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

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

On 23-Jun-21 10:55 AM, Ananyev, Konstantin wrote:
> 
> 
>> -----Original Message-----
>> From: Burakov, Anatoly <anatoly.burakov@intel.com>
>> Sent: Wednesday, June 23, 2021 10:43 AM
>> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>; dev@dpdk.org; Richardson, Bruce <bruce.richardson@intel.com>
>> Cc: Loftus, Ciara <ciara.loftus@intel.com>; Hunt, David <david.hunt@intel.com>
>> Subject: Re: [PATCH v1 1/7] power_intrinsics: allow monitor checks inversion
>>
>> On 21-Jun-21 1:56 PM, Ananyev, Konstantin wrote:
>>>
>>> Hi Anatoly,
>>>
>>>> Previously, the semantics of power monitor were such that we were
>>>> checking current value against the expected value, and if they matched,
>>>> then the sleep was aborted. This is somewhat inflexible, because it only
>>>> allowed us to check for a specific value.
>>>>
>>>> This commit adds an option to reverse the check, so that we can have
>>>> monitor sleep aborted if the expected value *doesn't* match what's in
>>>> memory. This allows us to both implement all currently implemented
>>>> driver code, as well as support more use cases which don't easily map to
>>>> previous semantics (such as waiting on writes to AF_XDP counter value).
>>>>
>>>> Since the old behavior is the default, no need to adjust existing
>>>> implementations.
>>>>
>>>> Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
>>>> ---
>>>>    lib/eal/include/generic/rte_power_intrinsics.h | 4 ++++
>>>>    lib/eal/x86/rte_power_intrinsics.c             | 5 ++++-
>>>>    2 files changed, 8 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/lib/eal/include/generic/rte_power_intrinsics.h b/lib/eal/include/generic/rte_power_intrinsics.h
>>>> index dddca3d41c..1006c2edfc 100644
>>>> --- a/lib/eal/include/generic/rte_power_intrinsics.h
>>>> +++ b/lib/eal/include/generic/rte_power_intrinsics.h
>>>> @@ -31,6 +31,10 @@ struct rte_power_monitor_cond {
>>>>                           *   4, or 8. Supplying any other value will result in
>>>>                           *   an error.
>>>>                           */
>>>> +     uint8_t invert;  /**< Invert check for expected value (e.g. instead of
>>>> +                       *   checking if `val` matches something, check if
>>>> +                       *   `val` *doesn't* match a particular value)
>>>> +                       */
>>>>    };
>>>>
>>>>    /**
>>>> diff --git a/lib/eal/x86/rte_power_intrinsics.c b/lib/eal/x86/rte_power_intrinsics.c
>>>> index 39ea9fdecd..5d944e9aa4 100644
>>>> --- a/lib/eal/x86/rte_power_intrinsics.c
>>>> +++ b/lib/eal/x86/rte_power_intrinsics.c
>>>> @@ -117,7 +117,10 @@ rte_power_monitor(const struct rte_power_monitor_cond *pmc,
>>>>                 const uint64_t masked = cur_value & pmc->mask;
>>>>
>>>>                 /* if the masked value is already matching, abort */
>>>> -             if (masked == pmc->val)
>>>> +             if (!pmc->invert && masked == pmc->val)
>>>> +                     goto end;
>>>> +             /* same, but for inverse check */
>>>> +             if (pmc->invert && masked != pmc->val)
>>>>                         goto end;
>>>>         }
>>>>
>>>
>>> Hmm..., such approach looks too 'patchy'...
>>> Can we at least replace 'inver' with something like:
>>> enum rte_power_monitor_cond_op {
>>>           _EQ, NEQ,...
>>> };
>>> Then at least new comparions ops can be added in future.
>>> Even better I think would be to just leave to PMD to provide a comparison callback.
>>> Will make things really simple and generic:
>>> struct rte_power_monitor_cond {
>>>        volatile void *addr;
>>>        int (*cmp)(uint64_t val);
>>>        uint8_t size;
>>> };
>>> And then in rte_power_monitor(...):
>>> ....
>>> const uint64_t cur_value = __get_umwait_val(pmc->addr, pmc->size);
>>> if (pmc->cmp(cur_value) != 0)
>>>           goto end;
>>> ....
>>>
>>
>> I like the idea of a callback, but these are supposed to be
>> intrinsic-like functions, so putting too much into them is contrary to
>> their goal, and it's going to make the API hard to use in simpler cases
>> (e.g. when we're explicitly calling rte_power_monitor as opposed to
>> letting the RX callback do it for us). For example, event/dlb code calls
>> rte_power_monitor explicitly.
> 
> Good point, I didn't know that.
> Would be interesting to see how do they use it.

To be fair, it should be possible to rewrite their code using a 
callback. Perhaps adding a (void *) parameter for any custom data 
related to the callback (because C doesn't have closures...), but 
otherwise it should be doable, so the question isn't that it's 
impossible to rewrite event/dlb code to use callbacks, it's more of an 
issue with complicating usage of already-not-quite-straightforward API 
even more.

> 
>>
>> It's going to be especially "fun" to do these indirect function calls
>> from inside transactional region on call to multi-monitor.
> 
> But the callback is not supposed to do any memory reads/writes.
> Just mask/compare of the provided value with some constant.

Yeah, but with callbacks we can't really control that, can we? I mean i 
guess a *sane* implementation wouldn't do that, but still, it's 
theoretically possible to perform more complex checks and even touch 
some unrelated data in the process.

> 
>> I'm not
>> opposed to having a callback here, but maybe others have more thoughts
>> on this?
>>
>> --
>> Thanks,
>> Anatoly


-- 
Thanks,
Anatoly