Re: [PATCH v2 3/3] docs: cgroup-v1: clarify that domain levels are system-specific

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 4/1/24 4:05 PM, Vitalii Bursov wrote:
> 
> 
> On 01.04.24 07:05, Shrikanth Hegde wrote:
>>
>>
>> On 3/31/24 9:31 PM, Vitalii Bursov wrote:
>>> Add a clarification that domain levels are system-specific
>>> and where to check for system details.
>>>
>>> Add CPU clusters to the scheduler domain levels table.
>>>
>>> Signed-off-by: Vitalii Bursov <vitaly@bursov.com>
>>> ---
>>>  Documentation/admin-guide/cgroup-v1/cpusets.rst | 16 +++++++++++-----
>>>  1 file changed, 11 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/Documentation/admin-guide/cgroup-v1/cpusets.rst b/Documentation/admin-guide/cgroup-v1/cpusets.rst
>>> index 7d3415eea..d16a3967d 100644
>>> --- a/Documentation/admin-guide/cgroup-v1/cpusets.rst
>>> +++ b/Documentation/admin-guide/cgroup-v1/cpusets.rst
>>> @@ -568,19 +568,25 @@ on the next tick.  For some applications in special situation, waiting
>>>  
>>>  The 'cpuset.sched_relax_domain_level' file allows you to request changing
>>>  this searching range as you like.  This file takes int value which
>>> -indicates size of searching range in levels ideally as follows,
>>> +indicates size of searching range in levels approximately as follows,
>>>  otherwise initial value -1 that indicates the cpuset has no request.
>>>  
>>>  ====== ===========================================================
>>>    -1   no request. use system default or follow request of others.
>>>     0   no search.
>>>     1   search siblings (hyperthreads in a core).
>>> -   2   search cores in a package.
>>> -   3   search cpus in a node [= system wide on non-NUMA system]
>>> -   4   search nodes in a chunk of node [on NUMA system]
>>> -   5   search system wide [on NUMA system]
>>> +   2   search cpu clusters
>>> +   3   search cores in a package.
>>> +   4   search cpus in a node [= system wide on non-NUMA system]
>>> +   5   search nodes in a chunk of node [on NUMA system]
>>> +   6   search system wide [on NUMA system]
>>
>> I think above block of documentation need not change. SD_CLUSTER is a software 
>> construct, not a sched domain per se. 
>>
> 
> I added "cpu clusters" because the original table:
> ====== ===========================================================
>   -1   no request. use system default or follow request of others.
>    0   no search.
>    1   search siblings (hyperthreads in a core).
>    2   search cores in a package.
>    3   search cpus in a node [= system wide on non-NUMA system]
>    4   search nodes in a chunk of node [on NUMA system]
>    5   search system wide [on NUMA system]
> ====== ===========================================================
> does not match to what I see on a few systems I checked.
> 
> AMD Ryzen and the same dual-CPU Intel server with NUMA disabled:
>   level:0 - SMT
>   level:2 - MC
>   level:3 - PKG
> 
> Server with NUMA enabled:
>   level:0 - SMT
>   level:2 - MC
>   level:5 - NUMA
> 

None of these are "cpu clusters". 

From what i know, the description for the above are.
SMT - multi-threads/hyperthreads
MC - Multi-Core 
PKG - Package/Socket level 
NUMA - Node level. When you enable, PKG gets degenerated since pkg mask and numa mask would 
have been same. 

 

> So, for the relax level original table:
>   1 -> enables 0 SMP -> OK
>   2 -> enables 1 unknown -> does not enable cores in a package
>   3 -> enables 2 MC -> OK for NUMA, but not system wide on non-NUMA system
>   5 -> enables 4 unknown -> does not enable system wide on NUMA
> 
> The updated table
> ====== ===========================================================
>   -1   no request. use system default or follow request of others.
>    0   no search.
>    1   search siblings (hyperthreads in a core).
>    2   search cpu clusters
>    3   search cores in a package.
>    4   search cpus in a node [= system wide on non-NUMA system]
>    5   search nodes in a chunk of node [on NUMA system]
>    6   search system wide [on NUMA system]
> ====== ===========================================================
> would work like this:
>   1 -> enables 0 SMP -> OK
>   2 -> enables 1 unknown -> does nothing new
>   3 -> enables 2 MC -> OK, cores in a package for NUMA and non-NUMA system
>   4 -> enables 3 PKG -> OK on non-NUMA system

It wont, PKG domain itself wont be there. It gets removed.

>   6 -> enables 5 NUMA -> OK
> 
> I think it would look more correct on "average" systems, but anyway,
> please confirm and I'll remove the table update in an updated patch.
> 

IMHO, the table need not get updated. Just adding a paragraph pointing 
to refer to the sysfs files is good enough. 


> Thanks
> 
>> IMO the next paragraph that is added is good enough and the above change can be removed.
> 
>>>  ====== ===========================================================
>>>  
>>> +Not all levels can be present and values can change depending on the
>>> +system architecture and kernel configuration. Check
>>> +/sys/kernel/debug/sched/domains/cpu*/domain*/ for system-specific
>>> +details.
>>> +
>>>  The system default is architecture dependent.  The system default
>>>  can be changed using the relax_domain_level= boot parameter.
>>>