linux-kernel - Re: [PATCH V1 10/13] selftests/resctrl: Change Cache Allocation Technology (CAT) test

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Date:   Wed, 11 Mar 2020 10:03:08 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        shuah@...nel.org, linux-kselftest@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        tony.luck@...el.com, babu.moger@....com, james.morse@....com,
        ravi.v.shankar@...el.com, fenghua.yu@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V1 10/13] selftests/resctrl: Change Cache Allocation
 Technology (CAT) test

Hi Sai,

On 3/10/2020 6:59 PM, Sai Praneeth Prakhya wrote:
> On Tue, 2020-03-10 at 15:14 -0700, Reinette Chatre wrote:
>> Hi Sai,
>>
>> Not just specific to this patch but I think the prevalent use of global
>> variables that are initialized/used or allocated/released from a variety
>> of places within the code is creating traps. I seemed to have stumbled
>> on a few during this review so far but it is hard to keep track of and I
>> am not confident that I caught them all. Having the code be symmetrical
>> (allocate and free from same area or initialize and use from same area)
>> does help to avoid such complexity.
> 
> Sure! makes sense. I will try to wrap them up in some meaningful structures to
> pass around functions and will see if everything still works as expected. If
> not, I will comment why a particular variable needs to be global.
> 
>> This patch and the patch that follows are both quite large and difficult
>> to keep track of all the collected changes. There seems to be
>> opportunity for separating it into logical changes. Some of my comments
>> may be just because I could not keep track of all that is changed at the
>> same time.
> 
> Ok.. makes sense. The main reason this patch and the next patch are large 
> because they do two things
> 1. Remove previous CAT/CQM test case
> 2. Add new CAT/CQM test cases
> 
> Since the new test cases are not just logical extensions or fixing some bugs
> in previous test cases, the patch might not be readable. I am thinking to
> split this at-least like this
> 1. A patch to remove CAT test case
> 2. A patch to remove CQM test case
> 3. Patches that just add CAT and CQM (without other changes)
> 
> Please let me know if you think otherwise

I think this patch can be split up into logical changes without breaking
the tests along the way. In my original review I identified two changes
that can be split out. Other things that can be split out:
- have CAT test take shareable bits into account
- enable measurement of cache references (addition of this new perf
event attribute, hooks to get measurements, etc.)
- transition CAT test to use "perf rate" measurement instead of "perf count"
- etc.


> 
>> On 3/6/2020 7:40 PM, Sai Praneeth Prakhya wrote:

[SNIP]

>>> -static struct perf_event_attr pea_llc_miss;
>>> +static struct perf_event_attr pea_llc_miss, pea_llc_access;
>>>  static struct read_format rf_cqm;
>>> -static int fd_lm;
>>> +static int fd_lm, fd_la;
>>>  char llc_occup_path[1024];
>>>  
>>>  static void initialize_perf_event_attr(void)
>>> @@ -27,15 +27,30 @@ static void initialize_perf_event_attr(void)
>>>  	pea_llc_miss.inherit = 1;
>>>  	pea_llc_miss.exclude_guest = 1;
>>>  	pea_llc_miss.disabled = 1;
>>> +
>>> +	pea_llc_access.type = PERF_TYPE_HARDWARE;
>>> +	pea_llc_access.size = sizeof(struct perf_event_attr);
>>> +	pea_llc_access.read_format = PERF_FORMAT_GROUP;
>>> +	pea_llc_access.exclude_kernel = 1;
>>> +	pea_llc_access.exclude_hv = 1;
>>> +	pea_llc_access.exclude_idle = 1;
>>> +	pea_llc_access.exclude_callchain_kernel = 1;
>>> +	pea_llc_access.inherit = 1;
>>> +	pea_llc_access.exclude_guest = 1;
>>> +	pea_llc_access.disabled = 1;
>>> +
>>
>> This initialization appears to duplicate the initialization done above.
>> Perhaps this function could be a wrapper that calls an initialization
>> function with pointer to perf_event_attr that initializes structure the
>> same?
> 
> I did think about a wrapper but since pea_llc_access and pea_llc_miss are
> global variables, I thought passing them as variables might not look good (why
> do we want to pass a global variable?). I will try and see if I can make these
> local variables.

My goal was to avoid the duplicated code to initialize them identically.
It is not clear to me why you think that would not look good. Perhaps I
have not thought it through correctly ...

Reinette