linux-kernel - Re: [PATCH v4 7/7] selftests/resctrl: Adjust effective L3 cache size when SNC enabled

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Date:   Mon, 28 Aug 2023 10:06:32 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Tony Luck <tony.luck@...el.com>
CC:     Fenghua Yu <fenghua.yu@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>, <x86@...nel.org>,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Babu Moger <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <patches@...ts.linux.dev>
Subject: Re: [PATCH v4 7/7] selftests/resctrl: Adjust effective L3 cache size
 when SNC enabled

Hi Tony,

On 8/25/2023 10:56 AM, Tony Luck wrote:
> On Fri, Aug 11, 2023 at 10:33:43AM -0700, Reinette Chatre wrote:
>> Hi Tony,
>>
>> On 7/22/2023 12:07 PM, Tony Luck wrote:

...

>>> @@ -190,6 +245,8 @@ int get_cache_size(int cpu_no, char *cache_type, unsigned long *cache_size)
>>>  			break;
>>>  	}
>>>  
>>> +	if (cache_num == 3)
>>> +		*cache_size /= snc_ways();
>>>  	return 0;
>>>  }
>>>  
>>
>> I am surprised that this small change is sufficient. The resctrl
>> selftests are definitely not NUMA aware and the CAT and CMT tests
>> are not taking that into account when picking CPUs to run on. From
>> what I understand LLC occupancy counters need to be added in this
>> scenario but I do not see that done either.
> 
> This is a first step (the tests are definitely going to fail if
> they have incorrect information about the cache size).
> 
> For a fully reliable set of tests some major surgery will be required
> to bind to CPUs and memory to control allocation and access.
> 

What is the plan for making the tests more reliable? What is the
use of this patch if it is just the first step?

Reinette