> From: Feifei Wang [mailto:feifei.wa...@arm.com]
> Sent: Tuesday, 5 December 2023 04.13
>
> 在 2023/12/4 15:41, Morten Brørup 写道:
> >> From: Feifei Wang [mailto:feifei.wa...@arm.com]
> >> Sent: Monday, 4 December 2023 03.39
> >>
> >> For 'rte_pktmbuf_prefree_seg' function, 'rte_mbuf_refcnt_read(m) ==
> 1'
> >> and '__rte_mbuf_refcnt_update(m, -1) == 0' are the same cases where
> >> mbuf's refcnt value should be 1. Thus we can simplify the code and
> >> remove the redundant part.
> >>
> >> Furthermore, according to [1], when the mbuf is stored inside the
> >> mempool, the m->refcnt value should be 1. And then it is detached
> >> from its parent for an indirect mbuf. Thus change the description of
> >> 'rte_pktmbuf_prefree_seg' function.
> >>
> >> [1]
> https://patches.dpdk.org/project/dpdk/patch/20170404162807.20157-4-
> >> olivier.m...@6wind.com/
> >>
> >> Suggested-by: Ruifeng Wang <ruifeng.w...@arm.com>
> >> Signed-off-by: Feifei Wang <feifei.wa...@arm.com>
> >> ---
> >> lib/mbuf/rte_mbuf.h | 22 +++-------------------
> >> 1 file changed, 3 insertions(+), 19 deletions(-)
> >>
> >> diff --git a/lib/mbuf/rte_mbuf.h b/lib/mbuf/rte_mbuf.h
> >> index 286b32b788..42e9b50d51 100644
> >> --- a/lib/mbuf/rte_mbuf.h
> >> +++ b/lib/mbuf/rte_mbuf.h
> >> @@ -1328,7 +1328,7 @@ static inline int
> >> __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf *m)
> >> *
> >> * 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
> >> + * It decreases the reference counter, and if it reaches 1, it is
> > No, the original description is correct.
> > However, the reference counter is set to 1 when put back in the pool,
> as a shortcut so it isn't needed to be set back to 1 when allocated
> from the pool.
>
> Ok.
>
> for 'else if (__rte_mbuf_refcnt_update(m, -1) == 0)' case, it is easy
> to
> understand.
>
> but for '(likely(rte_mbuf_refcnt_read(m) == 1))' case, I think this
> will
> create misleading. dpdk users maybe difficult to understand why the
> code
> can not match the function description.
>
> Maybe we need some explanation here?
Agree. It is quite counterintuitive (but a clever optimization!) that the
reference counter is 1 instead of 0 when free.
Something like:
static __rte_always_inline struct rte_mbuf *
rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
{
__rte_mbuf_sanity_check(m, 0);
if (likely(rte_mbuf_refcnt_read(m) == 1)) {
+ /* This branch is a performance optimized variant of the branch
below.
+ * If the reference counter would reach 0 when decrementing it,
+ * do not decrement it to 0 and then initialize it to 1;
+ * just leave it at 1, thereby avoiding writing to memory.
+ */
if (!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 (m->next != NULL)
m->next = NULL;
if (m->nb_segs != 1)
m->nb_segs = 1;
+ /* No need to initialize the reference counter; it is already
1. */
return m;
} else if (__rte_mbuf_refcnt_update(m, -1) == 0) {
if (!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 (m->next != NULL)
m->next = NULL;
if (m->nb_segs != 1)
m->nb_segs = 1;
+ /* Initialize the reference counter to 1, so
+ * incrementing it is unnecessary when allocating the mbuf.
+ */
rte_mbuf_refcnt_set(m, 1);
return m;
}
return NULL;
}
Alternatively, add a function to do the initialization work:
static __rte_always_inline struct rte_mbuf *
rte_pktmbuf_prefree_seg_last_ref(struct rte_mbuf *m, const bool init_refcnt)
{
if (!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 (m->next != NULL)
m->next = NULL;
if (m->nb_segs != 1)
m->nb_segs = 1;
+ /* The reference counter must be initialized to 1 when the mbuf is free,
+ * so incrementing to 1 is unnecessary when allocating the mbuf.
+ */
if (init_refcnt)
rte_mbuf_refcnt_set(m, 1);
}
static __rte_always_inline struct rte_mbuf *
rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
{
__rte_mbuf_sanity_check(m, 0);
if (likely(rte_mbuf_refcnt_read(m) == 1)) {
+ /* This branch is a performance optimized variant of the branch
below.
+ * If the reference counter would reach 0 when decrementing it,
+ * do not decrement it to 0 and then initialize it to 1;
+ * just leave it at 1, thereby avoiding writing to memory.
+ */
return rte_pktmbuf_prefree_seg_last_ref(m, false);
} else if (__rte_mbuf_refcnt_update(m, -1) == 0) {
return rte_pktmbuf_prefree_seg_last_ref(m, true);
}
return NULL;
}
And while we're at it, we could add unlikely() to the second comparison:
if (unlikely(__rte_mbuf_refcnt_update(m, -1) == 0))
