Date: Mon, 22 Jan 2024 08:56:12 +0100
From: Maciej Wieczór-Retman <maciej.wieczor-retman@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>
CC: Fenghua Yu <fenghua.yu@...el.com>, Shuah Khan <shuah@...nel.org>,
	<ilpo.jarvinen@...ux.intel.com>, <linux-kernel@...r.kernel.org>,
	<linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] selftests/resctrl: Add non-contiguous CBMs CAT
 test

Hi!

On 2024-01-19 at 08:39:31 -0800, Reinette Chatre wrote:
>Hi Maciej,
>
>On 1/18/2024 11:37 PM, Maciej Wieczór-Retman wrote:
>> On 2024-01-18 at 09:15:46 -0800, Reinette Chatre wrote:
>>> On 1/18/2024 4:02 AM, Maciej Wieczór-Retman wrote:
>>>> On 2024-01-17 at 10:49:06 -0800, Reinette Chatre wrote:
>>>>> On 1/17/2024 12:26 AM, Maciej Wieczór-Retman wrote:
>>>>>> On 2024-01-08 at 14:42:11 -0800, Reinette Chatre wrote:
>>>>>>> On 12/12/2023 6:52 AM, Maciej Wieczor-Retman wrote:
>>>
>>>>>>>> +	bit_center = count_bits(full_cache_mask) / 2;
>>>>>>>> +	cont_mask = full_cache_mask >> bit_center;
>>>>>>>> +
>>>>>>>> +	/* Contiguous mask write check. */
>>>>>>>> +	snprintf(schemata, sizeof(schemata), "%lx", cont_mask);
>>>>>>>> +	ret = write_schemata("", schemata, uparams->cpu, test->resource);
>>>>>>>> +	if (ret)
>>>>>>>> +		return ret;
>>>>>>>
>>>>>>> How will user know what failed? I am seeing this single test exercise a few scenarios
>>>>>>> and it is not obvious to me if the issue will be clear if this test,
>>>>>>> noncont_cat_run_test(), fails.
>>>>>>
>>>>>> write_schemata() either succeeds with '0' or errors out with a negative value. If
>>>>>> the contiguous mask write fails, write_schemata should print out what was wrong
>>>>>> and I believe that the test will report an error rather than failure.
>>>>>
>>>>> Right. I am trying to understand whether the user will be able to decipher what failed
>>>>> in case there is an error. Seems like in this case the user is expected to look at the
>>>>> source code of the test to understand what the test was trying to do at the time it
>>>>> encountered the failure. In this case user may be "lucky" that this test only has
>>>>> one write_schemata() call _not_ followed by a ksft_print_msg() so user can use that
>>>>> reasoning to figure out which write_schemata() failed to further dig what test was
>>>>> trying to do. 
>>>>
>>>> When a write_schemata() is executed the string that is being written gets
>>>> printed. If there are multiple calls in a single tests and one fails I'd imagine
>>>> it would be easy for the user to figure out which one failed.
>>>
>>> It would be easy for the user the figure out if (a) it is obvious to the user
>>> what schema a particular write_schema() call attempted to write and (b) all the
>>> write_schema() calls attempt to write different schema.
>> 
>> Okay, your comment made me wonder if on error the schemata still is printed. I
>> double checked in the code and whether write_schemata() fails or not it has a
>> goto path where before returning it will print out the schema. So I believe that
>> satisfies your (a) condition.
>
>Let me try with an example.
>Scenario 1:
>The test has the following code:
>	...
>	write_schemata(..., "0xfff", ...);
>	...
>	write_schemata(..., "0xf0f", ...);
>	...
>
>Scenario 2:
>The test has the following code:
>	...
>	write_schemata(..., schemata, ...);
>	...
>	write_schemata(..., schemata, ...);
>	...
>
>Any failure of write_schemata() in scenario 1 will be easy to trace. As you
>state, write_schemata() prints the schemata attempted and it will thus be
>easy to look at the code to see which write_schemata() call failed since it
>is obvious from the code which schemata was attempted.
>A failure of one of the write_schemata() in scenario 2 will not be as easy
>to trace since the user first needs to determine what the value of "schemata"
>is at each call and that may depend on the platform, bit shifting done in test,
>and state of system state at time of test.

Doing things similar to scenario 1 would be great from a debugging perspective
but since the masks can have different sizes putting literals there seems
impossible.

Maybe the code could be improved by putting an example CBM in the comment above
a write_schemata() call? "For a 12 bit maximum CBM value, the contiguous
schemata will look like '0x3f'" and "For a 12 bit maximum CBM value, the
non-contiguous schemata will look like '0xf0f'"

This seems like the closest I could get to what you're
showing in scenario 1 (which I assume would be the best).

>> As for (b) depends on what you meant. Other tests that run more than one
>> write_schemata() use different ones every time (CAT, MBM, MBA). Do you suggest
>> that the non-contiguous test should attempt more schematas? For example shift
>> the bit hole from one side to the other? I assumed one CBM with a centered bit
>> hole would be enough to check if non-contiguous CBM feature works properly and
>> more CBMs would be redundant.
>
>Let me try with an example.
>Scenario 1:
>The test has the following code:
>	...
>	write_schemata(..., "0xfff", ...);
>	...
>	write_schemata(..., "0xf0f", ...);
>	...
>
>Scenario 2:
>The test has the following code:
>	...
>	write_schemata(..., "0xfff", ...);
>	...
>	write_schemata(..., "0xfff", ...);
>	...
>
>A failure of either write_schemata() in scenario 1 will be easy to trace since 
>the schemata attempted is different in each case. The schemata printed by the
>write_schemata() error message can thus easily be connected to the specific
>write_schemata() call.
>A failure of either write_schemata() in scenario 2 is not so obvious since they
>both attempted the same schemata so the error message printed by write_schemata()
>could belong to either. 

I believe my code follows the first scenario example (since one schemata is half
the full CBM, and the other one is the full CBM with a hole in the middle).

I'm sorry to drag this thread out but I want to be sure if I'm right or are you
suggesting something and I missed it?

>
>Reinette

-- 
Kind regards
Maciej Wieczór-Retman

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