linux-kernel - Re: [PATCH v3 1/4] sched/fair: add SD

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Date: Tue, 6 Feb 2024 16:56:02 +0800
From: kuiliang Shi <seakeel@...il.com>
To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
 Valentin Schneider <vschneid@...hat.com>
Cc: alexs@...nel.org, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Daniel Bristot de Oliveira <bristot@...hat.com>,
 linux-kernel@...r.kernel.org, sshegde@...ux.ibm.com
Subject: Re: [PATCH v3 1/4] sched/fair: add SD_CLUSTER in comments



On 2/6/24 10:46 AM, Ricardo Neri wrote:
> On Fri, Feb 02, 2024 at 03:27:32PM +0100, Valentin Schneider wrote:
>>
>> Subject nit: the prefix should be sched/topology
>>
>> On 01/02/24 19:54, alexs@...nel.org wrote:
>>> From: Alex Shi <alexs@...nel.org>
>>>
>>> The description of SD_CLUSTER is missing. Add it.
>>>
>>> Signed-off-by: Alex Shi <alexs@...nel.org>
>>> To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
>>> To: Valentin Schneider <vschneid@...hat.com>
>>> To: Vincent Guittot <vincent.guittot@...aro.org>
>>> To: Juri Lelli <juri.lelli@...hat.com>
>>> To: Peter Zijlstra <peterz@...radead.org>
>>> To: Ingo Molnar <mingo@...hat.com>
>>> ---
>>>  kernel/sched/topology.c | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>>> index 10d1391e7416..8b45f16a1890 100644
>>> --- a/kernel/sched/topology.c
>>> +++ b/kernel/sched/topology.c
>>> @@ -1554,6 +1554,7 @@ static struct cpumask		***sched_domains_numa_masks;
>>>   * function:
>>>   *
>>>   *   SD_SHARE_CPUCAPACITY   - describes SMT topologies
>>> + *   SD_CLUSTER             - describes CPU Cluster topologies
>>
>> So I know this is the naming we've gone for the "Cluster" naming, but this
>> comment isn't really explaining anything.
>>
>> include/linux/sched/sd_flags.h has a bit more info already:
>>  * Domain members share CPU cluster (LLC tags or L2 cache)
> 
> I also thought of this, but I didn't want to suggest to repeat in topolog.c
> what is described in sd_flags.h.
> 
> Maybe it is better to remove the descriptions of all flags here and instead
> direct the reader to sd_flags.h?

yes, good idea for getting their meaning directly from the header file. 
So is it fine for next sending?

    sched/fair: remove duplicate comments for SD_ flags
    
    Originally, it missed SD_CLUSTER flag description, but comparing to copy
    the flags meaning from sd_flags.h, reader is better to get info from
    there.
    
    Keep SD_ASYM_PACKING unchange for a point from another way.
    
    Signed-off-by: Alex Shi <alexs@...nel.org>
    To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
    To: Valentin Schneider <vschneid@...hat.com>
    To: Vincent Guittot <vincent.guittot@...aro.org>
    To: Juri Lelli <juri.lelli@...hat.com>
    To: Peter Zijlstra <peterz@...radead.org>
    To: Ingo Molnar <mingo@...hat.com>

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 10d1391e7416..96a24bf954ad 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1551,11 +1551,7 @@ static struct cpumask            ***sched_domains_numa_masks;
  *
  * These flags are purely descriptive of the topology and do not prescribe
  * behaviour. Behaviour is artificial and mapped in the below sd_init()
- * function:
- *
- *   SD_SHARE_CPUCAPACITY   - describes SMT topologies
- *   SD_SHARE_PKG_RESOURCES - describes shared caches
- *   SD_NUMA                - describes NUMA topologies
+ * function, for details in include/linux/sched/sd_flags.h:
  *
  * Odd one out, which beside describing the topology has a quirk also
  * prescribes the desired behaviour that goes along with it:
> 
>>
>> I had to go through a bit of git history to remember what the CLUSTER thing
>> was about, how about this:
>>
>> * SD_CLUSTER             - describes shared shared caches, cache tags or busses
> 
> AFAIK, this describes a subset of CPUs in the package that share a
> resource, likely L2 cache.
> 
>> * SD_SHARE_PKG_RESOURCES - describes shared LLC cache
>>
>> And looking at this it would make sense to:
>>   rename SD_CLUSTER into SD_SHARE_PKG_RESOURCES
> 
> but not all CPUs in the package share the resource
> 
>>   rename SD_SHARE_PKG_RESOURCES into SD_SHARE_LLC
>> but that's another topic...
>>