[PATCH v2] net: use bulk free in kfree_skb_list

[PATCH v2] net: use bulk free in kfree_skb_list

[Felix Fietkau]

Since we're freeing multiple skbs, we might as well use bulk free to save a
few cycles. Use the same conditions for bulk free as in napi_consume_skb.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
---
v2: call kmem_cache_free_bulk once the skb array is full instead of
    falling back to kfree_skb
 net/core/skbuff.c | 40 ++++++++++++++++++++++++++++++++++++----
 1 file changed, 36 insertions(+), 4 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 2415d9cb9b89..1eeaa264d2a4 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -666,12 +666,44 @@ EXPORT_SYMBOL(kfree_skb);
 
 void kfree_skb_list(struct sk_buff *segs)
 {
-	while (segs) {
-		struct sk_buff *next = segs->next;
+	struct sk_buff *next = segs;
+	void *skbs[16];
+	int n_skbs = 0;
 
-		kfree_skb(segs);
-		segs = next;
+	while ((segs = next) != NULL) {
+		next = segs->next;
+
+		if (!skb_unref(segs))
+			continue;
+
+		if (segs->fclone != SKB_FCLONE_UNAVAILABLE) {
+			kfree_skb(segs);
+			continue;
+		}
+
+		trace_kfree_skb(segs, __builtin_return_address(0));
+
+		/* drop skb->head and call any destructors for packet */
+		skb_release_all(segs);
+
+#ifdef CONFIG_SLUB
+		/* SLUB writes into objects when freeing */
+		prefetchw(segs);
+#endif
+
+		skbs[n_skbs++] = segs;
+
+		if (n_skbs < ARRAY_SIZE(skbs))
+			continue;
+
+		kmem_cache_free_bulk(skbuff_head_cache, n_skbs, skbs);
+		n_skbs = 0;
 	}
+
+	if (!n_skbs)
+		return;
+
+	kmem_cache_free_bulk(skbuff_head_cache, n_skbs, skbs);
 }
 EXPORT_SYMBOL(kfree_skb_list);
 
-- 
2.17.0

Re: [PATCH v2] net: use bulk free in kfree_skb_list

[Jesper Dangaard Brouer]

On Sun, 24 Mar 2019 17:56:44 +0100
Felix Fietkau <nbd@nbd.name> wrote:

> Since we're freeing multiple skbs, we might as well use bulk free to save a
> few cycles. Use the same conditions for bulk free as in napi_consume_skb.
> 
> Signed-off-by: Felix Fietkau <nbd@nbd.name>
> ---
> v2: call kmem_cache_free_bulk once the skb array is full instead of
>     falling back to kfree_skb

Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

Re: [PATCH v2] net: use bulk free in kfree_skb_list

[Eric Dumazet]

On 03/24/2019 09:56 AM, Felix Fietkau wrote:
> Since we're freeing multiple skbs, we might as well use bulk free to save a
> few cycles. Use the same conditions for bulk free as in napi_consume_skb.
> 

I do not believe kfree_skb_list() is used in the fast path, so do we really
need to make it so complex ?

Re: [PATCH v2] net: use bulk free in kfree_skb_list

[Felix Fietkau]

On 2019-03-25 09:51, Eric Dumazet wrote:
> 
> 
> On 03/24/2019 09:56 AM, Felix Fietkau wrote:
>> Since we're freeing multiple skbs, we might as well use bulk free to save a
>> few cycles. Use the same conditions for bulk free as in napi_consume_skb.
>> 
> 
> I do not believe kfree_skb_list() is used in the fast path, so do we really
> need to make it so complex ?
mac80211 uses it to free the fraglist from A-MSDU aggregated packets in
the tx status path. That's one fast path where it gets used right now
and the reason it was showing up in my perf traces.

- Felix

Re: [PATCH v2] net: use bulk free in kfree_skb_list

[Eric Dumazet]

On 03/25/2019 02:09 AM, Felix Fietkau wrote:
> On 2019-03-25 09:51, Eric Dumazet wrote:
>>
>>
>> On 03/24/2019 09:56 AM, Felix Fietkau wrote:
>>> Since we're freeing multiple skbs, we might as well use bulk free to save a
>>> few cycles. Use the same conditions for bulk free as in napi_consume_skb.
>>>
>>
>> I do not believe kfree_skb_list() is used in the fast path, so do we really
>> need to make it so complex ?
> mac80211 uses it to free the fraglist from A-MSDU aggregated packets in
> the tx status path. That's one fast path where it gets used right now
> and the reason it was showing up in my perf traces.

This is not drop monitor friendly then....

TX completion should use consume_skb() or dev_kfree_skb()

BTW, I wonder what drop-monitor signal bulk free is sending ?

Re: [PATCH v2] net: use bulk free in kfree_skb_list

[Felix Fietkau]

On 2019-03-25 10:31, Eric Dumazet wrote:
> 
> 
> On 03/25/2019 02:09 AM, Felix Fietkau wrote:
>> On 2019-03-25 09:51, Eric Dumazet wrote:
>>>
>>>
>>> On 03/24/2019 09:56 AM, Felix Fietkau wrote:
>>>> Since we're freeing multiple skbs, we might as well use bulk free to save a
>>>> few cycles. Use the same conditions for bulk free as in napi_consume_skb.
>>>>
>>>
>>> I do not believe kfree_skb_list() is used in the fast path, so do we really
>>> need to make it so complex ?
>> mac80211 uses it to free the fraglist from A-MSDU aggregated packets in
>> the tx status path. That's one fast path where it gets used right now
>> and the reason it was showing up in my perf traces.
> 
> This is not drop monitor friendly then....
> 
> TX completion should use consume_skb() or dev_kfree_skb()
> 
> BTW, I wonder what drop-monitor signal bulk free is sending ?
Good point about the drop monitor. Would you prefer that I replace this
patch with one that adds a consume_skb_list function and another one
that makes mac80211 use it?

- Felix

Re: [PATCH v2] net: use bulk free in kfree_skb_list

[Jesper Dangaard Brouer]

On Mon, 25 Mar 2019 10:35:35 +0100 Felix Fietkau <nbd@nbd.name> wrote:
> On 2019-03-25 10:31, Eric Dumazet wrote:
> > On 03/25/2019 02:09 AM, Felix Fietkau wrote:  
> >> On 2019-03-25 09:51, Eric Dumazet wrote:  
> >>> On 03/24/2019 09:56 AM, Felix Fietkau wrote:  
> >>>>
> >>>> Since we're freeing multiple skbs, we might as well use bulk
> >>>> free to save a few cycles. Use the same conditions for bulk free
> >>>> as in napi_consume_skb. 
> >>>
> >>> I do not believe kfree_skb_list() is used in the fast path, so do
> >>> we really need to make it so complex ?  
> >>
> >> mac80211 uses it to free the fraglist from A-MSDU aggregated
> >> packets in the tx status path. That's one fast path where it gets
> >> used right now and the reason it was showing up in my perf
> >> traces.  

Notice that kfree_skb_list(to_free) is used in (what I consider)
"fast-path" when the qdisc system drops packets, happens via to_free in
bottom of __dev_xmit_skb().  And fq_codel (which is default in most
distro's today) have an optimization (in fq_codel_drop()) for bulk
freeing SKBs, which will also benefit from this.

Thus, I still think it will be valuable to optimize kfree_skb_list().


> > This is not drop monitor friendly then....
> > 
> > TX completion should use consume_skb() or dev_kfree_skb()
> > 
> > BTW, I wonder what drop-monitor signal bulk free is sending ?  
>
> Good point about the drop monitor. Would you prefer that I replace
> this patch with one that adds a consume_skb_list function and another
> one that makes mac80211 use it?

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

Re: [PATCH v2] net: use bulk free in kfree_skb_list

[Paolo Abeni]

Hi,

On Sun, 2019-03-24 at 17:56 +0100, Felix Fietkau wrote:
> Since we're freeing multiple skbs, we might as well use bulk free to save a
> few cycles. Use the same conditions for bulk free as in napi_consume_skb.
> 
> Signed-off-by: Felix Fietkau <nbd@nbd.name>
> ---
> v2: call kmem_cache_free_bulk once the skb array is full instead of
>     falling back to kfree_skb
>  net/core/skbuff.c | 40 ++++++++++++++++++++++++++++++++++++----
>  1 file changed, 36 insertions(+), 4 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 2415d9cb9b89..1eeaa264d2a4 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -666,12 +666,44 @@ EXPORT_SYMBOL(kfree_skb);
>  
>  void kfree_skb_list(struct sk_buff *segs)
>  {
> -	while (segs) {
> -		struct sk_buff *next = segs->next;
> +	struct sk_buff *next = segs;
> +	void *skbs[16];
> +	int n_skbs = 0;
>  
> -		kfree_skb(segs);
> -		segs = next;
> +	while ((segs = next) != NULL) {
> +		next = segs->next;
> +
> +		if (!skb_unref(segs))
> +			continue;
> +
> +		if (fclone != SKB_FCLONE_UNAVAILABLE) {
> +			kfree_skb(segs);
> +			continue;
> +		}

I think you should swap the order of skb_unref() and the above check,
or skbs with 'fclone != SKB_FCLONE_UNAVAILABLE' will go twice in
skb_unref() (kfree_skb() calls skb_unref(), too).

Other than that LGTM,

Thanks,

Paolo