From: Stephen Hemminger <stephen@networkplumber.org>
To: Cristian Dumitrescu <cristian.dumitrescu@intel.com>,
Jasvinder Singh <jasvinder.singh@intel.com>
Cc: dev@dpdk.org
Subject: DPDK PIE computation is broken
Date: Mon, 23 May 2022 15:49:46 -0700 [thread overview]
Message-ID: <20220523154946.5a4bcb33@hermes.local> (raw)
Was looking at where rte_rand() is currently used in DPDK.
Found that rte_pie uses it (that is expected.
But it also using it wrong on a couple of levels.
First. Look at:
#define RTE_RAND_MAX ~0LLU /**< Max value of the random number */
_rte_pie_drop(const struct rte_pie_config *pie_cfg,
struct rte_pie *pie)
{
uint64_t rand_value;
...
rand_value = rte_rand()/RTE_RAND_MAX;
if ((double)rand_value < pie->drop_prob) {
pie->accu_prob = 0;
return 1;
}
/* No drop */
return 0;
Since rand_value is 64 bit unsigned value the division happens as unsigned
Therefore for all values of rte_rand(). rand_value is going to be 0
Converting it to double just makes it zero in another form.
How was PIE tested? Why was this not seen.
Second, the obvious fix would be to make rand_value a double.
But doing floating point divide in the packet path would be a major
performance loss. Saw this earlier with the packet_sched code.
reply other threads:[~2022-05-23 22:49 UTC|newest]
Thread overview: [no followups] expand[flat|nested] mbox.gz Atom feed
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