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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 1 Jun 2014 15:21:52 +0300

On Sun, Jun 01, 2014 at 02:39:39PM +0300, Igor Royzis wrote:
> 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.

Absolutely, but we need to find a way to address Eric's comments.

> > 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 <[email-masked]> 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 <[email-masked]> 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 <[email-masked]>
> >> > Signed-off-by: Anton Nayshtut <[email-masked]>
> >>
> >> 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