Re: [dpdk-dev] [PATCH v2 2/2] mbuf: add bulk free function

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

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 <mb@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?