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

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

On 07-Jul-21 11:04 AM, David Hunt wrote:
> 
> On 5/7/2021 4:22 PM, Anatoly Burakov 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
>>
>>   doc/guides/nics/features.rst           |  10 +
>>   doc/guides/prog_guide/power_man.rst    |  65 ++--
>>   doc/guides/rel_notes/release_21_08.rst |   3 +
>>   lib/power/rte_power_pmd_mgmt.c         | 452 +++++++++++++++++++------
>>   4 files changed, 394 insertions(+), 136 deletions(-)
>>
> 
> --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--;
> 
> 
> Hi Anatoly,
> 
>     I don't think the logic around this is bulletproof yet, in my 
> testing I'm seeing n_queues_ready_to_sleep wrap around (i.e. decremented 
> while already zero).
> 
> Rgds,
> Dave.
> 
> 
> --snip--

Thanks for your testing!

It seems that number of empty polls is not a reliable indicator of 
whether the queue is ready to sleep, because if we get a non-empty poll 
right after sleep, we'll have empty poll counter still at high value, 
which will cause the n_queues_ready_to_sleep to decrement, even though 
it's at zero because we just had a sleep.

Using n_sleeps and sleep_target is better in this case. I'll submit a v7 
with this fix. Thanks!

-- 
Thanks,
Anatoly