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Date: Wed, 2 Feb 2022 13:08:27 +0100 From: Alexander Lobakin <alexandr.lobakin@...el.com> To: Paolo Abeni <pabeni@...hat.com> Cc: Alexander Lobakin <alexandr.lobakin@...el.com>, netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>, Kees Cook <keescook@...omium.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org> Subject: Re: [RFC PATCH 2/3] net: gro: minor optimization for dev_gro_receive() From: Paolo Abeni <pabeni@...hat.com> Date: Wed, 02 Feb 2022 11:09:41 +0100 > Hello, Hi! > > On Tue, 2022-01-18 at 18:39 +0100, Alexander Lobakin wrote: > > From: Paolo Abeni <pabeni@...hat.com> > > Date: Tue, 18 Jan 2022 17:31:00 +0100 > > > > > On Tue, 2022-01-18 at 16:56 +0100, Alexander Lobakin wrote: > > > > From: Paolo Abeni <pabeni@...hat.com> > > > > Date: Tue, 18 Jan 2022 16:24:19 +0100 > > > > > > > > > While inspecting some perf report, I noticed that the compiler > > > > > emits suboptimal code for the napi CB initialization, fetching > > > > > and storing multiple times the memory for flags bitfield. > > > > > This is with gcc 10.3.1, but I observed the same with older compiler > > > > > versions. > > > > > > > > > > We can help the compiler to do a nicer work e.g. initially setting > > > > > all the bitfield to 0 using an u16 alias. The generated code is quite > > > > > smaller, with the same number of conditional > > > > > > > > > > Before: > > > > > objdump -t net/core/gro.o | grep " F .text" > > > > > 0000000000000bb0 l F .text 0000000000000357 dev_gro_receive > > > > > > > > > > After: > > > > > 0000000000000bb0 l F .text 000000000000033c dev_gro_receive > > > > > > > > > > Signed-off-by: Paolo Abeni <pabeni@...hat.com> > > > > > --- > > > > > include/net/gro.h | 13 +++++++++---- > > > > > net/core/gro.c | 16 +++++----------- > > > > > 2 files changed, 14 insertions(+), 15 deletions(-) > > > > > > > > > > diff --git a/include/net/gro.h b/include/net/gro.h > > > > > index 8f75802d50fd..a068b27d341f 100644 > > > > > --- a/include/net/gro.h > > > > > +++ b/include/net/gro.h > > > > > @@ -29,14 +29,17 @@ struct napi_gro_cb { > > > > > /* Number of segments aggregated. */ > > > > > u16 count; > > > > > > > > > > - /* Start offset for remote checksum offload */ > > > > > - u16 gro_remcsum_start; > > > > > + /* Used in ipv6_gro_receive() and foo-over-udp */ > > > > > + u16 proto; > > > > > > > > > > /* jiffies when first packet was created/queued */ > > > > > unsigned long age; > > > > > > > > > > - /* Used in ipv6_gro_receive() and foo-over-udp */ > > > > > - u16 proto; > > > > > + /* portion of the cb set to zero at every gro iteration */ > > > > > + u32 zeroed_start[0]; > > > > > + > > > > > + /* Start offset for remote checksum offload */ > > > > > + u16 gro_remcsum_start; > > > > > > > > > > /* This is non-zero if the packet may be of the same flow. */ > > > > > u8 same_flow:1; > > > > > @@ -70,6 +73,8 @@ struct napi_gro_cb { > > > > > /* GRO is done by frag_list pointer chaining. */ > > > > > u8 is_flist:1; > > > > > > > > > > + u32 zeroed_end[0]; > > > > > > > > This should be wrapped in struct_group() I believe, or compilers > > > > will start complaining soon. See [0] for the details. > > > > Adding Kees to the CCs. > > > > > > Thank you for the reference. That really slipped-off my mind. > > > > > > This patch does not use memcpy() or similar, just a single direct > > > assignement. Would that still require struct_group()? > > > > Oof, sorry, I saw start/end and overlooked that it's only for > > a single assignment. > > Then it shouldn't cause warnings, but maybe use an anonymous > > union instead? > > > > union { > > u32 zeroed; > > struct { > > u16 gro_remcsum_start; > > ... > > }; > > }; > > __wsum csum; > > > > Use can still use a BUILD_BUG_ON() in this case, like > > sizeof(zeroed) != offsetof(csum) - offsetof(zeroed). > > Please forgive me for the very long delay. I'm looking again at this > stuff for formal non-rfc submission. Sure, not a problem at all (: > > I like the anonymous union less, because it will move around much more > code - making the patch less readable - and will be more fragile e.g. > some comment alike "please don't move around 'csum'" would be needed. We still need comments around zeroed_{start,end}[0] for now. I used offsetof(csum) as offsetofend(is_flist) which I'd prefer here unfortunately expands to offsetof(is_flist) + sizeof(is_flist), and the latter causes an error of using sizeof() against a bitfield. > > No strong opinion anyway, so if you really prefer that way I can adapt. > Please let me know. I don't really have a strong preference here, I just suspect that zero-length array will produce or already produce -Warray-bounds warnings, and empty-struct constructs like struct { } zeroed_start; u16 gro_remcsum_start; ... struct { } zeroed_end; memset(NAPI_GRO_CB(skb)->zeroed_start, 0, offsetofend(zeroed_end) - offsetsof(zeroed_start)); will trigger Fortify compile-time errors from Kees' KSPP tree. I think we could use __struct_group(/* no tag */, zeroed, /* no attrs */, u16 gro_remcsum_start; ... u8 is_flist:1; ); __wsum csum; BUILD_BUG_ON(sizeof_field(struct napi_gro_cb, zeroed) != sizeof(u32)); BUILD_BUG_ON(!IS_ALIGNED(offsetof(struct napi_gro_cb, zeroed), sizeof(u32))); /* Avoid slow unaligned acc */ *(u32 *)&NAPI_GRO_CB(skb)->zeroed = 0; This doesn't depend on `csum`, doesn't need `struct { }` or `struct zero[0]` markers and still uses a direct assignment. Also adding Gustavo, maybe he'd like to leave a comment here. > > Thanks! > > Paolo Thanks, Al
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