Re: [PATCH v5 0/8] Add support for Sub-NUMA cluster (SNC) systems

[Reinette Chatre]

Hi Tony,

On 9/12/2023 9:01 AM, Tony Luck wrote:
> On Mon, Sep 11, 2023 at 01:23:35PM -0700, Reinette Chatre wrote:
>> Hi Tony,
>>
>> On 8/29/2023 4:44 PM, Tony Luck wrote:
>>> The Sub-NUMA cluster feature on some Intel processors partitions
>>> the CPUs that share an L3 cache into two or more sets. This plays
>>> havoc with the Resource Director Technology (RDT) monitoring features.
>>> Prior to this patch Intel has advised that SNC and RDT are incompatible.
>>>
>>> Some of these CPU support an MSR that can partition the RMID
>>> counters in the same way. This allows for monitoring features
>>> to be used (with the caveat that memory accesses between different
>>> SNC NUMA nodes may still not be counted accuratlely.
>>
>> Same typo as in V4.
> 
> Sorry. Will fix and re-post.
> 
>>>
>>> Note that this patch series improves resctrl reporting considerably
>>> on systems with SNC enabled, but there will still be some anomalies
>>> for processes accessing memory from other sub-NUMA nodes.
>>
>> I have the same question as with V4 that was not answered in that email
>> thread nor in this new version.
>> https://lore.kernel.org/lkml/e350514e-76ed-14ea-3e74-c0852658182f@xxxxxxxxx/
> 
> Non-SNC systems already have an issue when reporting memory bandwidth
> for a task that Linux may migrate the task to a CPU on a different node
> which means that logging for that task will also move to different files
> in the mon_data/mon_L3_*/ for the new node.

It is not obvious to me that this is an issue. From what I understand
the data remains accurate.

How does this map to the earlier "may still not be counted
accurately"? 

> 
> With SNC enabled, migration between NUMA nodes on the same socket may happen
> much more frequently because:
> 1) The CPUs on the other NUMA nodes in the socket are in the same Linux
>    L3 cache domain. So Linux regard the migration as "cheap".
> 2) The ACPI SLIT table on SNC enabled systems may also report the
>    latency for remote access to another NUMA node on the same socket
>    as significantly lower than the latency for cross-socket access. On
>    my test system the SLIT distance for same socket nodes is 0xC,
>    compared to 0x15 for cross-socket distance. This will also lead
>    to Linux being more likely to migrate a task to a CPU on another
>    SNC NUMA node in the same socket.
> 
> To avoid migration issues, users may use sched_setaffinity(2) to bind
> tasks to the subset of CPUs that share an SNC NUMA node.
> 
> I can write this up in a new cover letter.
> 
>> I stop my review of this series here.
> 
> Reinette
> 
> Should I repost the whole series as v6 with the new cover letter. The
> only change to the patches so far is to the selftest reported by
> Shaopeng Tan[1].
> 

Is this an assurance that the cover letter in no way reflects how 
feedback was addressed in the rest of this series?

Reinette