From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id D579647048; Mon, 15 Dec 2025 17:14:37 +0100 (CET) Received: from mails.dpdk.org (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id 84E64402D9; Mon, 15 Dec 2025 17:14:37 +0100 (CET) Received: from dkmailrelay1.smartsharesystems.com (smartserver.smartsharesystems.com [77.243.40.215]) by mails.dpdk.org (Postfix) with ESMTP id CD2AB40151; Mon, 15 Dec 2025 17:14:36 +0100 (CET) Received: from smartserver.smartsharesystems.com (smartserver.smartsharesys.local [192.168.4.10]) by dkmailrelay1.smartsharesystems.com (Postfix) with ESMTP id BD8FD202E1; Mon, 15 Dec 2025 17:14:36 +0100 (CET) Content-class: urn:content-classes:message MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Subject: RE: mbuf fast-free requirements analysis Date: Mon, 15 Dec 2025 17:14:33 +0100 Message-ID: <98CBD80474FA8B44BF855DF32C47DC35F655E5@smartserver.smartshare.dk> X-MimeOLE: Produced By Microsoft Exchange V6.5 In-Reply-To: <0d1645e1c83f4ec4ad676095b910845c@huawei.com> X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: mbuf fast-free requirements analysis Thread-Index: AdxtsuI54Ph/Or7ES4uJIkNuHGVH/AAHMGowAACuIkA= References: <98CBD80474FA8B44BF855DF32C47DC35F655E0@smartserver.smartshare.dk> <0d1645e1c83f4ec4ad676095b910845c@huawei.com> From: =?iso-8859-1?Q?Morten_Br=F8rup?= To: "Konstantin Ananyev" , , , 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: Monday, 15 December 2025 15.41 >=20 > > > > 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. >=20 > 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); } /** * 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 >=20