Interestingly, John, our doc maintainer is not Cc'ed. I add him. Please use --cc-cmd devtools/get-maintainer.sh I am expecting a review from an x86 maintainer as well. If no maintainer replies, ping them.
07/07/2020 11:50, Phil Yang: > Add deprecating the generic rte_atomic_xx APIs to c11 atomic built-ins > guide and examples. [...] > +Atomic Operations: Use C11 Atomic Built-ins > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +DPDK `generic rte_atomic > <https://github.com/DPDK/dpdk/blob/v20.02/lib/librte_eal/common/include/generic/rte_atomic.h>`_ > operations are Why this github link on 20.02? Please try to keep lines small. > +implemented by `__sync built-ins > <https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html>`_. Long links should be on their own line to avoid long lines. > +These __sync built-ins result in full barriers on aarch64, which are > unnecessary > +in many use cases. They can be replaced by `__atomic built-ins > <https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html>`_ that > +conform to the C11 memory model and provide finer memory order control. > + > +So replacing the rte_atomic operations with __atomic built-ins might improve > +performance for aarch64 machines. `More details > <https://www.dpdk.org/wp-content/uploads/sites/35/2019/10/StateofC11Code.pdf>`_. "More details." Please make a sentence. > + > +Some typical optimization cases are listed below: > + > +Atomicity > +^^^^^^^^^ > + > +Some use cases require atomicity alone, the ordering of the memory operations > +does not matter. For example the packets statistics in the `vhost > <https://github.com/DPDK/dpdk/blob/v20.02/examples/vhost/main.c#L796>`_ > example application. Again github. If you really want a web link, use code.dpdk.org or doc.dpdk.org/api But why giving code example at all? > + > +It just updates the number of transmitted packets, no subsequent logic > depends > +on these counters. So the RELAXED memory ordering is sufficient: > + > +.. code-block:: c > + > + static __rte_always_inline void > + virtio_xmit(struct vhost_dev *dst_vdev, struct vhost_dev *src_vdev, > + struct rte_mbuf *m) > + { > + ... > + ... > + if (enable_stats) { > + __atomic_add_fetch(&dst_vdev->stats.rx_total_atomic, 1, > __ATOMIC_RELAXED); > + __atomic_add_fetch(&dst_vdev->stats.rx_atomic, ret, > __ATOMIC_RELAXED); > + ... > + } > + } I don't see how adding real code helps here. Why not just mentioning __atomic_add_fetch and __ATOMIC_RELAXED? > + > +One-way Barrier > +^^^^^^^^^^^^^^^ > + > +Some use cases allow for memory reordering in one way while requiring memory > +ordering in the other direction. > + > +For example, the memory operations before the `lock > <https://github.com/DPDK/dpdk/blob/v20.02/lib/librte_eal/common/include/generic/rte_spinlock.h#L66>`_ > can move to the > +critical section, but the memory operations in the critical section cannot > move > +above the lock. In this case, the full memory barrier in the CAS operation > can > +be replaced to ACQUIRE. On the other hand, the memory operations after the > +`unlock > <https://github.com/DPDK/dpdk/blob/v20.02/lib/librte_eal/common/include/generic/rte_spinlock.h#L88>`_ > can move to the critical section, but the memory operations in the > +critical section cannot move below the unlock. So the full barrier in the > STORE > +operation can be replaced with RELEASE. Again github links instead of our doxygen. > + > +Reader-Writer Concurrency > +^^^^^^^^^^^^^^^^^^^^^^^^^ No blank line here? > +Lock-free reader-writer concurrency is one of the common use cases in DPDK. > + > +The payload or the data that the writer wants to communicate to the reader, > +can be written with RELAXED memory order. However, the guard variable should > +be written with RELEASE memory order. This ensures that the store to guard > +variable is observable only after the store to payload is observable. > +Refer to `rte_hash insert > <https://github.com/DPDK/dpdk/blob/v20.02/lib/librte_hash/rte_cuckoo_hash.c#L737>`_ > for an example. Hum... > + > +.. code-block:: c > + > + static inline int32_t > + rte_hash_cuckoo_insert_mw(const struct rte_hash *h, > + ... > + int32_t *ret_val) > + { > + ... > + ... > + > + /* Insert new entry if there is room in the primary > + * bucket. > + */ > + for (i = 0; i < RTE_HASH_BUCKET_ENTRIES; i++) { > + /* Check if slot is available */ > + if (likely(prim_bkt->key_idx[i] == EMPTY_SLOT)) { > + prim_bkt->sig_current[i] = sig; > + /* Store to signature and key should not > + * leak after the store to key_idx. i.e. > + * key_idx is the guard variable for signature > + * and key. > + */ > + __atomic_store_n(&prim_bkt->key_idx[i], > + new_idx, > + __ATOMIC_RELEASE); > + break; > + } > + } > + > + ... > + } > + > +Correspondingly, on the reader side, the guard variable should be read > +with ACQUIRE memory order. The payload or the data the writer communicated, > +can be read with RELAXED memory order. This ensures that, if the store to > +guard variable is observable, the store to payload is also observable. Refer > to `rte_hash lookup > <https://github.com/DPDK/dpdk/blob/v20.02/lib/librte_hash/rte_cuckoo_hash.c#L1215>`_ > for an example. > + > +.. code-block:: c > + > + static inline int32_t > + search_one_bucket_lf(const struct rte_hash *h, const void *key, uint16_t > sig, > + void **data, const struct rte_hash_bucket *bkt) > + { > + ... > + > + for (i = 0; i < RTE_HASH_BUCKET_ENTRIES; i++) { > + .... > + if (bkt->sig_current[i] == sig) { > + key_idx = __atomic_load_n(&bkt->key_idx[i], > + __ATOMIC_ACQUIRE); > + if (key_idx != EMPTY_SLOT) { > + k = (struct rte_hash_key *) ((char *)keys + > + key_idx * h->key_entry_size); > + > + if (rte_hash_cmp_eq(key, k->key, h) == 0) { > + if (data != NULL) { > + *data = __atomic_load_n(&k->pdata, > + __ATOMIC_ACQUIRE); > + } > + > + /* > + * Return index where key is stored, > + * subtracting the first dummy index > + */ > + return key_idx - 1; > + } > + ... > + } > + NACK for the big chunks of real code. Please use words and avoid code. If you insist on keeping code in doc, I will make you responsible of updating all the code we have already in the doc :)
