Re: [dpdk-dev] [PATCH v6 5/7] power: support callbacks for multiple Rx queues

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

On 07-Jul-21 11:11 AM, Ananyev, Konstantin wrote:
>>>
>>>> Currently, there is a hard limitation on the PMD power management
>>>> support that only allows it to support a single queue per lcore. This is
>>>> not ideal as most DPDK use cases will poll multiple queues per core.
>>>>
>>>> The PMD power management mechanism relies on ethdev Rx callbacks, so it
>>>> is very difficult to implement such support because callbacks are
>>>> effectively stateless and have no visibility into what the other ethdev
>>>> devices are doing. This places limitations on what we can do within the
>>>> framework of Rx callbacks, but the basics of this implementation are as
>>>> follows:
>>>>
>>>> - Replace per-queue structures with per-lcore ones, so that any device
>>>>     polled from the same lcore can share data
>>>> - Any queue that is going to be polled from a specific lcore has to be
>>>>     added to the list of queues to poll, so that the callback is aware of
>>>>     other queues being polled by the same lcore
>>>> - Both the empty poll counter and the actual power saving mechanism is
>>>>     shared between all queues polled on a particular lcore, and is only
>>>>     activated when all queues in the list were polled and were determined
>>>>     to have no traffic.
>>>> - The limitation on UMWAIT-based polling is not removed because UMWAIT
>>>>     is incapable of monitoring more than one address.
>>>>
>>>> Also, while we're at it, update and improve the docs.
>>>>
>>>> Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
>>>> ---
>>>>
>>>> Notes:
>>>>       v6:
>>>>       - Track each individual queue sleep status (Konstantin)
>>>>       - Fix segfault (Dave)
>>>>
>>>>       v5:
>>>>       - Remove the "power save queue" API and replace it with mechanism suggested by
>>>>         Konstantin
>>>>
>>>>       v3:
>>>>       - Move the list of supported NICs to NIC feature table
>>>>
>>>>       v2:
>>>>       - Use a TAILQ for queues instead of a static array
>>>>       - Address feedback from Konstantin
>>>>       - Add additional checks for stopped queues
>>>>
>>
>> <snip>
>>
>>> ....
>>>> +static inline void
>>>> +queue_reset(struct pmd_core_cfg *cfg, struct queue_list_entry *qcfg)
>>>> +{
>>>> +     const bool is_ready_to_sleep = qcfg->n_empty_polls > EMPTYPOLL_MAX;
>>>> +
>>>> +     /* reset empty poll counter for this queue */
>>>> +     qcfg->n_empty_polls = 0;
>>>> +     /* reset the queue sleep counter as well */
>>>> +     qcfg->n_sleeps = 0;
>>>> +     /* remove the queue from list of cores ready to sleep */
>>>> +     if (is_ready_to_sleep)
>>>> +             cfg->n_queues_ready_to_sleep--;
>>>> +     /*
>>>> +      * no need change the lcore sleep target counter because this lcore will
>>>> +      * reach the n_sleeps anyway, and the other cores are already counted so
>>>> +      * there's no need to do anything else.
>>>> +      */
>>>> +}
>>>> +
>>>> +static inline bool
>>>> +queue_can_sleep(struct pmd_core_cfg *cfg, struct queue_list_entry *qcfg)
>>>> +{
>>>> +     /* this function is called - that means we have an empty poll */
>>>> +     qcfg->n_empty_polls++;
>>>> +
>>>> +     /* if we haven't reached threshold for empty polls, we can't sleep */
>>>> +     if (qcfg->n_empty_polls <= EMPTYPOLL_MAX)
>>>> +             return false;
>>>> +
>>>> +     /*
>>>> +      * we've reached a point where we are able to sleep, but we still need
>>>> +      * to check if this queue has already been marked for sleeping.
>>>> +      */
>>>> +     if (qcfg->n_sleeps == cfg->sleep_target)
>>>> +             return true;
>>>> +
>>>> +     /* mark this queue as ready for sleep */
>>>> +     qcfg->n_sleeps = cfg->sleep_target;
>>>> +     cfg->n_queues_ready_to_sleep++;
>>>
>>> So, assuming there is no incoming traffic, should it be:
>>> 1) poll_all_queues(times=EMPTYPOLL_MAX); sleep; poll_all_queues(times=1); sleep; poll_all_queues(times=1); sleep; ...
>>> OR
>>> 2) poll_all_queues(times=EMPTYPOLL_MAX); sleep; poll_all_queues(times= EMPTYPOLL_MAX); sleep; poll_all_queues(times=
>> EMPTYPOLL_MAX); sleep; ...
>>> ?
>>>
>>> My initial thought was 2) but might be the intention is 1)?
>>
>>
>> The intent is 1), not 2). There's no need to wait for more empty polls
>> once we pass the threshold - we keep sleeping until there's traffic.
>>
> 
> Ok, then:
> Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
> 
> Probably worth to put extra explanation here on in the doc,
> to help people avoid wrong assumptions😉
> 

I don't see value in going into such details. What would be the point? 
Like, what difference would this information make to anyone?

-- 
Thanks,
Anatoly