Date: Wed, 26 Jun 2024 09:46:01 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
CC: <fenghua.yu@...el.com>, <shuah@...nel.org>,
	<linux-kselftest@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<ilpo.jarvinen@...ux.intel.com>, <tony.luck@...el.com>
Subject: Re: [PATCH v2 1/2] selftests/resctrl: Adjust effective L3 cache size
 with SNC enabled

Hi Maciej,

On 6/26/24 12:09 AM, Maciej Wieczor-Retman wrote:
> Hello!,
> 
> On 2024-06-25 at 09:28:55 -0700, Reinette Chatre wrote:
>> Hi Maciej,
>>
>> On 6/25/24 4:04 AM, Maciej Wieczor-Retman wrote:
>>> Hello,
>>> sorry it took me so long to get back to this. I prepared the next version with
>>> your comments applied and Tony's replies taken into account.
>>
>> Thank you very much for sticking with this.
>>
>>>
>>> I wanted to briefly discuss this before posting:
>>>
>>> On 2024-05-30 at 16:07:29 -0700, Reinette Chatre wrote:
>>>> On 5/15/24 4:18 AM, Maciej Wieczor-Retman wrote:
>>>>> +		return 1;
>>>>> +	}
>>>>> +
>>>>> +	for (i = 1; i <= MAX_SNC ; i++) {
>>>>> +		if (i * node_cpus >= cache_cpus)
>>>>> +			return i;
>>>>> +	}
>>>>
>>>> This is not obvious to me. From the function comments this seems to address the
>>>> scenarios when CPUs from other nodes are offline. It is not clear to me how
>>>> this loop addresses this. For example, let's say there are four SNC nodes
>>>> associated with a cache and only the node0 CPUs are online. The above would
>>>> detect this as "1", not "4", if I read this right?
>>>>
>>>> I wonder if it may not be easier to just follow what the kernel does
>>>> (in the new version).
>>>> User space can learn the number of online and present CPUs from
>>>> /sys/devices/system/cpu/online and /sys/devices/system/cpu/present
>>>> respectively. A simple string compare of the contents can be used to
>>>> determine if they are identical and a warning can be printed if they are not.
>>>> With a warning when accurate detection cannot be done the simple
>>>> check will do.
>>>>
>>>> Could you please add an informational message indicating how many SNC nodes
>>>> were indeed detected?
>>>
>>> Should the information "how many SNC nodes are detected?" get printed every time
>>> (by which I mean at the end of CMT and MBM tests) or only when we get the error
>>> "SNC enabled but kernel doesn't support it" happens? Of course in the first case
>>> if there is only 1 node detected nothing would be printed to avoid noise.
>>
>> I agree that it is not needed to print something about SNC if it is disabled.
>> hmmm ... so SNC impacts every test but it is only detected by default during CAT
>> and CMT test, with MBA and MBM "detection" only triggered if the test fails?
> 
> Yes, snc_ways() ran before starting CAT and CMT to adjust cache size variable.
> And then after CAT,CMT,MBM and MBA if the return value indicated failure.
> 
>>
>> What if the "SNC detection" is moved to be within run_single_test() but instead of
>> repeating the detection from scratch every time it rather works like get_vendor()
>> where the full detection is only done on first attempt? run_single_test() can detect if
>> SNC is enabled and (if number of SNC nodes > 1) print an informational message
>> that is inherited by all tests.
>> Any test that needs to know the number of SNC nodes can continue to use the
>> same function used for detection (that only does actual detection once).
>>
>> What do you think?
> 
> I think running the detection once at the start and then reusing the results is
> a good idea. You're proposing adding a value (global or passed through all the
> tests) that would get initialized on the first run_single_test()?

I was thinking about a solution similar to get_vendor() that uses a static local
variable. A global variable could work also.

> And then the SNC status (if enabled) + a warning if the detection could be wrong
> (because of the online/present cpus ratio) would happen before the test runs?

I do not think this was part of previous tests, but yes, this is a concern where
a warning would be helpful.

> On the warning placement I think it should be moved out of being printed only on
> failure. I did some experiments using "chcpu" to enable/disable cores and then
> run selftests. They didn't have any problems succeeding even though SNC
> detection detected different mode every time (I added a printf() around the line

I am not surprised here since there has not been much tuning of the CAT test.

> that cache size is modified to show what SNC mode is detected). While I
> understand these tests shouldn't fail since they just use a different portion of
> the cache I think the user should be informed it's not really NUMA aware if the
> detection was wrong:

Seems like there are two warnings to consider:
(a) SNC detection may be wrong.
(b) If SNC is enabled and kernel does not support SNC then the tests may fail.

For (a) I think that it is possible to know when SNC detection may be wrong. A test
similar to the kernel test that compares the "online" and "present" CPUs [1] can
be used. The /sys/devices/system/cpu/online and /sys/devices/system/cpu/present
files are available for this. A simpler way may be to just print a warning if
/sys/devices/system/cpu/offline is not empty and set the number of SNC nodes
to 1. Instead, a new "snc_unreliable" global can be set that can be used
to print additional information during test failure.

I do think that it is fair to print all the SNC details during detection but
I am concerned that those messages will be out of sight during test failures
and I thus do think it is useful to have extra information during test
failure.

Reinette

[1] https://lore.kernel.org/lkml/20240621223859.43471-18-tony.luck@intel.com/

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