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Message-ID: <CABGOaVRdsw=4nqBMR0h8JPEiunOEpHR+02H=HRbgt_TxhVviiA@mail.gmail.com>
Date: Mon, 2 Mar 2020 14:12:55 +0530
From: Yadu Kishore <kyk.segfault@...il.com>
To: David Miller <davem@...emloft.net>
Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v2] net: Make skb_segment not to compute checksum if
network controller supports checksumming
Hi all,
A small correction to the data I sent earlier:
Without patch :-
============
[Function = %cpu cycles]
skb_mac_gso_segment = 0.05
inet_gso_segment = 0.26
tcp4_gso_segment = 0.05
tcp_gso_segment = 0.17
skb_segment = 0.55
skb_copy_and_csum_bits = 0.60
do_csum = 7.43
memcpy = 3.81
__alloc_skb = 0.93
==================
SUM = 13.85
With patch :-
============
[Function = %cpu cycles]
skb_mac_gso_segment = 0.05
inet_gso_segment = 0.34
tcp4_gso_segment = 0.06
tcp_gso_segment = 0.26
skb_segment = 0.55
** skb_copy_bits = 0.62 ** <-- corrected
do_csum = 0.04
memcpy = 4.29
__alloc_skb = 0.73
==================
** SUM = 6.94 ** <-- corrected
Thanks,
Yadu Kishore
On Mon, Mar 2, 2020 at 12:22 PM Yadu Kishore <kyk.segfault@...il.com> wrote:
>
> > > Can you contrast this against a run with your changes? The thought is
> > > that the majority of this cost is due to the memory loads and stores, not
> > > the arithmetic ops to compute the checksum. When enabling checksum
> > > offload, the same stalls will occur, but will simply be attributed to
> > > memcpy instead of to do_csum.
>
> > Agreed.
>
> Below is the data from perf with and without the patch for the same
> TCP Tx iperf run: (network driver has NETIF_F_HW_CSUM enabled)
>
> Without patch :-
> ============
> [Function = %cpu cycles]
> skb_mac_gso_segment = 0.05
> inet_gso_segment = 0.26
> tcp4_gso_segment = 0.05
> tcp_gso_segment = 0.17
> skb_segment = 0.55
> skb_copy_and_csum_bits = 0.60
> do_csum = 7.43
> memcpy = 3.81
> __alloc_skb = 0.93
> ==================
> SUM = 13.85
>
>
> With patch :-
> ============
> [Function = %cpu cycles]
> skb_mac_gso_segment = 0.05
> inet_gso_segment = 0.34
> tcp4_gso_segment = 0.06
> tcp_gso_segment = 0.26
> skb_segment = 0.55
> skb_copy_and_csum_bits = 0.00
> do_csum = 0.04
> memcpy = 4.29
> __alloc_skb = 0.73
> ==================
> SUM = 6.32
>
> So, with the patch, from the above data we can see
> that the percentage of cpu cycles spent in do_csum
> has come down from 7.43% to 0.04%.
>
> > > > Is this not already handled by __copy_skb_header above? If ip_summed
> > > > has to be initialized, so have csum_start and csum_offset. That call
> > > > should have initialized all three.
>
> > > Thanks, I will look into why even though __copy_skb_header is being
> > > called, I am still
> > > seeing skb->ip_summed set to CHECKSUM_NONE in the network driver.
>
> > Thanks.
>
> My mistake. I had removed the 'skb->ip_summed = CHECKSUM_PARTIAL' line
> from the patch but had forgotten to enable NETIF_F_HW_CSUM in the network
> driver. Hence I was still seeing skb->ip_summed set to CHECKSUM_NONE.
> After re-enabling NETIF_F_HW_CSUM in the driver, I now see that
> skb->ip_summed is being set correctly to CHECKSUM_PARTIAL.
> So as suggested, the __copy_skb_header is indeed taking care of doing
> this and hence there is no need to explicitly set 'ip_summed' in the patch.
> Below is V2 of the patch with the changes.
>
>
> Problem:
> TCP checksum in the output path is not being offloaded during GSO
> in the following case:
> The network driver does not support scatter-gather but supports
> checksum offload with NETIF_F_HW_CSUM.
>
> Cause:
> skb_segment calls skb_copy_and_csum_bits if the network driver
> does not announce NETIF_F_SG. It does not check if the driver
> supports NETIF_F_HW_CSUM.
> So for devices which might want to offload checksum but do not support SG
> there is currently no way to do so if GSO is enabled.
>
> Solution:
> In skb_segment check if the network controller does checksum and if so
> call skb_copy_bits instead of skb_copy_and_csum_bits.
>
> Testing:
> Without the patch, ran iperf TCP traffic with NETIF_F_HW_CSUM enabled
> in the network driver. Observed the TCP checksum offload is not happening
> since the skbs received by the driver in the output path have
> skb->ip_summed set to CHECKSUM_NONE.
>
> With the patch ran iperf TCP traffic and observed that TCP checksum
> is being offloaded with skb->ip_summed set to CHECKSUM_PARTIAL.
> Also tested with the patch by disabling NETIF_F_HW_CSUM in the driver
> to cover the newly introduced if-else code path in skb_segment.
>
> Link: https://lore.kernel.org/netdev/CA+FuTSeYGYr3Umij+Mezk9CUcaxYwqEe5sPSuXF8jPE2yMFJAw@mail.gmail.com
> Signed-off-by: Yadu Kishore <kyk.segfault@...il.com>
> ---
> net/core/skbuff.c | 23 +++++++++++++++--------
> 1 file changed, 15 insertions(+), 8 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 1365a55..eca72bc 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -3926,14 +3926,21 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
> goto perform_csum_check;
>
> if (!sg) {
> - if (!nskb->remcsum_offload)
> - nskb->ip_summed = CHECKSUM_NONE;
> - SKB_GSO_CB(nskb)->csum =
> - skb_copy_and_csum_bits(head_skb, offset,
> - skb_put(nskb, len),
> - len, 0);
> - SKB_GSO_CB(nskb)->csum_start =
> - skb_headroom(nskb) + doffset;
> + if (!csum) {
> + if (!nskb->remcsum_offload)
> + nskb->ip_summed = CHECKSUM_NONE;
> + SKB_GSO_CB(nskb)->csum =
> + skb_copy_and_csum_bits(head_skb, offset,
> + skb_put(nskb,
> + len),
> + len, 0);
> + SKB_GSO_CB(nskb)->csum_start =
> + skb_headroom(nskb) + doffset;
> + } else {
> + skb_copy_bits(head_skb, offset,
> + skb_put(nskb, len),
> + len);
> + }
> continue;
> }
>
> --
> 2.7.4
>
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