Re: [PATCH 3/7] net/bonding: change mbuf pool and ring allocation

["Sanford, Robert" <rsanford@akamai.com>]

Hello Connor,

Thank you for the questions and comments. I will repeat the questions, followed by my answers.

Q: Could you be more detailed, why is mbuf pool caching not needed?

A: The short answer: under certain conditions, we can run out of
buffers from that small, LACPDU-mempool. We actually saw this occur
in production, on mostly-idle links.

For a long explanation, let's assume the following:
1. 1 tx-queue per bond and underlying ethdev ports.
2. 256 tx-descriptors (per ethdev port).
3. 257 mbufs in each port's LACPDU-pool, as computed by
bond_mode_8023ad_activate_slave(), and cache-size 32.
4. The "app" xmits zero packets to this bond for a long time.
5. In EAL intr thread context, LACP tx_machine() allocates 1 mbuf
(LACPDU) per second from the pool, and puts it into LACP tx-ring.
6. Every second, another thread, let's call it the tx-core, calls
tx-burst (with zero packets to xmit), finds 1 mbuf on LACP tx-ring,
and underlying ethdev PMD puts mbuf data into a tx-desc.
7. PMD tx-burst configured not to clean up used tx-descs until
there are almost none free, e.g., less than pool's cache-size *
CACHE_FLUSH_THRESH_MULTIPLIER (1.5).
8. When cleaning up tx-descs, we may leave up to 47 mbufs in the
tx-core's LACPDU-pool cache (not accessible from intr thread).

When the number of used tx-descs (0..255) + number of mbufs in the
cache (0..47) reaches 257, then allocation fails.

If I understand the LACP tx-burst code correctly, it would be
worse if nb_tx_queues > 1, because (assuming multiple tx-cores)
any queue/lcore could xmit an LACPDU. Thus, up to nb_tx_queues *
47 mbufs could be cached, and not accessible from tx_machine().

You would not see this problem if the app xmits other (non-LACP)
mbufs on a regular basis, to expedite the clean-up of tx-descs
including LACPDU mbufs (unless nb_tx_queues tx-core caches
could hold all LACPDU mbufs).

If we make mempool's cache size 0, then allocation will not fail.

A mempool cache for LACPDUs does not offer much additional speed:
during alloc, the intr thread does not have default mempool caches
(AFAIK); and the average time between frees is either 1 second (LACP
short timeouts) or 10 seconds (long timeouts), i.e., infrequent.

--------

Q: Why reserve one additional slot in the rx and tx rings?

A: rte_ring_create() requires the ring size N, to be a power of 2,
but it can only store N-1 items. Thus, if we want to store X items,
we need to ask for (at least) X+1. Original code fails when the real
desired size is a power of 2, because in such a case, align32pow2
does not round up.

For example, say we want a ring to hold 4:

    rte_ring_create(... rte_align32pow2(4) ...)

rte_align32pow2(4) returns 4, and we end up with a ring that only
stores 3 items.

    rte_ring_create(... rte_align32pow2(4+1) ...)

rte_align32pow2(5) returns 8, and we end up with a ring that
stores up to 7 items, more than we need, but acceptable.

--------

Q: I found the comment for BOND_MODE_8023AX_SLAVE_RX_PKTS is
wrong, could you fix it in this patch?

A: Yes, I will fix it in the next version of the patch.

--
Regards,
Robert Sanford


On 12/16/21, 4:01 AM, "Min Hu (Connor)" <humin29@huawei.com> wrote:

    Hi, Robert,

    在 2021/12/16 2:19, Robert Sanford 写道:
    > - Turn off mbuf pool caching to avoid mbufs lingering in pool caches.
    >    At most, we transmit one LACPDU per second, per port.
    Could you be more detailed, why does mbuf pool caching is not needed?

    > - Fix calculation of ring sizes, taking into account that a ring of
    >    size N holds up to N-1 items.
    Same to that, why should resvere another items ?
    > 
    By the way, I found the comment for BOND_MODE_8023AX_SLAVE_RX_PKTS is
    is wrong, could you fix it in this patch?
    > Signed-off-by: Robert Sanford <rsanford@akamai.com>
    > ---
    >   drivers/net/bonding/rte_eth_bond_8023ad.c | 14 ++++++++------
    >   1 file changed, 8 insertions(+), 6 deletions(-)
    > 
    > diff --git a/drivers/net/bonding/rte_eth_bond_8023ad.c b/drivers/net/bonding/rte_eth_bond_8023ad.c
    > index 43231bc..83d3938 100644
    > --- a/drivers/net/bonding/rte_eth_bond_8023ad.c
    > +++ b/drivers/net/bonding/rte_eth_bond_8023ad.c
    > @@ -1101,9 +1101,7 @@ bond_mode_8023ad_activate_slave(struct rte_eth_dev *bond_dev,
    >   	}
    >   
    >   	snprintf(mem_name, RTE_DIM(mem_name), "slave_port%u_pool", slave_id);
    > -	port->mbuf_pool = rte_pktmbuf_pool_create(mem_name, total_tx_desc,
    > -		RTE_MEMPOOL_CACHE_MAX_SIZE >= 32 ?
    > -			32 : RTE_MEMPOOL_CACHE_MAX_SIZE,
    > +	port->mbuf_pool = rte_pktmbuf_pool_create(mem_name, total_tx_desc, 0,
    >   		0, element_size, socket_id);
    >   
    >   	/* Any memory allocation failure in initialization is critical because
    > @@ -1113,19 +1111,23 @@ bond_mode_8023ad_activate_slave(struct rte_eth_dev *bond_dev,
    >   			slave_id, mem_name, rte_strerror(rte_errno));
    >   	}
    >   
    > +	/* Add one extra because ring reserves one. */
    >   	snprintf(mem_name, RTE_DIM(mem_name), "slave_%u_rx", slave_id);
    >   	port->rx_ring = rte_ring_create(mem_name,
    > -			rte_align32pow2(BOND_MODE_8023AX_SLAVE_RX_PKTS), socket_id, 0);
    > +			rte_align32pow2(BOND_MODE_8023AX_SLAVE_RX_PKTS + 1),
    > +			socket_id, 0);
    >   
    >   	if (port->rx_ring == NULL) {
    >   		rte_panic("Slave %u: Failed to create rx ring '%s': %s\n", slave_id,
    >   			mem_name, rte_strerror(rte_errno));
    >   	}
    >   
    > -	/* TX ring is at least one pkt longer to make room for marker packet. */
    > +	/* TX ring is at least one pkt longer to make room for marker packet.
    > +	 * Add one extra because ring reserves one. */
    >   	snprintf(mem_name, RTE_DIM(mem_name), "slave_%u_tx", slave_id);
    >   	port->tx_ring = rte_ring_create(mem_name,
    > -			rte_align32pow2(BOND_MODE_8023AX_SLAVE_TX_PKTS + 1), socket_id, 0);
    > +			rte_align32pow2(BOND_MODE_8023AX_SLAVE_TX_PKTS + 2),
    > +			socket_id, 0);
    >   
    >   	if (port->tx_ring == NULL) {
    >   		rte_panic("Slave %u: Failed to create tx ring '%s': %s\n", slave_id,
    >