[PATCH bpf-next V2] bpf/xdp: optimize bpf_xdp_pointer to avoid reading sinfo

[PATCH bpf-next V2] bpf/xdp: optimize bpf_xdp_pointer to avoid reading sinfo

[Jesper Dangaard Brouer]

Currently we observed a significant performance degradation in
samples/bpf xdp1 and xdp2, due XDP multibuffer "xdp.frags" handling,
added in commit 772251742262 ("samples/bpf: fixup some tools to be able
to support xdp multibuffer").

This patch reduce the overhead by avoiding to read/load shared_info
(sinfo) memory area, when XDP packet don't have any frags. This improves
performance because sinfo is located in another cacheline.

Function bpf_xdp_pointer() is used by BPF helpers bpf_xdp_load_bytes()
and bpf_xdp_store_bytes(). As a help to reviewers, xdp_get_buff_len() can
potentially access sinfo, but it uses xdp_buff_has_frags() flags bit check
to avoid accessing sinfo in no-frags case.

The likely/unlikely instrumentation lays out asm code such that sinfo
access isn't interleaved with no-frags case (checked on GCC 12.2.1-4).
The generated asm code is more compact towards the no-frags case.

The BPF kfunc bpf_dynptr_slice() also use bpf_xdp_pointer(). Thus, it
should also take effect for that.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 net/core/filter.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 968139f4a1ac..961db5bd2f94 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3948,20 +3948,21 @@ void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off,
 
 void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset, u32 len)
 {
-	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
 	u32 size = xdp->data_end - xdp->data;
+	struct skb_shared_info *sinfo;
 	void *addr = xdp->data;
 	int i;
 
 	if (unlikely(offset > 0xffff || len > 0xffff))
 		return ERR_PTR(-EFAULT);
 
-	if (offset + len > xdp_get_buff_len(xdp))
+	if (unlikely(offset + len > xdp_get_buff_len(xdp)))
 		return ERR_PTR(-EINVAL);
 
-	if (offset < size) /* linear area */
+	if (likely((offset < size))) /* linear area */
 		goto out;
 
+	sinfo = xdp_get_shared_info_from_buff(xdp);
 	offset -= size;
 	for (i = 0; i < sinfo->nr_frags; i++) { /* paged area */
 		u32 frag_size = skb_frag_size(&sinfo->frags[i]);

Re: [PATCH bpf-next V2] bpf/xdp: optimize bpf_xdp_pointer to avoid reading sinfo

[Lorenzo Bianconi]

[-- Attachment #1: Type: text/plain, Size: 2407 bytes --]

> Currently we observed a significant performance degradation in
> samples/bpf xdp1 and xdp2, due XDP multibuffer "xdp.frags" handling,
> added in commit 772251742262 ("samples/bpf: fixup some tools to be able
> to support xdp multibuffer").
> 
> This patch reduce the overhead by avoiding to read/load shared_info
> (sinfo) memory area, when XDP packet don't have any frags. This improves
> performance because sinfo is located in another cacheline.
> 
> Function bpf_xdp_pointer() is used by BPF helpers bpf_xdp_load_bytes()
> and bpf_xdp_store_bytes(). As a help to reviewers, xdp_get_buff_len() can
> potentially access sinfo, but it uses xdp_buff_has_frags() flags bit check
> to avoid accessing sinfo in no-frags case.
> 
> The likely/unlikely instrumentation lays out asm code such that sinfo
> access isn't interleaved with no-frags case (checked on GCC 12.2.1-4).
> The generated asm code is more compact towards the no-frags case.
> 
> The BPF kfunc bpf_dynptr_slice() also use bpf_xdp_pointer(). Thus, it
> should also take effect for that.
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  net/core/filter.c |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 968139f4a1ac..961db5bd2f94 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3948,20 +3948,21 @@ void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off,
>  
>  void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset, u32 len)
>  {
> -	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
>  	u32 size = xdp->data_end - xdp->data;
> +	struct skb_shared_info *sinfo;
>  	void *addr = xdp->data;
>  	int i;
>  
>  	if (unlikely(offset > 0xffff || len > 0xffff))
>  		return ERR_PTR(-EFAULT);
>  
> -	if (offset + len > xdp_get_buff_len(xdp))
> +	if (unlikely(offset + len > xdp_get_buff_len(xdp)))
>  		return ERR_PTR(-EINVAL);
>  
> -	if (offset < size) /* linear area */
> +	if (likely((offset < size))) /* linear area */

nit: you can drop a round bracket here. Other than that:

Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>

>  		goto out;
>  
> +	sinfo = xdp_get_shared_info_from_buff(xdp);
>  	offset -= size;
>  	for (i = 0; i < sinfo->nr_frags; i++) { /* paged area */
>  		u32 frag_size = skb_frag_size(&sinfo->frags[i]);
> 
> 

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Re: [PATCH bpf-next V2] bpf/xdp: optimize bpf_xdp_pointer to avoid reading sinfo

[Alexei Starovoitov]

On Thu, Jun 1, 2023 at 1:34 PM Lorenzo Bianconi <lorenzo@kernel.org> wrote:
>
> > Currently we observed a significant performance degradation in
> > samples/bpf xdp1 and xdp2, due XDP multibuffer "xdp.frags" handling,
> > added in commit 772251742262 ("samples/bpf: fixup some tools to be able
> > to support xdp multibuffer").
> >
> > This patch reduce the overhead by avoiding to read/load shared_info
> > (sinfo) memory area, when XDP packet don't have any frags. This improves
> > performance because sinfo is located in another cacheline.
> >
> > Function bpf_xdp_pointer() is used by BPF helpers bpf_xdp_load_bytes()
> > and bpf_xdp_store_bytes(). As a help to reviewers, xdp_get_buff_len() can
> > potentially access sinfo, but it uses xdp_buff_has_frags() flags bit check
> > to avoid accessing sinfo in no-frags case.
> >
> > The likely/unlikely instrumentation lays out asm code such that sinfo
> > access isn't interleaved with no-frags case (checked on GCC 12.2.1-4).
> > The generated asm code is more compact towards the no-frags case.
> >
> > The BPF kfunc bpf_dynptr_slice() also use bpf_xdp_pointer(). Thus, it
> > should also take effect for that.
> >
> > Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> > ---
> >  net/core/filter.c |    7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 968139f4a1ac..961db5bd2f94 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -3948,20 +3948,21 @@ void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off,
> >
> >  void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset, u32 len)
> >  {
> > -     struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> >       u32 size = xdp->data_end - xdp->data;
> > +     struct skb_shared_info *sinfo;
> >       void *addr = xdp->data;
> >       int i;
> >
> >       if (unlikely(offset > 0xffff || len > 0xffff))
> >               return ERR_PTR(-EFAULT);
> >
> > -     if (offset + len > xdp_get_buff_len(xdp))
> > +     if (unlikely(offset + len > xdp_get_buff_len(xdp)))
> >               return ERR_PTR(-EINVAL);
> >
> > -     if (offset < size) /* linear area */
> > +     if (likely((offset < size))) /* linear area */
>
> nit: you can drop a round bracket here. Other than that:

Fixed while applying. Thanks everyone.

> Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
>
> >               goto out;
> >
> > +     sinfo = xdp_get_shared_info_from_buff(xdp);
> >       offset -= size;
> >       for (i = 0; i < sinfo->nr_frags; i++) { /* paged area */
> >               u32 frag_size = skb_frag_size(&sinfo->frags[i]);
> >
> >

Re: [PATCH bpf-next V2] bpf/xdp: optimize bpf_xdp_pointer to avoid reading sinfo

[Toke Høiland-Jørgensen]

Jesper Dangaard Brouer <brouer@redhat.com> writes:

> Currently we observed a significant performance degradation in
> samples/bpf xdp1 and xdp2, due XDP multibuffer "xdp.frags" handling,
> added in commit 772251742262 ("samples/bpf: fixup some tools to be able
> to support xdp multibuffer").
>
> This patch reduce the overhead by avoiding to read/load shared_info
> (sinfo) memory area, when XDP packet don't have any frags. This improves
> performance because sinfo is located in another cacheline.
>
> Function bpf_xdp_pointer() is used by BPF helpers bpf_xdp_load_bytes()
> and bpf_xdp_store_bytes(). As a help to reviewers, xdp_get_buff_len() can
> potentially access sinfo, but it uses xdp_buff_has_frags() flags bit check
> to avoid accessing sinfo in no-frags case.
>
> The likely/unlikely instrumentation lays out asm code such that sinfo
> access isn't interleaved with no-frags case (checked on GCC 12.2.1-4).
> The generated asm code is more compact towards the no-frags case.
>
> The BPF kfunc bpf_dynptr_slice() also use bpf_xdp_pointer(). Thus, it
> should also take effect for that.
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

Thanks for fixing this!

Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>

Re: [PATCH bpf-next V2] bpf/xdp: optimize bpf_xdp_pointer to avoid reading sinfo

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Thu, 01 Jun 2023 18:21:54 +0200 you wrote:
> Currently we observed a significant performance degradation in
> samples/bpf xdp1 and xdp2, due XDP multibuffer "xdp.frags" handling,
> added in commit 772251742262 ("samples/bpf: fixup some tools to be able
> to support xdp multibuffer").
> 
> This patch reduce the overhead by avoiding to read/load shared_info
> (sinfo) memory area, when XDP packet don't have any frags. This improves
> performance because sinfo is located in another cacheline.
> 
> [...]

Here is the summary with links:
  - [bpf-next,V2] bpf/xdp: optimize bpf_xdp_pointer to avoid reading sinfo
    https://git.kernel.org/bpf/bpf-next/c/411486626e57

You are awesome, thank you!
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