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Mon, 22 Dec 2025 15:22:06 +0000 Received: from dubpeml500001.china.huawei.com ([7.214.147.241]) by dubpeml500001.china.huawei.com ([7.214.147.241]) with mapi id 15.02.1544.011; Mon, 22 Dec 2025 15:22:06 +0000 From: Konstantin Ananyev To: =?iso-8859-1?Q?Morten_Br=F8rup?= , "dev@dpdk.org" , "techboard@dpdk.org" , "bruce.richardson@intel.com" Subject: RE: mbuf fast-free requirements analysis Thread-Topic: mbuf fast-free requirements analysis Thread-Index: AdxtsuI54Ph/Or7ES4uJIkNuHGVH/AAHMGowAACuIkAAzd1lUAAd58awAHUVwFA= Date: Mon, 22 Dec 2025 15:22:06 +0000 Message-ID: <4ef23a6663774a119d3087fb21ede984@huawei.com> References: <98CBD80474FA8B44BF855DF32C47DC35F655E0@smartserver.smartshare.dk> <0d1645e1c83f4ec4ad676095b910845c@huawei.com> <98CBD80474FA8B44BF855DF32C47DC35F655E5@smartserver.smartshare.dk> <98CBD80474FA8B44BF855DF32C47DC35F65601@smartserver.smartshare.dk> In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35F65601@smartserver.smartshare.dk> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [10.206.138.220] Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org > > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com] > > Sent: Friday, 19 December 2025 18.08 > > > > > > > > > > Executive Summary: > > > > > > > > > > My analysis shows that the mbuf library is not a barrier for > > fast- > > > > freeing > > > > > segmented packet mbufs, and thus fast-free of jumbo frames is > > > > possible. > > > > > > > > > > > > > > > Detailed Analysis: > > > > > > > > > > The purpose of the mbuf fast-free Tx optimization is to reduce > > > > > rte_pktmbuf_free_seg() to something much simpler in the ethdev > > > > drivers, by > > > > > eliminating the code path related to indirect mbufs. > > > > > Optimally, we want to simplify the ethdev driver's function that > > > > frees the > > > > > transmitted mbufs, so it can free them directly to their mempool > > > > without > > > > > accessing the mbufs themselves. > > > > > > > > > > If the driver cannot access the mbuf itself, it cannot determine > > > > which > > > > > mempool it belongs to. > > > > > We don't want the driver to access every mbuf being freed; but if > > all > > > > > mbufs of a Tx queue belong to the same mempool, the driver can > > > > determine > > > > > which mempool by looking into just one of the mbufs. > > > > > > > > > > REQUIREMENT 1: The mbufs of a Tx queue must come from the same > > > > mempool. > > > > > > > > > > > > > > > When an mbuf is freed to its mempool, some of the fields in the > > mbuf > > > > must > > > > > be initialized. > > > > > So, for fast-free, this must be done by the driver's function > > that > > > > > prepares the Tx descriptor. > > > > > This is a requirement to the driver, not a requirement to the > > > > application. > > > > > > > > > > Now, let's dig into the code for freeing an mbuf. > > > > > Note: For readability purposes, I'll cut out some code and > > comments > > > > > unrelated to this topic. > > > > > > > > > > static __rte_always_inline void > > > > > rte_pktmbuf_free_seg(struct rte_mbuf *m) > > > > > { > > > > > m =3D rte_pktmbuf_prefree_seg(m); > > > > > if (likely(m !=3D NULL)) > > > > > rte_mbuf_raw_free(m); > > > > > } > > > > > > > > > > > > > > > rte_mbuf_raw_free(m) is simple, so nothing to gain there: > > > > > > > > > > /** > > > > > * Put mbuf back into its original mempool. > > > > > * > > > > > * The caller must ensure that the mbuf is direct and properly > > > > > * reinitialized (refcnt=3D1, next=3DNULL, nb_segs=3D1), as done = by > > > > > * rte_pktmbuf_prefree_seg(). > > > > > */ > > > > > static __rte_always_inline void > > > > > rte_mbuf_raw_free(struct rte_mbuf *m) > > > > > { > > > > > rte_mbuf_history_mark(m, RTE_MBUF_HISTORY_OP_LIB_FREE); > > > > > rte_mempool_put(m->pool, m); > > > > > } > > > > > > > > > > Note that the description says that the mbuf must be direct. > > > > > This is not entirely accurate; the mbuf is allowed to use a > > pinned > > > > > external buffer, if the mbuf holds the only reference to it. > > > > > (Most of the mbuf library functions have this documentation > > > > inaccuracy, > > > > > which should be fixed some day.) > > > > > > > > > > So, the fast-free optimization really comes down to > > > > > rte_pktmbuf_prefree_seg(m), which must not return NULL. > > > > > > > > > > Let's dig into that. > > > > > > > > > > /** > > > > > * Decrease reference counter and unlink a mbuf segment > > > > > * > > > > > * This function does the same than a free, except that it does > > not > > > > > * return the segment to its pool. > > > > > * It decreases the reference counter, and if it reaches 0, it is > > > > > * detached from its parent for an indirect mbuf. > > > > > * > > > > > * @return > > > > > * - (m) if it is the last reference. It can be recycled or > > freed. > > > > > * - (NULL) if the mbuf still has remaining references on it. > > > > > */ > > > > > static __rte_always_inline struct rte_mbuf * > > > > > rte_pktmbuf_prefree_seg(struct rte_mbuf *m) > > > > > { > > > > > bool refcnt_not_one; > > > > > > > > > > refcnt_not_one =3D unlikely(rte_mbuf_refcnt_read(m) !=3D 1); > > > > > if (refcnt_not_one && __rte_mbuf_refcnt_update(m, -1) !=3D 0) > > > > > return NULL; > > > > > > > > > > if (unlikely(!RTE_MBUF_DIRECT(m))) { > > > > > rte_pktmbuf_detach(m); > > > > > if (RTE_MBUF_HAS_EXTBUF(m) && > > > > > RTE_MBUF_HAS_PINNED_EXTBUF(m) && > > > > > __rte_pktmbuf_pinned_extbuf_decref(m)) > > > > > return NULL; > > > > > } > > > > > > > > > > if (refcnt_not_one) > > > > > rte_mbuf_refcnt_set(m, 1); > > > > > if (m->nb_segs !=3D 1) > > > > > m->nb_segs =3D 1; > > > > > if (m->next !=3D NULL) > > > > > m->next =3D NULL; > > > > > > > > > > return m; > > > > > } > > > > > > > > > > This function can only succeed (i.e. return non-NULL) when > > 'refcnt' > > > > is 1 > > > > > (or reaches 0). > > > > > > > > > > REQUIREMENT 2: The driver must hold the only reference to the > > mbuf, > > > > > i.e. 'm->refcnt' must be 1. > > > > > > > > > > > > > > > When the function succeeds, it initializes the mbuf fields as > > > > required by > > > > > rte_mbuf_raw_free() before returning. > > > > > > > > > > Now, since the driver has exclusive access to the mbuf, it is > > free to > > > > > initialize the 'm->next' and 'm->nb_segs' at any time. > > > > > It could do that when preparing the Tx descriptor. > > > > > > > > > > This is very interesting, because it means that fast-free does > > not > > > > > prohibit segmented packets! > > > > > (But the driver must have sufficient Tx descriptors for all > > segments > > > > in > > > > > the mbuf.) > > > > > > > > > > > > > > > Now, lets dig into rte_pktmbuf_prefree_seg()'s block handling > > non- > > > > direct > > > > > mbufs, i.e. cloned mbufs and mbufs with external buffer: > > > > > > > > > > if (unlikely(!RTE_MBUF_DIRECT(m))) { > > > > > rte_pktmbuf_detach(m); > > > > > if (RTE_MBUF_HAS_EXTBUF(m) && > > > > > RTE_MBUF_HAS_PINNED_EXTBUF(m) && > > > > > __rte_pktmbuf_pinned_extbuf_decref(m)) > > > > > return NULL; > > > > > } > > > > > > > > > > Starting with rte_pktmbuf_detach(): > > > > > > > > > > static inline void rte_pktmbuf_detach(struct rte_mbuf *m) > > > > > { > > > > > struct rte_mempool *mp =3D m->pool; > > > > > uint32_t mbuf_size, buf_len; > > > > > uint16_t priv_size; > > > > > > > > > > if (RTE_MBUF_HAS_EXTBUF(m)) { > > > > > /* > > > > > * The mbuf has the external attached buffer, > > > > > * we should check the type of the memory pool where > > > > > * the mbuf was allocated from to detect the pinned > > > > > * external buffer. > > > > > */ > > > > > uint32_t flags =3D rte_pktmbuf_priv_flags(mp); > > > > > > > > > > if (flags & RTE_PKTMBUF_POOL_F_PINNED_EXT_BUF) { > > > > > /* > > > > > * The pinned external buffer should not be > > > > > * detached from its backing mbuf, just exit. > > > > > */ > > > > > return; > > > > > } > > > > > __rte_pktmbuf_free_extbuf(m); > > > > > } else { > > > > > __rte_pktmbuf_free_direct(m); > > > > > } > > > > > priv_size =3D rte_pktmbuf_priv_size(mp); > > > > > mbuf_size =3D (uint32_t)(sizeof(struct rte_mbuf) + > > priv_size); > > > > > buf_len =3D rte_pktmbuf_data_room_size(mp); > > > > > > > > > > m->priv_size =3D priv_size; > > > > > m->buf_addr =3D (char *)m + mbuf_size; > > > > > rte_mbuf_iova_set(m, rte_mempool_virt2iova(m) + mbuf_size); > > > > > m->buf_len =3D (uint16_t)buf_len; > > > > > rte_pktmbuf_reset_headroom(m); > > > > > m->data_len =3D 0; > > > > > m->ol_flags =3D 0; > > > > > } > > > > > > > > > > The only quick and simple code path through this function is when > > the > > > > mbuf > > > > > uses a pinned external buffer: > > > > > if (RTE_MBUF_HAS_EXTBUF(m)) { > > > > > uint32_t flags =3D rte_pktmbuf_priv_flags(mp); > > > > > if (flags & RTE_PKTMBUF_POOL_F_PINNED_EXT_BUF) > > > > > return; > > > > > > > > > > REQUIREMENT 3: The mbuf must not be cloned or use a non-pinned > > > > external > > > > > buffer. > > > > > > > > > > > > > > > Continuing with the next part of rte_pktmbuf_prefree_seg()'s > > block: > > > > > if (RTE_MBUF_HAS_EXTBUF(m) && > > > > > RTE_MBUF_HAS_PINNED_EXTBUF(m) && > > > > > __rte_pktmbuf_pinned_extbuf_decref(m)) > > > > > return NULL; > > > > > > > > > > Continuing with the next part of the block in > > > > rte_pktmbuf_prefree_seg(): > > > > > > > > > > /** > > > > > * @internal Handle the packet mbufs with attached pinned > > external > > > > buffer > > > > > * on the mbuf freeing: > > > > > * > > > > > * - return zero if reference counter in shinfo is one. It means > > > > there is > > > > > * no more reference to this pinned buffer and mbuf can be > > returned > > > > to > > > > > * the pool > > > > > * > > > > > * - otherwise (if reference counter is not one), decrement > > > > reference > > > > > * counter and return non-zero value to prevent freeing the > > backing > > > > mbuf. > > > > > * > > > > > * Returns non zero if mbuf should not be freed. > > > > > */ > > > > > static inline int __rte_pktmbuf_pinned_extbuf_decref(struct > > rte_mbuf > > > > *m) > > > > > { > > > > > struct rte_mbuf_ext_shared_info *shinfo; > > > > > > > > > > /* Clear flags, mbuf is being freed. */ > > > > > m->ol_flags =3D RTE_MBUF_F_EXTERNAL; > > > > > shinfo =3D m->shinfo; > > > > > > > > > > /* Optimize for performance - do not dec/reinit */ > > > > > if (likely(rte_mbuf_ext_refcnt_read(shinfo) =3D=3D 1)) > > > > > return 0; > > > > > > > > > > /* > > > > > * Direct usage of add primitive to avoid > > > > > * duplication of comparing with one. > > > > > */ > > > > > if (likely(rte_atomic_fetch_add_explicit(&shinfo->refcnt, - > > 1, > > > > > rte_memory_order_acq_rel) - 1)) > > > > > return 1; > > > > > > > > > > /* Reinitialize counter before mbuf freeing. */ > > > > > rte_mbuf_ext_refcnt_set(shinfo, 1); > > > > > return 0; > > > > > } > > > > > > > > > > Essentially, if the mbuf does use a pinned external buffer, > > > > > rte_pktmbuf_prefree_seg() only succeeds if that pinned external > > > > buffer is > > > > > only referred to by the mbuf. > > > > > > > > > > REQUIREMENT 4: If the mbuf uses a pinned external buffer, the > > mbuf > > > > must > > > > > hold the only reference to that pinned external buffer, i.e. in > > that > > > > case, > > > > > 'm->shinfo->refcnt' must be 1. > > > > > > > > > > > > > > > Please review. > > > > > > > > > > If I'm not mistaken, the mbuf library is not a barrier for fast- > > > > freeing > > > > > segmented packet mbufs, and thus fast-free of jumbo frames is > > > > possible. > > > > > > > > > > We need a driver developer to confirm that my suggested approach > > - > > > > > resetting the mbuf fields, incl. 'm->nb_segs' and 'm->next', when > > > > > preparing the Tx descriptor - is viable. > > > > > > > > Great analysis, makes a lot of sense to me. > > > > Shall we add then a special API to make PMD maintainers life a bit > > > > easier: > > > > Something like rte_mbuf_fast_free_prep(mp, mb), that will > > optionally > > > > check > > > > that requirements outlined above are satisfied for given mbuf and > > > > also reset mbuf fields to expected values? > > > > > > Good idea, Konstantin. > > > > > > Detailed suggestion below. > > > Note that __rte_mbuf_raw_sanity_check_mp() is used to checks the > > requirements > > > after 'nb_segs' and 'next' have been initialized. > > > > > > /** > > > * Reinitialize an mbuf for freeing back into the mempool. > > > * > > > * The caller must ensure that the mbuf comes from the specified > > mempool, > > > * is direct and only referred to by the caller (refcnt=3D1). > > > * > > > * This function is used by drivers in their transmit function for > > mbuf fast release > > > * when the transmit descriptor is initialized, > > > * so the driver can call rte_mbuf_raw_free() > > > * when the packet segment has been transmitted. > > > * > > > * @see RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE > > > * > > > * @param mp > > > * The mempool to which the mbuf belong. > > > * @param m > > > * The mbuf being reinitialized. > > > */ > > > static __rte_always_inline void > > > rte_mbuf_raw_prefree_seg(const struct rte_mempool *mp, struct > > rte_mbuf *m) > > > { > > > if (m->nb_segs !=3D 1) > > > m->nb_segs =3D 1; > > > if (m->next !=3D NULL) > > > m->next =3D NULL; > > > > > > __rte_mbuf_raw_sanity_check_mp(m, mp); > > > rte_mbuf_history_mark(mbuf, > > > RTE_MBUF_HISTORY_OP_LIB_PREFREE_RAW); > > > } > > > > Thanks Morten, though should we really panic if condition is not met? > > Might be just do check first and return an error. >=20 > __rte_mbuf_raw_sanity_check_mp() is a no-op unless > RTE_LIBRTE_MBUF_DEBUG is enabled. Yep, I noticed. >=20 > Using it everywhere in alloc/free in the mbuf library is the convention. >=20 > And if we don't do it here, the __rte_mbuf_raw_sanity_check_mp() in > rte_mbuf_raw_free() will panic later instead. Why?=20 This new routine (rte_mbuf_raw_prefree_seg) can check that mbuf satisfies f= ast-free criteria before updating it, and if conditions are not met simply return an error. Then the driver has several options: 1) Drop the packet silently 2) Refuse to send it 3) Switch to some slower but always working code-path (without fast-free) 4) Panic > Better to fail early. > > > > > > > > /** > > > * Reinitialize a bulk of mbufs for freeing back into the mempool. > > > * > > > * The caller must ensure that the mbufs come from the specified > > mempool, > > > * are direct and only referred to by the caller (refcnt=3D1). > > > * > > > * This function is used by drivers in their transmit function for > > mbuf fast release > > > * when the transmit descriptors are initialized, > > > * so the driver can call rte_mbuf_raw_free_bulk() > > > * when the packet segments have been transmitted. > > > * > > > * @see RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE > > > * > > > * @param mp > > > * The mempool to which the mbufs belong. > > > * @param mbufs > > > * Array of pointers to mbufs being reinitialized. > > > * The array must not contain NULL pointers. > > > * @param count > > > * Array size. > > > */ > > > static __rte_always_inline void > > > rte_mbuf_raw_prefree_seg_bulk(const struct rte_mempool *mp, struct > > rte_mbuf > > > **mbufs, unsigned int count) > > > { > > > for (unsigned int idx =3D 0; idx < count; idx++) { > > > struct rte_mbuf *m =3D mbufs[idx]; > > > > > > if (m->nb_segs !=3D 1) > > > m->nb_segs =3D 1; > > > if (m->next !=3D NULL) > > > m->next =3D NULL; > > > > > > __rte_mbuf_raw_sanity_check_mp(m, mp); > > > } > > > rte_mbuf_history_mark_bulk(mbufs, count, > > > RTE_MBUF_HISTORY_OP_LIB_PREFREE_RAW); > > > } > > > > > > > Konstantin > > > > > > > > >=20