Re: [dpdk-dev] [PATCH v3 6/7] power: support monitoring multiple Rx queues

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

On 29-Jun-21 1:07 AM, Ananyev, Konstantin wrote:
> 
> 
>>>> Use the new multi-monitor intrinsic to allow monitoring multiple ethdev
>>>> Rx queues while entering the energy efficient power state. The multi
>>>> version will be used unconditionally if supported, and the UMWAIT one
>>>> will only be used when multi-monitor is not supported by the hardware.
>>>>
>>>> Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
>>>> ---
>>>>    doc/guides/prog_guide/power_man.rst |  9 ++--
>>>>    lib/power/rte_power_pmd_mgmt.c      | 76 ++++++++++++++++++++++++++++-
>>>>    2 files changed, 80 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/doc/guides/prog_guide/power_man.rst b/doc/guides/prog_guide/power_man.rst
>>>> index fac2c19516..3245a5ebed 100644
>>>> --- a/doc/guides/prog_guide/power_man.rst
>>>> +++ b/doc/guides/prog_guide/power_man.rst
>>>> @@ -221,13 +221,16 @@ power saving whenever empty poll count reaches a certain number.
>>>>    The "monitor" mode is only supported in the following configurations and scenarios:
>>>>
>>>>    * If ``rte_cpu_get_intrinsics_support()`` function indicates that
>>>> +  ``rte_power_monitor_multi()`` function is supported by the platform, then
>>>> +  monitoring multiple Ethernet Rx queues for traffic will be supported.
>>>> +
>>>> +* If ``rte_cpu_get_intrinsics_support()`` function indicates that only
>>>>      ``rte_power_monitor()`` is supported by the platform, then monitoring will be
>>>>      limited to a mapping of 1 core 1 queue (thus, each Rx queue will have to be
>>>>      monitored from a different lcore).
>>>>
>>>> -* If ``rte_cpu_get_intrinsics_support()`` function indicates that the
>>>> -  ``rte_power_monitor()`` function is not supported, then monitor mode will not
>>>> -  be supported.
>>>> +* If ``rte_cpu_get_intrinsics_support()`` function indicates that neither of the
>>>> +  two monitoring functions are supported, then monitor mode will not be supported.
>>>>
>>>>    * Not all Ethernet devices support monitoring, even if the underlying
>>>>      platform may support the necessary CPU instructions. Please refer to
>>>> diff --git a/lib/power/rte_power_pmd_mgmt.c b/lib/power/rte_power_pmd_mgmt.c
>>>> index 7762cd39b8..aab2d4f1ee 100644
>>>> --- a/lib/power/rte_power_pmd_mgmt.c
>>>> +++ b/lib/power/rte_power_pmd_mgmt.c
>>>> @@ -155,6 +155,24 @@ queue_list_remove(struct pmd_core_cfg *cfg, const union queue *q)
>>>>         return 0;
>>>>    }
>>>>
>>>> +static inline int
>>>> +get_monitor_addresses(struct pmd_core_cfg *cfg,
>>>> +             struct rte_power_monitor_cond *pmc)
>>>> +{
>>>> +     const struct queue_list_entry *qle;
>>>> +     size_t i = 0;
>>>> +     int ret;
>>>> +
>>>> +     TAILQ_FOREACH(qle, &cfg->head, next) {
>>>> +             struct rte_power_monitor_cond *cur = &pmc[i];
>>>
>>> Looks like you never increment 'i' value inside that function.
>>> Also it probably will be safer to add 'num' parameter to check that
>>> we will never over-run pmc[] boundaries.
>>
>> Will fix in v4, good catch!
>>
>>>
>>>> +             const union queue *q = &qle->queue;
>>>> +             ret = rte_eth_get_monitor_addr(q->portid, q->qid, cur);
>>>> +             if (ret < 0)
>>>> +                     return ret;
>>>> +     }
>>>> +     return 0;
>>>> +}
>>>> +
>>>>    static void
>>>>    calc_tsc(void)
>>>>    {
>>>> @@ -183,6 +201,48 @@ calc_tsc(void)
>>>>         }
>>>>    }
>>>>
>>>> +static uint16_t
>>>> +clb_multiwait(uint16_t port_id, uint16_t qidx,
>>>> +             struct rte_mbuf **pkts __rte_unused, uint16_t nb_rx,
>>>> +             uint16_t max_pkts __rte_unused, void *addr __rte_unused)
>>>> +{
>>>> +     const unsigned int lcore = rte_lcore_id();
>>>> +     const union queue q = {.portid = port_id, .qid = qidx};
>>>> +     const bool empty = nb_rx == 0;
>>>> +     struct pmd_core_cfg *q_conf;
>>>> +
>>>> +     q_conf = &lcore_cfg[lcore];
>>>> +
>>>> +     /* early exit */
>>>> +     if (likely(!empty)) {
>>>> +             q_conf->empty_poll_stats = 0;
>>>> +     } else {
>>>> +             /* do we care about this particular queue? */
>>>> +             if (!queue_is_power_save(q_conf, &q))
>>>> +                     return nb_rx;
>>>
>>> I still don't understand the need of 'special' power_save queue here...
>>> Why we can't just have a function:
>>>
>>> get_number_of_queues_whose_sequential_empty_polls_less_then_threshold(struct pmd_core_cfg *lcore_cfg),
>>> and then just:
>>>
>>> /* all queues have at least EMPTYPOLL_MAX sequential empty polls */
>>> if (get_number_of_queues_whose_sequential_empty_polls_less_then_threshold(q_conf) == 0) {
>>>       /* go into power-save mode here */
>>> }
>>
>> Okay, let's go through this step by step :)
>>
>> Let's suppose we have three queues - q0, q1 and q2. We want to sleep
>> whenever there's no traffic on *all of them*, however we cannot know
>> that until we have checked all of them.
>>
>> So, let's suppose that q0, q1 and q2 were empty all this time, but now
>> some traffic arrived at q2 while we're still checking q0. We see that q0
>> is empty, and all of the queues were empty for the last N polls, so we
>> think we will be safe to sleep at q0 despite the fact that traffic has
>> just arrived at q2.
>> This is not an issue with MONITOR mode because we will be able to see if
>> current Rx ring descriptor is busy or not via the NIC callback, *but
>> this is not possible* with PAUSE and SCALE modes, because they don't
>> have the sneaky lookahead function of MONITOR! So, with PAUSE and SCALE
>> modes, it is possible to end up in a situation where you *think* you
>> don't have any traffic, but you actually do, you just haven't checked
>> the relevant queue yet.
> 
> I think such situation is unavoidable.
> Yes, traffic can arrive to *any* queue at *any* time.
> With your example above - user choose q2 as 'special' queue, but
> traffic actually arrives on q0 or q1.
> And yes, if user choose PAUSE or SCALE methods he *can* miss the traffic,
> because as you said for these methods there is no notification mechanisms.
> I think there are just unavoidable limitations with these power-save methods.
> 
>> In order to prevent this from happening, we do not sleep on every queue,
>> instead we sleep *once* per loop.
> 
> Yes, totally agree we shouldn't sleep on *every* queue.
> We need to go to sleep when there is no traffic on *any* of queues we monitor.
> 
>> That is, we check q0, check q1, check
>> q2, and only then we decide whether we want to sleep or not.
> 
>> Of course, with such scheme it is still possible to e.g. sleep in q2
>> while there's traffic waiting in q0,
> 
> Yes, exactly.
> 
>> but worst case is less bad with
>> this scheme, because we'll be doing at worst 1 extra sleep.
> 
> Hmm, I think it would be one extra sleep anyway.
> 
>> Whereas with what you're suggesting, if we had e.g. 10 queues to poll,
>> and we checked q1 but traffic has just arrived at q0, we'll be sleeping
>> at q1, then we'll be sleeping at q2, then we'll be sleeping at q3, then
>> we'll be sleeping at q4, then we'll be sleeping at q5.... and 9 sleeps
>> later we finally reach q0 and find out after all this time that we
>> shouldn't have slept in the first place.
> 
> Ah ok, I think I understand now what you are saying.
> Sure, to avoid such situation, we'll need to maintain extra counters and
> update them properly when we go to sleep.
> I should state it clearly at the beginning.
> It might be easier to explain what I meant by code snippet:
> 
> lcore_conf needs 2 counters:
> uint64_t   nb_queues_ready_to_sleep;
> uint64_t   nb_sleeps;
> 
> Plus each queue needs 2 counters:
> uint64_t nb_empty_polls;
> uint64_t nb_sleeps;
> 
> Now, at rx_callback():
> 
> /* check did sleep happen since previous call,
>       if yes, then reset queue counters */
> if (queue->nb_sleeps != lcore_conf->nb_sleeps) {
>      queue->nb_sleeps = lcore_conf->nb_sleeps;
>      queue->nb_empty_polls = 0;
> }
> 
>   /* packet arrived, reset counters */
>   if (nb_rx != 0) {
>     /* queue is not 'ready_to_sleep' any more */
>     if (queue->nb_empty_polls > EMPTYPOLL_MAX)
>         lcore_conf-> nb_queues_ready_to_sleep--;
>     queue->nb_empty_polls = 0;
> 
> /* empty poll */
> } else {
>      /* queue reaches EMPTYPOLL_MAX threshold, mark it as 'ready_to_sleep' */
>      if (queue->nb_empty_polls == EMPTYPOLL_MAX)
>         lcore_conf-> nb_queues_ready_to_sleep++;
>      queue->nb_empty_polls++;
> }
> 
>     /* no traffic on any queue for at least EMPTYPOLL_MAX iterations */
>     if (lcore_conf-> nb_queues_ready_to_sleep == lcore_conf->n_queues) {
>        /* update counters and sleep */
>        lcore_conf->nb_sleeps++;
>        lcore_conf-> nb_queues_ready_to_sleep = 0;
>        goto_sleep();
>     }
> }
> 
Actually, i don't think this is going to work, because i can see no 
(easy) way to get from lcore to specific queue. I mean, you could have 
an O(N) for loop that will loop over the list of queues every time we 
enter the callback, but i don't think that's such a good idea.

-- 
Thanks,
Anatoly