[dpdk-dev] [PATCH] mbuf: optimize first reference increment in rte_pktmbuf_attach

[dpdk-dev] [PATCH] mbuf: optimize first reference increment in rte_pktmbuf_attach

[Olivier Matz]

As it's done in __rte_pktmbuf_prefree_seg(), we can avoid using an
atomic increment in rte_pktmbuf_attach() by checking if we are the
only owner of the mbuf first.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
---
 lib/librte_mbuf/rte_mbuf.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index ab6de67..cea35b7 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -838,7 +838,11 @@ static inline void rte_pktmbuf_attach(struct rte_mbuf *mi, struct rte_mbuf *m)
 	else
 		md = rte_mbuf_from_indirect(m);
 
-	rte_mbuf_refcnt_update(md, 1);
+	/* optimize the case where we are the only owner */
+	if (likely(rte_mbuf_refcnt_read(md) == 1))
+		rte_mbuf_refcnt_set(md, 2);
+	else
+		rte_mbuf_refcnt_update(md, 1);
 	mi->priv_size = m->priv_size;
 	mi->buf_physaddr = m->buf_physaddr;
 	mi->buf_addr = m->buf_addr;
-- 
2.1.4

Re: [dpdk-dev] [PATCH] mbuf: optimize first reference increment in rte_pktmbuf_attach

[Bruce Richardson]

On Mon, Jun 01, 2015 at 11:32:25AM +0200, Olivier Matz wrote:
> As it's done in __rte_pktmbuf_prefree_seg(), we can avoid using an
> atomic increment in rte_pktmbuf_attach() by checking if we are the
> only owner of the mbuf first.
> 
> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> ---
>  lib/librte_mbuf/rte_mbuf.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index ab6de67..cea35b7 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -838,7 +838,11 @@ static inline void rte_pktmbuf_attach(struct rte_mbuf *mi, struct rte_mbuf *m)
>  	else
>  		md = rte_mbuf_from_indirect(m);
>  
> -	rte_mbuf_refcnt_update(md, 1);
> +	/* optimize the case where we are the only owner */
> +	if (likely(rte_mbuf_refcnt_read(md) == 1))
> +		rte_mbuf_refcnt_set(md, 2);
> +	else
> +		rte_mbuf_refcnt_update(md, 1);
>  	mi->priv_size = m->priv_size;
>  	mi->buf_physaddr = m->buf_physaddr;
>  	mi->buf_addr = m->buf_addr;
> -- 
> 2.1.4
> 
Why not make the change inside rte_mbuf_refcnt_update itself? If it is ever
called with a current refcnt of 1, it should always be safe to do the update
without a cmpset.

/Bruce

Re: [dpdk-dev] [PATCH] mbuf: optimize first reference increment in rte_pktmbuf_attach

[Olivier MATZ]

Hi Bruce,

On 06/03/2015 12:59 PM, Bruce Richardson wrote:
> On Mon, Jun 01, 2015 at 11:32:25AM +0200, Olivier Matz wrote:
>> As it's done in __rte_pktmbuf_prefree_seg(), we can avoid using an
>> atomic increment in rte_pktmbuf_attach() by checking if we are the
>> only owner of the mbuf first.
>>
>> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
>> ---
>>  lib/librte_mbuf/rte_mbuf.h | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
>> index ab6de67..cea35b7 100644
>> --- a/lib/librte_mbuf/rte_mbuf.h
>> +++ b/lib/librte_mbuf/rte_mbuf.h
>> @@ -838,7 +838,11 @@ static inline void rte_pktmbuf_attach(struct rte_mbuf *mi, struct rte_mbuf *m)
>>  	else
>>  		md = rte_mbuf_from_indirect(m);
>>  
>> -	rte_mbuf_refcnt_update(md, 1);
>> +	/* optimize the case where we are the only owner */
>> +	if (likely(rte_mbuf_refcnt_read(md) == 1))
>> +		rte_mbuf_refcnt_set(md, 2);
>> +	else
>> +		rte_mbuf_refcnt_update(md, 1);
>>  	mi->priv_size = m->priv_size;
>>  	mi->buf_physaddr = m->buf_physaddr;
>>  	mi->buf_addr = m->buf_addr;
>> -- 
>> 2.1.4
>>
> Why not make the change inside rte_mbuf_refcnt_update itself? If it is ever
> called with a current refcnt of 1, it should always be safe to do the update
> without a cmpset.

Good idea, I'll see if I can do that in a new version.

Thanks,
Olivier


> 
> /Bruce
> 

[dpdk-dev] [PATCH v2] mbuf: optimize rte_mbuf_refcnt_update

[Olivier Matz]

In __rte_pktmbuf_prefree_seg(), there was an optimization to avoid using
a costly atomic operation when updating the mbuf reference counter if
its value is 1. Indeed, it means that we are the only owner of the mbuf,
and therefore nobody can change it at the same time.

We can generalize this optimization directly in rte_mbuf_refcnt_update()
so the other callers of this function, like rte_pktmbuf_attach(), can
also take advantage of this optimization.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
---
 lib/librte_mbuf/rte_mbuf.h | 57 +++++++++++++++++++++++-----------------------
 1 file changed, 28 insertions(+), 29 deletions(-)

diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index ab6de67..6c9cfd6 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -426,21 +426,6 @@ if (!(exp)) {                                                        \
 #ifdef RTE_MBUF_REFCNT_ATOMIC
 
 /**
- * Adds given value to an mbuf's refcnt and returns its new value.
- * @param m
- *   Mbuf to update
- * @param value
- *   Value to add/subtract
- * @return
- *   Updated value
- */
-static inline uint16_t
-rte_mbuf_refcnt_update(struct rte_mbuf *m, int16_t value)
-{
-	return (uint16_t)(rte_atomic16_add_return(&m->refcnt_atomic, value));
-}
-
-/**
  * Reads the value of an mbuf's refcnt.
  * @param m
  *   Mbuf to read
@@ -466,6 +451,33 @@ rte_mbuf_refcnt_set(struct rte_mbuf *m, uint16_t new_value)
 	rte_atomic16_set(&m->refcnt_atomic, new_value);
 }
 
+/**
+ * Adds given value to an mbuf's refcnt and returns its new value.
+ * @param m
+ *   Mbuf to update
+ * @param value
+ *   Value to add/subtract
+ * @return
+ *   Updated value
+ */
+static inline uint16_t
+rte_mbuf_refcnt_update(struct rte_mbuf *m, int16_t value)
+{
+	/*
+	 * The atomic_add is an expensive operation, so we don't want to
+	 * call it in the case where we know we are the uniq holder of
+	 * this mbuf (i.e. ref_cnt == 1). Otherwise, an atomic
+	 * operation has to be used because concurrent accesses on the
+	 * reference counter can occur.
+	 */
+	if (likely(rte_mbuf_refcnt_read(m) == 1)) {
+		rte_mbuf_refcnt_set(m, 1 + value);
+		return 1 + value;
+	}
+
+	return (uint16_t)(rte_atomic16_add_return(&m->refcnt_atomic, value));
+}
+
 #else /* ! RTE_MBUF_REFCNT_ATOMIC */
 
 /**
@@ -895,20 +907,7 @@ __rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
 {
 	__rte_mbuf_sanity_check(m, 0);
 
-	/*
-	 * Check to see if this is the last reference to the mbuf.
-	 * Note: the double check here is deliberate. If the ref_cnt is "atomic"
-	 * the call to "refcnt_update" is a very expensive operation, so we
-	 * don't want to call it in the case where we know we are the holder
-	 * of the last reference to this mbuf i.e. ref_cnt == 1.
-	 * If however, ref_cnt != 1, it's still possible that we may still be
-	 * the final decrementer of the count, so we need to check that
-	 * result also, to make sure the mbuf is freed properly.
-	 */
-	if (likely (rte_mbuf_refcnt_read(m) == 1) ||
-			likely (rte_mbuf_refcnt_update(m, -1) == 0)) {
-
-		rte_mbuf_refcnt_set(m, 0);
+	if (likely(rte_mbuf_refcnt_update(m, -1) == 0)) {
 
 		/* if this is an indirect mbuf, then
 		 *  - detach mbuf
-- 
2.1.4

Re: [dpdk-dev] [PATCH v2] mbuf: optimize rte_mbuf_refcnt_update

[Bruce Richardson]

On Mon, Jun 08, 2015 at 04:57:22PM +0200, Olivier Matz wrote:
> In __rte_pktmbuf_prefree_seg(), there was an optimization to avoid using
> a costly atomic operation when updating the mbuf reference counter if
> its value is 1. Indeed, it means that we are the only owner of the mbuf,
> and therefore nobody can change it at the same time.
> 
> We can generalize this optimization directly in rte_mbuf_refcnt_update()
> so the other callers of this function, like rte_pktmbuf_attach(), can
> also take advantage of this optimization.
> 
> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>

Acked-by: Bruce Richardson <bruce.richardson@intel.com>

Re: [dpdk-dev] [PATCH v2] mbuf: optimize rte_mbuf_refcnt_update

[Thomas Monjalon]

2015-06-09 13:57, Bruce Richardson:
> On Mon, Jun 08, 2015 at 04:57:22PM +0200, Olivier Matz wrote:
> > In __rte_pktmbuf_prefree_seg(), there was an optimization to avoid using
> > a costly atomic operation when updating the mbuf reference counter if
> > its value is 1. Indeed, it means that we are the only owner of the mbuf,
> > and therefore nobody can change it at the same time.
> > 
> > We can generalize this optimization directly in rte_mbuf_refcnt_update()
> > so the other callers of this function, like rte_pktmbuf_attach(), can
> > also take advantage of this optimization.
> > 
> > Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> 
> Acked-by: Bruce Richardson <bruce.richardson@intel.com>

Applied, thanks