Re: [dpdk-dev] [PATCH v1 1/7] power_intrinsics: allow monitor checks inversion

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

On 24-Jun-21 10:47 AM, Ananyev, Konstantin wrote:
> 

<snip>

>>>> I did a quick prototype for this, and i don't think it is going to work.
>>>>
>>>> Callbacks with just "current value" as argument will be pretty limited
>>>> and will only really work for cases where we know what we are expecting.
>>>> However, for cases like event/dlb or net/mlx5, the expected value is (or
>>>> appears to be) dependent upon some internal device data, and is not
>>>> constant like in case of net/ixgbe for example.
>>>>
>>>> This can be fixed by passing an opaque pointer, either by storing it in
>>>> the monitor condition, or by passing it directly to rte_power_monitor at
>>>> invocation time.
>>>>
>>>> The latter doesn't work well because when we call rte_power_monitor from
>>>> inside the rte_power library, we lack the context necessary to get said
>>>> opaque pointer.
>>>>
>>>> The former doesn't work either, because the only place where we can get
>>>> this argument is inside get_monitor_addr, but the opaque pointer must
>>>> persist after we exit that function in order to avoid use-after-free -
>>>> which means that it either has to be statically allocated (which means
>>>> it's not thread-safe for a non-trivial case), or dynamically allocated
>>>> (which a big no-no on a hotpath).
>>>
>>> If I get you right, expected_value (and probably mask) can be variable ones.
>>> So for callback approach to work we need to pass all this as parameters
>>> to PMD comparison callback:
>>> int pmc_callback(uint64_t real_val, uint64_t expected_val, uint64_t mask)
>>> Correct?
>>
>> If we have both expected value, mask, and current value, then what's the
>> point of the callback? The point of the callback would be to pass just
>> the current value, and let the callback decide what's the expected value
>> and how to compare it.
> 
> For me the main point of callback is to hide PMD specific comparison semantics.
> Basically they provide us with some values in struct rte_power_monitor_cond,
> and then it is up to them how to interpret them in their comparison function.
> All we'll do for them: will read the value at address provided.
> I understand that it looks like an overkill, as majority of these comparison functions
> will be like:
> int cmp_callback(uint64_t real_val, uint64_t expected_val, uint64_t mask)
> {
>          return ((real_val & mask) == expected_val);
> }
> Though qsort() and bsearch() work in a similar manner, and everyone seems ok with it.
> 
>>
>> So, we can either let callback handle expected values itself by having
>> an opaque callback-specific argument (which means it has to persist
>> between .get_monitor_addr() and rte_power_monitor() calls),
> 
> But that's what we doing already - PMD fills rte_power_monitor_cond values
> for us, we store them somewhere and then use them to decide should we go to sleep or not.
> All callback does - moves actual values interpretation back to PMD:
> Right now:
> PMD:      provide PMC values
> POWER: store PMC values somewhere
>                  read the value at address provided in PMC
>                  interpret PMC values and newly read value and make the decision
> 
> With callback:
> PMD:      provide PMC values
> POWER: store PMC values somewhere
>                  read the value at address provided in PMC
> PMD:      interpret PMC values and newly read value and make the decision
> 
> Or did you mean something different here?
> 
>> or we do the
>> comparisons inside rte_power_monitor(), and store the expected/mask
>> values in the monitor condition, and *don't* have any callbacks at all.
>> Are you suggesting an alternative to the above two options?
> 
> As I said in my first mail - we can just replace 'inverse' with 'op'.
> That at least will make this API extendable, if someone will need
> something different in future.
> 
> Another option is

Right, so the idea is store the PMD-specific data in the monitor 
condition, and leave it to the callback to interpret it.

The obvious question then is, how many values is enough? Two? Three? 
Four? This option doesn't really solve the basic issue, it just kicks 
the can down the road. I guess three values should be enough for 
everyone (tm) ? :D

I don't like the 'op' thing because if the goal is to be flexible, it's 
unnecessarily limiting *and* makes the API even more complex to use. I 
would rather have a number of PMD-specific values and leave it up to the 
callback to interpret them, because at least that way we're not limited 
to predefined operations on the monitor condition data.

-- 
Thanks,
Anatoly