From: Krupa Ramakrishnan krupa.ramakrishnan@amd.com
mainline inclusion from mainline-v5.16-rc1 commit 54d032ced98378bcb9d32dd5e378b7e402b36ad8 category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/I4T0ML CVE: NA
-----------------------------------------------
In build_zonelists(), when the fallback list is built for the nodes, the node load gets reinitialized during each iteration. This results in nodes with same distances occupying the same slot in different node fallback lists rather than appearing in the intended round- robin manner. This results in one node getting picked for allocation more compared to other nodes with the same distance.
As an example, consider a 4 node system with the following distance matrix.
Node 0 1 2 3 ---------------- 0 10 12 32 32 1 12 10 32 32 2 32 32 10 12 3 32 32 12 10
For this case, the node fallback list gets built like this:
Node Fallback list --------------------- 0 0 1 2 3 1 1 0 3 2 2 2 3 0 1 3 3 2 0 1 <-- Unexpected fallback order
In the fallback list for nodes 2 and 3, the nodes 0 and 1 appear in the same order which results in more allocations getting satisfied from node 0 compared to node 1.
The effect of this on remote memory bandwidth as seen by stream benchmark is shown below:
Case 1: Bandwidth from cores on nodes 2 & 3 to memory on nodes 0 & 1 (numactl -m 0,1 ./stream_lowOverhead ... --cores <from 2, 3>) Case 2: Bandwidth from cores on nodes 0 & 1 to memory on nodes 2 & 3 (numactl -m 2,3 ./stream_lowOverhead ... --cores <from 0, 1>)
---------------------------------------- BANDWIDTH (MB/s) TEST Case 1 Case 2 ---------------------------------------- COPY 57479.6 110791.8 SCALE 55372.9 105685.9 ADD 50460.6 96734.2 TRIADD 50397.6 97119.1 ----------------------------------------
The bandwidth drop in Case 1 occurs because most of the allocations get satisfied by node 0 as it appears first in the fallback order for both nodes 2 and 3.
This can be fixed by accumulating the node load in build_zonelists() rather than reinitializing it during each iteration. With this the nodes with the same distance rightly get assigned in the round robin manner.
In fact this was how it was originally until commit f0c0b2b808f2 ("change zonelist order: zonelist order selection logic") dropped the load accumulation and resorted to initializing the load during each iteration.
While zonelist ordering was removed by commit c9bff3eebc09 ("mm, page_alloc: rip out ZONELIST_ORDER_ZONE"), the change to the node load accumulation in build_zonelists() remained. So essentially this patch reverts back to the accumulated node load logic.
After this fix, the fallback order gets built like this:
Node Fallback list ------------------ 0 0 1 2 3 1 1 0 3 2 2 2 3 0 1 3 3 2 1 0 <-- Note the change here
The bandwidth in Case 1 improves and matches Case 2 as shown below.
---------------------------------------- BANDWIDTH (MB/s) TEST Case 1 Case 2 ---------------------------------------- COPY 110438.9 110107.2 SCALE 105930.5 105817.5 ADD 97005.1 96159.8 TRIADD 97441.5 96757.1 ----------------------------------------
The correctness of the fallback list generation has been verified for the above node configuration where the node 3 starts as memory-less node and comes up online only during memory hotplug.
[bharata@amd.com: Added changelog, review, test validation]
Link: https://lkml.kernel.org/r/20210830121603.1081-3-bharata@amd.com Fixes: f0c0b2b808f2 ("change zonelist order: zonelist order selection logic") Signed-off-by: Krupa Ramakrishnan krupa.ramakrishnan@amd.com Co-developed-by: Sadagopan Srinivasan Sadagopan.Srinivasan@amd.com Signed-off-by: Sadagopan Srinivasan Sadagopan.Srinivasan@amd.com Signed-off-by: Bharata B Rao bharata@amd.com Acked-by: Mel Gorman mgorman@suse.de Reviewed-by: Anshuman Khandual anshuman.khandual@arm.com Cc: KAMEZAWA Hiroyuki kamezawa.hiroyu@jp.fujitsu.com Cc: Lee Schermerhorn lee.schermerhorn@hp.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Peng Liu liupeng256@huawei.com Reviewed-by: Kefeng Wang wangkefeng.wang@huawei.com Signed-off-by: Zheng Zengkai zhengzengkai@huawei.com --- mm/page_alloc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 4e67b4506238..ef761d5c7025 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -6058,7 +6058,7 @@ static void build_zonelists(pg_data_t *pgdat) */ if (node_distance(local_node, node) != node_distance(local_node, prev_node)) - node_load[node] = load; + node_load[node] += load;
node_order[nr_nodes++] = node; prev_node = node;