Re: [dpdk-dev] [EXT] Re: [PATCH 3/3] l3fwd-power: add interrupt-only mode

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 15-Jun-20 12:43 PM, Jerin Jacob wrote:
> On Mon, Jun 15, 2020 at 5:02 PM Burakov, Anatoly
> <anatoly.burakov@intel.com> wrote:
>>
>> On 02-Jun-20 1:16 PM, Harman Kalra wrote:
>>> On Tue, Jun 02, 2020 at 03:53:07PM +0530, Harman Kalra wrote:
>>>> On Mon, Jun 01, 2020 at 01:50:26PM +0100, Burakov, Anatoly wrote:
>>>>> On 30-May-20 11:02 AM, Harman Kalra wrote:
>>>>>> On Fri, May 29, 2020 at 03:19:45PM +0100, Burakov, Anatoly wrote:
>>>>>>> External Email
>>>>>>>
>>>>>>> ----------------------------------------------------------------------
>>>>>>> On 29-May-20 2:19 PM, Harman Kalra wrote:
>>>>>>>
>>>>>>>>>          if (ret < 0)
>>>>>>>>>                  rte_exit(EXIT_FAILURE, "Invalid L3FWD parameters\n");
>>>>>>>>> -       if (app_mode != APP_MODE_TELEMETRY && init_power_library())
>>>>>>>>> +       if (app_mode == APP_MODE_DEFAULT)
>>>>>>>>> +               app_mode = APP_MODE_LEGACY;
>>>>>>>>> +
>>>>>>>>> +       /* only legacy and empty poll mode rely on power library */
>>>>>>>>> +       if ((app_mode == APP_MODE_LEGACY || app_mode == APP_MODE_EMPTY_POLL) &&
>>>>>>>>> +                       init_power_library())
>>>>>>>>>                  rte_exit(EXIT_FAILURE, "init_power_library failed\n");
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> Rather than just exiting from here can we have a else condition to
>>>>>>>> automatically enter into the "interrupt only" mode.
>>>>>>>> Please correct me if I am missing something.
>>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> Thanks for your review. I don't think silently proceeding is a good idea. If
>>>>>>> the user wants interrupt-only mode, they should request it. Silently falling
>>>>>>> back to interrupt-only mode will create an illusion of successful
>>>>>>> initialization and set the wrong expectation for how the application will
>>>>>>> behave.
>>>>>>>
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> Thanks for the explanation which even I also believe is logically perfect.
>>>>>>
>>>>>> But since l3fwd-power is an old application and has many users around
>>>>>> which are currently using this app in interrupt only mode without giving
>>>>>> an extra argument. But suddenly they will start getting failure messages with
>>>>>> the new patchset.
>>>>>>
>>>>>> My only intent with else condition was backward compatibility.
>>>>>> Or may be we can have more descriptive failure message, something like
>>>>>> "init_power_library failed, check manual for other modes".
>>>>>>
>>>>>> Thanks
>>>>>> Harman
>>>>>>
>>>>>>
>>>>>
>>>>> I think we can compormise on an informative log message suggesting to use
>>>>> interrupt mode. I'm not keen on reverting to previous buggy behavior :)
>>>>>
>>>> Hi
>>>>
>>>> I am not insisting to revert to previous behavior, I am just trying to
>>>> highlight some probable issues that many users might face as its an old
>>>> application.
>>>> Since many arm based soc might not be supporting frequency scaling, can
>>>> we add the following check as soon as the application starts, probe the
>>>> platform if it supports frequency scaling, if not automatically set the
>>>> mode to interrupt mode, something like:
>>>> if (access("/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor",
>>>>                       F_OK))
>>>>       app_mode = APP_MODE_INTERRUPT;
>>>
>>> Sorry, no direct check in application but we can introduce a new API in
>>> power library:
>>>      bool rte_is_freq_scaling() {
>>>           return  access("/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor",
>>>                           F_OK);
>>>      }
>>>
>>> and in the application we can use "rte_is_freq_scaling()" at the start.
>>>
>>
>> What you're suggesting here is effectively what you have already
>> suggested: silently fall back to interrupt-only mode if power lib init
>> failed. I already outlined why i don't think it's a good approach.
> 
> Is probing "/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor"
> file presence,
> detects the power lib availability . Right?  Not the failure. Right?
> IMO, it make sense to have following case:
> # first check, Is the system is capable of power lib if so use power lib
> # if the system is not capable of using power lib use interrupt mode.
> 
> I think, there is difference between system capable of using power lib
> vs power lib not available or power lib failure.

I am of the opinion that if a test sets up an unrealistic expectation of 
how an application should behave, it's a problem with the test, not with 
the application.

If the system is not capable of running with power lib - the application 
shouldn't be requested to run in such mode.

"The application behaved that way before" - yes, it did. It was a bug in 
the application, that it allowed users to effectively misuse the 
application and use it despite the fact that it was in a half-working 
state. This problem has been addressed by 1) not allowing the 
application to run in half-working state, and 2) adding a new mode where 
the old "expected" behavior is *actually* expected and is "full working 
state" now.

Therefore, all users who were previously misusing the application to do 
something it was not designed to do because of a bug in the 
implementation, should now fix their usage and use the correct mode - 
and such breakage is IMO necessary to call attention to earlier misuse 
in the tools, and to correct this usage.

What bothers me about your suggestion is that it is impossible to fail 
the test if the wrong mode was requested (as in, if we request the 
power-lib mode on a system that doesn't have freq scaling) - it instead 
silently falls back to a mode that is almost guaranteed to work.

-- 
Thanks,
Anatoly