Re: [PATCH] Fixed zero copy GSO without orphaning the fragments

From: Igor Royzis
Date: Sun Jun 01 2014 - 07:39:45 EST

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The patch discussion seems got lost due to a delay it took us to get
the numbers. We believe that a 24% improvement in VM's network
performance (and probably the better improvement the more guests are
running on a host) is worth commenting and getting to some conclusion.

> Before your patch :
>
> sizeof(struct skb_shared_info)=0x140
> offsetof(struct skb_shared_info, frags[1])=0x40
>
> SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) -> 0x140
>
> After your patch :
>
> sizeof(struct skb_shared_info)=0x148
> offsetof(struct skb_shared_info, frags[1])=0x48
>
> SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) -> 0x180
>
> Thats a serious bump, because it increases all skb truesizes, and
> typical skb with one fragment will use 2 cache lines instead of one in
> struct skb_shared_info, so this adds memory pressure in fast path.
>
> So while this patch might increase performance for some workloads,
> it generally decreases performance on many others.

Would moving the parent fragment pointer from skb_shared_info to
skbuff solve this issue?

Regards,
-Igor

On Sun, May 25, 2014 at 1:54 PM, Igor Royzis <igorr@xxxxxxxxxxx> wrote:
>> If true, I'd like to see some performance numbers please.
>
> The numbers have been obtained by running iperf between 2 QEMU Win2012
> VMs, 4 vCPU/ 4GB RAM each.
> iperf parameters: -w 256K -l 256K -t 300
> Original kernel 3.15.0-rc5: 34.4 Gbytes transferred, 984
> Mbits/sec bandwidth.
> Kernel 3.15.0-rc5 with our patch: 42.5 Gbytes transferred, 1.22
> Gbits/sec bandwidth.
>
> Overall improvement is about 24%.
> Below are raw iperf outputs.
>
> kernel 3.15.0-rc5:
> C:\iperf>iperf -c 192.168.11.2 -w 256K -l 256K -t 300
> ------------------------------------------------------------
> Client connecting to 192.168.11.2, TCP port 5001
> TCP window size: 256 KByte
> ------------------------------------------------------------
> [ 3] local 192.168.11.1 port 49167 connected with 192.168.11.2 port 5001
> [ ID] Interval Transfer Bandwidth
> [ 3] 0.0-300.7 sec 34.4 GBytes 984 Mbits/sec
>
> kernel 3.15.0-rc5-patched:
> C:\iperf>iperf -c 192.168.11.2 -w 256K -l 256K -t 300
> ------------------------------------------------------------
> Client connecting to 192.168.11.2, TCP port 5001
> TCP window size: 256 KByte
> ------------------------------------------------------------
> [ 3] local 192.168.11.1 port 49167 connected with 192.168.11.2 port 5001
> [ ID] Interval Transfer Bandwidth
> [ 3] 0.0-300.7 sec 42.5 GBytes 1.22 Gbits/sec
>
> On Tue, May 20, 2014 at 2:50 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>>
>> On Tue, May 20, 2014 at 02:24:21PM +0300, Igor Royzis wrote:
>> > Fix accessing GSO fragments memory (and a possible corruption therefore) after
>> > reporting completion in a zero copy callback. The previous fix in the commit 1fd819ec
>> > orphaned frags which eliminates zero copy advantages. The fix makes the completion
>> > called after all the fragments were processed avoiding unnecessary orphaning/copying
>> > from userspace.
>> >
>> > The GSO fragments corruption issue was observed in a typical QEMU/KVM VM setup that
>> > hosts a Windows guest (since QEMU virtio-net Windows driver doesn't support GRO).
>> > The fix has been verified by running the HCK OffloadLSO test.
>> >
>> > Signed-off-by: Igor Royzis <igorr@xxxxxxxxxxx>
>> > Signed-off-by: Anton Nayshtut <anton@xxxxxxxxxxx>
>>
>> OK but with 1fd819ec there's no corruption, correct?
>> So this patch is in fact an optimization?
>> If true, I'd like to see some performance numbers please.
>>
>> Thanks!
>>
>> > ---
>> > include/linux/skbuff.h | 1 +
>> > net/core/skbuff.c | 18 +++++++++++++-----
>> > 2 files changed, 14 insertions(+), 5 deletions(-)
>> >
>> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>> > index 08074a8..8c49edc 100644
>> > --- a/include/linux/skbuff.h
>> > +++ b/include/linux/skbuff.h
>> > @@ -287,6 +287,7 @@ struct skb_shared_info {
>> > struct sk_buff *frag_list;
>> > struct skb_shared_hwtstamps hwtstamps;
>> > __be32 ip6_frag_id;
>> > + struct sk_buff *zcopy_src;
>> >
>> > /*
>> > * Warning : all fields before dataref are cleared in __alloc_skb()
>> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>> > index 1b62343..6fa6342 100644
>> > --- a/net/core/skbuff.c
>> > +++ b/net/core/skbuff.c
>> > @@ -610,14 +610,18 @@ EXPORT_SYMBOL(__kfree_skb);
>> > */
>> > void kfree_skb(struct sk_buff *skb)
>> > {
>> > + struct sk_buff *zcopy_src;
>> > if (unlikely(!skb))
>> > return;
>> > if (likely(atomic_read(&skb->users) == 1))
>> > smp_rmb();
>> > else if (likely(!atomic_dec_and_test(&skb->users)))
>> > return;
>> > + zcopy_src = skb_shinfo(skb)->zcopy_src;
>> > trace_kfree_skb(skb, __builtin_return_address(0));
>> > __kfree_skb(skb);
>> > + if (unlikely(zcopy_src))
>> > + kfree_skb(zcopy_src);
>> > }
>> > EXPORT_SYMBOL(kfree_skb);
>> >
>> > @@ -662,14 +666,18 @@ EXPORT_SYMBOL(skb_tx_error);
>> > */
>> > void consume_skb(struct sk_buff *skb)
>> > {
>> > + struct sk_buff *zcopy_src;
>> > if (unlikely(!skb))
>> > return;
>> > if (likely(atomic_read(&skb->users) == 1))
>> > smp_rmb();
>> > else if (likely(!atomic_dec_and_test(&skb->users)))
>> > return;
>> > + zcopy_src = skb_shinfo(skb)->zcopy_src;
>> > trace_consume_skb(skb);
>> > __kfree_skb(skb);
>> > + if (unlikely(zcopy_src))
>> > + consume_skb(zcopy_src);
>> > }
>> > EXPORT_SYMBOL(consume_skb);
>> >
>> > @@ -2867,7 +2875,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>> > skb_frag_t *frag = skb_shinfo(head_skb)->frags;
>> > unsigned int mss = skb_shinfo(head_skb)->gso_size;
>> > unsigned int doffset = head_skb->data - skb_mac_header(head_skb);
>> > - struct sk_buff *frag_skb = head_skb;
>> > unsigned int offset = doffset;
>> > unsigned int tnl_hlen = skb_tnl_header_len(head_skb);
>> > unsigned int headroom;
>> > @@ -2913,7 +2920,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>> > i = 0;
>> > nfrags = skb_shinfo(list_skb)->nr_frags;
>> > frag = skb_shinfo(list_skb)->frags;
>> > - frag_skb = list_skb;
>> > pos += skb_headlen(list_skb);
>> >
>> > while (pos < offset + len) {
>> > @@ -2975,6 +2981,11 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>> > nskb->data - tnl_hlen,
>> > doffset + tnl_hlen);
>> >
>> > + if (skb_shinfo(head_skb)->tx_flags & SKBTX_DEV_ZEROCOPY) {
>> > + skb_shinfo(nskb)->zcopy_src = head_skb;
>> > + atomic_inc(&head_skb->users);
>> > + }
>> > +
>> > if (nskb->len == len + doffset)
>> > goto perform_csum_check;
>> >
>> > @@ -3001,7 +3012,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>> > i = 0;
>> > nfrags = skb_shinfo(list_skb)->nr_frags;
>> > frag = skb_shinfo(list_skb)->frags;
>> > - frag_skb = list_skb;
>> >
>> > BUG_ON(!nfrags);
>> >
>> > @@ -3016,8 +3026,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>> > goto err;
>> > }
>> >
>> > - if (unlikely(skb_orphan_frags(frag_skb, GFP_ATOMIC)))
>> > - goto err;
>> >
>> > *nskb_frag = *frag;
>> > __skb_frag_ref(nskb_frag);
>> > --
>> > 1.7.9.5
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Next message: walter harms: "Re: [PATCH 4/11] via-ircc: Remove useless return variables"
Previous message: Mark Brown: "Re: [PATCH] regulator: core: allow non-exact matches in regulator_set_voltage_time()"
Next in thread: Michael S. Tsirkin: "Re: [PATCH] Fixed zero copy GSO without orphaning the fragments"
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