On 2019-09-26 10:30, Bruce Richardson wrote:
On Wed, Sep 25, 2019 at 09:02:28PM +0200, Mattias Rönnblom wrote:
On 2019-09-25 14:03, Morten Brørup wrote:
Add function for freeing a bulk of mbufs.
Signed-off-by: Morten Brørup <m...@smartsharesystems.com>
---
lib/librte_mbuf/rte_mbuf.c | 35 +++++++++++++++++++++++++++++++++++
lib/librte_mbuf/rte_mbuf.h | 16 +++++-----------
2 files changed, 40 insertions(+), 11 deletions(-)
diff --git a/lib/librte_mbuf/rte_mbuf.c b/lib/librte_mbuf/rte_mbuf.c
index 37718d49c..b63a0eced 100644
--- a/lib/librte_mbuf/rte_mbuf.c
+++ b/lib/librte_mbuf/rte_mbuf.c
@@ -245,6 +245,41 @@ int rte_mbuf_check(const struct rte_mbuf *m, int is_header,
return 0;
}
+/**
+ * Maximum bulk of mbufs rte_pktmbuf_free_bulk() returns to mempool.
+ */
+#define RTE_PKTMBUF_FREE_BULK_SZ 64
+
+/* Free a bulk of mbufs back into their original mempools. */
+void rte_pktmbuf_free_bulk(struct rte_mbuf **mbufs, unsigned int count)
+{
+ struct rte_mbuf *m, *free[RTE_PKTMBUF_FREE_BULK_SZ];
+ unsigned int idx, nb_free = 0;
+
+ for (idx = 0; idx < count; idx++) {
+ m = mbufs[idx];
+ if (unlikely(m == NULL))
+ continue;
+
+ __rte_mbuf_sanity_check(m, 1);
+ m = rte_pktmbuf_prefree_seg(m);
+ if (unlikely(m == NULL))
+ continue;
+
+ if (nb_free >= RTE_PKTMBUF_FREE_BULK_SZ ||
+ (nb_free > 0 && m->pool != free[0]->pool)) {
Maybe an unlikely() would be in order here?
I'd caution against it, since it can penalize the cold branch a lot. If a
branch really is predictable the HW branch predictors generally are good
enough to handle it at runtime. So long as a path is a valid path for a
runtime app, i.e. not something like a fatal error only ever hit once in an
entire run, I'd tend to omit likely()/unlikely() calls unless profiling
shows a real performance difference.
Let's see if I understand you: your worry is that wrapping that
expression in an unlikely() will lead to code that is slower (than w/o
the hint), if during runtime the probability turns out to be 50/50?
Wouldn't leaving out unlikely() just lead to the compiler using its
fancy heuristics in an attempt to come to a conclusion, what path is the
more likely?
About HW branch prediction - I'm sure it's good, but still the compiler
needs to decided which code code path requires a branch, and which need
not. Even if HW branch prediction successfully predicted a branch being
taken, actually branching is going to be somewhat more expensive than to
not branch?
