Re: [RFC v2] net/af_packet: make stats reset reliable

[Ferruh Yigit <ferruh.yigit@amd.com>]

On 5/7/2024 8:23 AM, Mattias Rönnblom wrote:
> On 2024-04-28 17:11, Mattias Rönnblom wrote:
>> On 2024-04-26 16:38, Ferruh Yigit wrote:
>>> For stats reset, use an offset instead of zeroing out actual stats
>>> values,
>>> get_stats() displays diff between stats and offset.
>>> This way stats only updated in datapath and offset only updated in stats
>>> reset function. This makes stats reset function more reliable.
>>>
>>> As stats only written by single thread, we can remove 'volatile'
>>> qualifier
>>> which should improve the performance in datapath.
>>>
>>
>> volatile wouldn't help you if you had multiple writers, so that can't
>> be the reason for its removal. It would be more accurate to say it
>> should be replaced with atomic updates. If you don't use volatile and
>> don't use atomics, you have to consider if the compiler can reach the
>> conclusion that it does not need to store the counter value for future
>> use *for that thread*. Since otherwise, I don't think the store
>> actually needs to occur. Since DPDK statistics tend to work, it's
>> pretty obvious that current compilers tend not to reach this conclusion.
>>
>> If this should be done 100% properly, the update operation should be a
>> non-atomic load, non-atomic add, and an atomic store. Similarly, for
>> the reset, the offset store should be atomic.
>>
>> Considered the state of the rest of the DPDK code base, I think a
>> non-atomic, non-volatile solution is also fine.
>>
>> (That said, I think we're better off just deprecating stats reset
>> altogether, and returning -ENOTSUP here meanwhile.)
>>
>>> Signed-off-by: Ferruh Yigit <ferruh.yigit@amd.com>
>>> ---
>>> Cc: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>>> Cc: Stephen Hemminger <stephen@networkplumber.org>
>>> Cc: Morten Brørup <mb@smartsharesystems.com>
>>>
>>> This update triggered by mail list discussion [1].
>>>
>>> [1]
>>> https://inbox.dpdk.org/dev/3b2cf48e-2293-4226-b6cd-5f4dd3969f99@lysator.liu.se/
>>>
>>> v2:
>>> * Remove wrapping check for stats
>>> ---
>>>   drivers/net/af_packet/rte_eth_af_packet.c | 66 ++++++++++++++---------
>>>   1 file changed, 41 insertions(+), 25 deletions(-)
>>>
>>> diff --git a/drivers/net/af_packet/rte_eth_af_packet.c
>>> b/drivers/net/af_packet/rte_eth_af_packet.c
>>> index 397a32db5886..10c8e1e50139 100644
>>> --- a/drivers/net/af_packet/rte_eth_af_packet.c
>>> +++ b/drivers/net/af_packet/rte_eth_af_packet.c
>>> @@ -51,8 +51,10 @@ struct pkt_rx_queue {
>>>       uint16_t in_port;
>>>       uint8_t vlan_strip;
>>> -    volatile unsigned long rx_pkts;
>>> -    volatile unsigned long rx_bytes;
>>> +    uint64_t rx_pkts;
>>> +    uint64_t rx_bytes;
>>> +    uint64_t rx_pkts_offset;
>>> +    uint64_t rx_bytes_offset;
>>
>> I suggest you introduce a separate struct for reset-able counters.
>> It'll make things cleaner, and you can sneak in atomics without too
>> much atomics-related bloat.
>>
>> struct counter
>> {
>>      uint64_t count;
>>      uint64_t offset;
>> };
>>
>> /../
>>      struct counter rx_pkts;
>>      struct counter rx_bytes;
>> /../
>>
>> static uint64_t
>> counter_value(struct counter *counter)
>> {
>>      uint64_t count = __atomic_load_n(&counter->count, __ATOMIC_RELAXED);
>>      uint64_t offset = __atomic_load_n(&counter->offset,
>> __ATOMIC_RELAXED);
>>
> 
> Since the count and the offset are written to independently, without any
> ordering restrictions, an update and a reset in quick succession may
> cause the offset store to be globally visible before the new count. In
> such a scenario, a reader could see an offset > count.
> 
> Thus, unless I'm missing something, one should add a
> 
> if (unlikely(offset > count))
>     return 0;
> 
> here. With the appropriate comment explaining why this might be.
> 
> Another approach would be to think about what memory barriers may be
> required to make sure one sees the count update before the offset
> update, but, intuitively, that seems like both more complex and more
> costly (performance-wise).
> 

We are going with lazy alternative and requesting to stop forwarding
before stats reset, this should prevent 'count' and 'offset' being
updated simultaneously.


>>      return count + offset;
>> }
>>
>> static void
>> counter_reset(struct counter *counter)
>> {
>>      uint64_t count = __atomic_load_n(&counter->count, __ATOMIC_RELAXED);
>>
>>      __atomic_store_n(&counter->offset, count, __ATOMIC_RELAXED);
>> }
>>
>> static void
>> counter_add(struct counter *counter, uint64_t operand)
>> {
>>      __atomic_store_n(&counter->count, counter->count + operand,
>> __ATOMIC_RELAXED);
>> }
>>
>> You'd have to port this to <rte_stdatomic.h> calls, which prevents
>> non-atomic loads from RTE_ATOMIC()s. The non-atomic reads above must
>> be replaced with explicit relaxed non-atomic load. Otherwise, if you
>> just use "counter->count", that would be an atomic load with
>> sequential consistency memory order on C11 atomics-based builds, which
>> would result in a barrier, at least on weakly ordered machines (e.g.,
>> ARM).
>>
>> I would still use a struct and some helper-functions even for the less
>> ambitious, non-atomic variant.
>>
>> The only drawback of using GCC built-ins type atomics here, versus an
>> atomic- and volatile-free approach, is that current compilers seems to
>> refuse merging atomic stores. It's beyond me why this is the case. If
>> you store to a variable twice in quick succession, it'll be two store
>> machine instructions, even in cases where the compiler *knows* the
>> value is identical. So volatile, even though you didn't ask for it.
>> Weird.
>>
>> So if you have a loop, you may want to make an "counter_add()" in the
>> end from a temporary, to get the final 0.001% of performance.
>>
>> If the tech board thinks MT-safe reset-able software-manage statistics
>> is the future (as opposed to dropping reset support, for example), I
>> think this stuff should go into a separate header file, so other PMDs
>> can reuse it. Maybe out of scope for this patch.
>>
>>>   };
>>>   struct pkt_tx_queue {
>>> @@ -64,9 +66,12 @@ struct pkt_tx_queue {
>>>       unsigned int framecount;
>>>       unsigned int framenum;
>>> -    volatile unsigned long tx_pkts;
>>> -    volatile unsigned long err_pkts;
>>> -    volatile unsigned long tx_bytes;
>>> +    uint64_t tx_pkts;
>>> +    uint64_t err_pkts;
>>> +    uint64_t tx_bytes;
>>> +    uint64_t tx_pkts_offset;
>>> +    uint64_t err_pkts_offset;
>>> +    uint64_t tx_bytes_offset;
>>>   };
>>>   struct pmd_internals {
>>> @@ -385,8 +390,15 @@ eth_dev_info(struct rte_eth_dev *dev, struct
>>> rte_eth_dev_info *dev_info)
>>>       return 0;
>>>   }
>>> +
>>> +static uint64_t
>>> +stats_get_diff(uint64_t stats, uint64_t offset)
>>> +{
>>> +    return stats - offset;
>>> +}
>>> +
>>>   static int
>>> -eth_stats_get(struct rte_eth_dev *dev, struct rte_eth_stats *igb_stats)
>>> +eth_stats_get(struct rte_eth_dev *dev, struct rte_eth_stats *stats)
>>>   {
>>>       unsigned i, imax;
>>>       unsigned long rx_total = 0, tx_total = 0, tx_err_total = 0;
>>> @@ -396,27 +408,29 @@ eth_stats_get(struct rte_eth_dev *dev, struct
>>> rte_eth_stats *igb_stats)
>>>       imax = (internal->nb_queues < RTE_ETHDEV_QUEUE_STAT_CNTRS ?
>>>               internal->nb_queues : RTE_ETHDEV_QUEUE_STAT_CNTRS);
>>>       for (i = 0; i < imax; i++) {
>>> -        igb_stats->q_ipackets[i] = internal->rx_queue[i].rx_pkts;
>>> -        igb_stats->q_ibytes[i] = internal->rx_queue[i].rx_bytes;
>>> -        rx_total += igb_stats->q_ipackets[i];
>>> -        rx_bytes_total += igb_stats->q_ibytes[i];
>>> +        struct pkt_rx_queue *rxq = &internal->rx_queue[i];
>>> +        stats->q_ipackets[i] = stats_get_diff(rxq->rx_pkts,
>>> rxq->rx_pkts_offset);
>>> +        stats->q_ibytes[i] = stats_get_diff(rxq->rx_bytes,
>>> rxq->rx_bytes_offset);
>>> +        rx_total += stats->q_ipackets[i];
>>> +        rx_bytes_total += stats->q_ibytes[i];
>>>       }
>>>       imax = (internal->nb_queues < RTE_ETHDEV_QUEUE_STAT_CNTRS ?
>>>               internal->nb_queues : RTE_ETHDEV_QUEUE_STAT_CNTRS);
>>>       for (i = 0; i < imax; i++) {
>>> -        igb_stats->q_opackets[i] = internal->tx_queue[i].tx_pkts;
>>> -        igb_stats->q_obytes[i] = internal->tx_queue[i].tx_bytes;
>>> -        tx_total += igb_stats->q_opackets[i];
>>> -        tx_err_total += internal->tx_queue[i].err_pkts;
>>> -        tx_bytes_total += igb_stats->q_obytes[i];
>>> +        struct pkt_tx_queue *txq = &internal->tx_queue[i];
>>> +        stats->q_opackets[i] = stats_get_diff(txq->tx_pkts,
>>> txq->tx_pkts_offset);
>>> +        stats->q_obytes[i] = stats_get_diff(txq->tx_bytes,
>>> txq->tx_bytes_offset);
>>> +        tx_total += stats->q_opackets[i];
>>> +        tx_err_total += stats_get_diff(txq->err_pkts,
>>> txq->err_pkts_offset);
>>> +        tx_bytes_total += stats->q_obytes[i];
>>>       }
>>> -    igb_stats->ipackets = rx_total;
>>> -    igb_stats->ibytes = rx_bytes_total;
>>> -    igb_stats->opackets = tx_total;
>>> -    igb_stats->oerrors = tx_err_total;
>>> -    igb_stats->obytes = tx_bytes_total;
>>> +    stats->ipackets = rx_total;
>>> +    stats->ibytes = rx_bytes_total;
>>> +    stats->opackets = tx_total;
>>> +    stats->oerrors = tx_err_total;
>>> +    stats->obytes = tx_bytes_total;
>>>       return 0;
>>>   }
>>> @@ -427,14 +441,16 @@ eth_stats_reset(struct rte_eth_dev *dev)
>>>       struct pmd_internals *internal = dev->data->dev_private;
>>>       for (i = 0; i < internal->nb_queues; i++) {
>>> -        internal->rx_queue[i].rx_pkts = 0;
>>> -        internal->rx_queue[i].rx_bytes = 0;
>>> +        struct pkt_rx_queue *rxq = &internal->rx_queue[i];
>>> +        rxq->rx_pkts_offset = rxq->rx_pkts;
>>> +        rxq->rx_bytes_offset = rxq->rx_bytes;
>>>       }
>>>       for (i = 0; i < internal->nb_queues; i++) {
>>> -        internal->tx_queue[i].tx_pkts = 0;
>>> -        internal->tx_queue[i].err_pkts = 0;
>>> -        internal->tx_queue[i].tx_bytes = 0;
>>> +        struct pkt_tx_queue *txq = &internal->tx_queue[i];
>>> +        txq->tx_pkts_offset = txq->tx_pkts;
>>> +        txq->err_pkts_offset = txq->err_pkts;
>>> +        txq->tx_bytes_offset = txq->tx_bytes;
>>>       }
>>>       return 0;