > Jarno, since you original wrote this code, can you take a look to see > if everything still makes sense? > > > We keep the pre-allocated stats node at array index 0, which is initially > used by all CPUs, but if CPUs from multiple numa nodes start updating the > stats, we allocate additional stats nodes (up to one per numa node), and the > CPUs on node 0 keep using the preallocated entry. If stats cannot be > allocated from CPUs local node, then those CPUs keep using the entry at > index 0. Currently the code in net/openvswitch/flow.c will try to allocate > the local memory repeatedly, which may not be optimal when there is no > memory at the local node. > > Allocating the memory for the index 0 from other than node 0, as discussed > here, just means that the CPUs on node 0 will keep on using non-local memory > for stats. In a scenario where there are CPUs on two nodes (0, 1), but only > the node 1 has memory, a shared flow entry will still end up having separate > memory allocated for both nodes, but both of the nodes would be at node 1. > However, there is still a high likelihood that the memory allocations would > not share a cache line, which should prevent the nodes from invalidating > each other’s caches. Based on this I do not see a problem relaxing the > memory allocation for the default stats node. If node 0 has memory, however, > it would be better to allocate the memory from node 0. > > > Thanks for going through all of that. > > It seems like the question that is being raised is whether it actually > makes sense to try to get the initial memory on node 0, especially > since it seems to introduce some corner cases? Is there any reason why > the flow is more likely to hit node 0 than a randomly chosen one? > (Assuming that this is a multinode system, otherwise it's kind of a > moot point.) We could have a separate pointer to the default allocated > memory, so it wouldn't conflict with memory that was intentionally > allocated for node 0. > > > It would still be preferable to know from which node the default stats node > was allocated, and store it in the appropriate pointer in the array. We > could then add a new “default stats node index” that would be used to locate > the node in the array of pointers we already have. That way we would avoid > extra allocation and processing of the default stats node. I agree, that sounds reasonable to me. Will you make that change? Besides eliminating corner cases, it might help performance in some cases too by avoiding stressing memory bandwidth on node 0. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href