Re: [PATCH] mm: vmstat: Use zeroed stats for unpopulated zones

[Sandipan Das <sandipan@linux.ibm.com>]

On 07/05/20 12:39 pm, Michal Hocko wrote:
> On Wed 06-05-20 17:50:28, Vlastimil Babka wrote:
>> [...]
>>
>> How about something like this:
>>
>> diff --git a/Documentation/admin-guide/numastat.rst b/Documentation/admin-guide/numastat.rst
>> index aaf1667489f8..08ec2c2bdce3 100644
>> --- a/Documentation/admin-guide/numastat.rst
>> +++ b/Documentation/admin-guide/numastat.rst
>> @@ -6,6 +6,21 @@ Numa policy hit/miss statistics
>>  
>>  All units are pages. Hugepages have separate counters.
>>  
>> +The numa_hit, numa_miss and numa_foreign counters reflect how well processes
>> +are able to allocate memory from nodes they prefer. If they succeed, numa_hit
>> +is incremented on the preferred node, otherwise numa_foreign is incremented on
>> +the preferred node and numa_miss on the node where allocation succeeded.
>> +
>> +Usually preferred node is the one local to the CPU where the process executes,
>> +but restrictions such as mempolicies can change that, so there are also two
>> +counters based on CPU local node. local_node is similar to numa_hit and is
>> +incremented on allocation from a node by CPU on the same node. other_node is
>> +similar to numa_miss and is incremented on the node where allocation succeeds
>> +from a CPU from a different node. Note there is no counter analogical to
>> +numa_foreign.
>> +
>> +In more detail:
>> +
>>  =============== ============================================================
>>  numa_hit	A process wanted to allocate memory from this node,
>>  		and succeeded.
>> @@ -14,11 +29,13 @@ numa_miss	A process wanted to allocate memory from another node,
>>  		but ended up with memory from this node.
>>  
>>  numa_foreign	A process wanted to allocate on this node,
>> -		but ended up with memory from another one.
>> +		but ended up with memory from another node.
>>  
>> -local_node	A process ran on this node and got memory from it.
>> +local_node	A process ran on this node's CPU,
>> +		and got memory from this node.
>>  
>> -other_node	A process ran on this node and got memory from another node.
>> +other_node	A process ran on a different node's CPU
>> +		and got memory from this node.
>>  
>>  interleave_hit 	Interleaving wanted to allocate from this node
>>  		and succeeded.
>> @@ -28,3 +45,11 @@ For easier reading you can use the numastat utility from the numactl package
>>  (http://oss.sgi.com/projects/libnuma/). Note that it only works
>>  well right now on machines with a small number of CPUs.
>>  
>> +Note that on systems with memoryless nodes (where a node has CPUs but no
>> +memory) the numa_hit, numa_miss and numa_foreign statistics can be skewed
>> +heavily. In the current kernel implementation, if a process prefers a
>> +memoryless node (i.e.  because it is running on one of its local CPU), the
>> +implementation actually treats one of the nearest nodes with memory as the
>> +preferred node. As a result, such allocation will not increase the numa_foreign
>> +counter on the memoryless node, and will skew the numa_hit, numa_miss and
>> +numa_foreign statistics of the nearest node.
> 
> This is certainly an improvement. Thanks! The question whether we can
> identify where bogus numbers came from would be interesting as well.
> Maybe those are not worth fixing but it would be great to understand
> them at least. I have to say that the explanation via boot_pageset is
> not really clear to me.
> 

The documentation update will definitely help. Thanks for that.
I did collect some stack traces on a ppc64 guest for calls to zone_statistics()
in case of zones that are using the boot_pageset and most of them originate
from kmem_cache_init() with eventual calls to allocate_slab().

[    0.000000] [c00000000282b690] [c000000000402d98] zone_statistics+0x138/0x1d0                                                                                                
[    0.000000] [c00000000282b740] [c000000000401190] rmqueue_pcplist+0xf0/0x120                                                                                                 
[    0.000000] [c00000000282b7d0] [c00000000040b178] get_page_from_freelist+0x2f8/0x2100                                                                                        
[    0.000000] [c00000000282bb30] [c000000000401ae0] __alloc_pages_nodemask+0x1a0/0x2d0                                                                                         
[    0.000000] [c00000000282bbc0] [c00000000044b040] alloc_slab_page+0x70/0x580                                                                                                 
[    0.000000] [c00000000282bc20] [c00000000044b5f8] allocate_slab+0xa8/0x610                                                                                                   
...

In the remaining cases, the sources are ftrace_init() and early_trace_init().

Unless they are useful, can we avoid incrementing stats for zones using boot_pageset
inside zone_statistics()?


- Sandipan