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SubjectRe: [PATCH net-next 3/3] net: sched: make skip_sw actually skip software
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Hi Vlad,

On 2/16/24 08:47, Vlad Buslov wrote:
> On Thu 15 Feb 2024 at 16:04, Asbjørn Sloth Tønnesen <ast@fiberby.net> wrote:
>> TC filters come in 3 variants:
>> - no flag (no opinion, process wherever possible)
>> - skip_hw (do not process filter by hardware)
>> - skip_sw (do not process filter by software)
>>
>> However skip_sw is implemented so that the skip_sw
>> flag can first be checked, after it has been matched.
>>
>> IMHO it's common when using skip_sw, to use it on all rules.
>>
>> So if all filters in a block is skip_sw filters, then
>> we can bail early, we can thus avoid having to match
>> the filters, just to check for the skip_sw flag.
>>
>> +----------------------------+--------+--------+--------+
>> | Test description | Pre | Post | Rel. |
>> | | kpps | kpps | chg. |
>> +----------------------------+--------+--------+--------+
>> | basic forwarding + notrack | 1264.9 | 1277.7 | 1.01x |
>> | switch to eswitch mode | 1067.1 | 1071.0 | 1.00x |
>> | add ingress qdisc | 1056.0 | 1059.1 | 1.00x |
>> +----------------------------+--------+--------+--------+
>> | 1 non-matching rule | 927.9 | 1057.1 | 1.14x |
>> | 10 non-matching rules | 495.8 | 1055.6 | 2.13x |
>> | 25 non-matching rules | 280.6 | 1053.5 | 3.75x |
>> | 50 non-matching rules | 162.0 | 1055.7 | 6.52x |
>> | 100 non-matching rules | 87.7 | 1019.0 | 11.62x |
>> +----------------------------+--------+--------+--------+
>>
>> perf top (100 n-m skip_sw rules - pre patch):
>> 25.57% [kernel] [k] __skb_flow_dissect
>> 20.77% [kernel] [k] rhashtable_jhash2
>> 14.26% [kernel] [k] fl_classify
>> 13.28% [kernel] [k] fl_mask_lookup
>> 6.38% [kernel] [k] memset_orig
>> 3.22% [kernel] [k] tcf_classify
>>
>> perf top (100 n-m skip_sw rules - post patch):
>> 4.28% [kernel] [k] __dev_queue_xmit
>> 3.80% [kernel] [k] check_preemption_disabled
>> 3.68% [kernel] [k] nft_do_chain
>> 3.08% [kernel] [k] __netif_receive_skb_core.constprop.0
>> 2.59% [kernel] [k] mlx5e_xmit
>> 2.48% [kernel] [k] mlx5e_skb_from_cqe_mpwrq_nonlinear
>>
>> Test setup:
>> DUT: Intel Xeon D-1518 (2.20GHz) w/ Nvidia/Mellanox ConnectX-6 Dx 2x100G
>> Data rate measured on switch (Extreme X690), and DUT connected as
>> a router on a stick, with pktgen and pktsink as VLANs.
>> Pktgen was in range 12.79 - 12.95 Mpps across all tests.
>>
>> Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
>> ---
>> include/net/pkt_cls.h | 5 +++++
>> net/core/dev.c | 3 +++
>> 2 files changed, 8 insertions(+)
>>
>> diff --git a/include/net/pkt_cls.h b/include/net/pkt_cls.h
>> index a4ee43f493bb..a065da4df7ff 100644
>> --- a/include/net/pkt_cls.h
>> +++ b/include/net/pkt_cls.h
>> @@ -74,6 +74,11 @@ static inline bool tcf_block_non_null_shared(struct tcf_block *block)
>> return block && block->index;
>> }
>>
>> +static inline bool tcf_block_has_skip_sw_only(struct tcf_block *block)
>> +{
>> + return block && atomic_read(&block->filtercnt) == atomic_read(&block->skipswcnt);
>> +}
>
> Note that this introduces a read from heavily contended cache-line on
> data path for all classifiers, including the ones that don't support
> offloads. Wonder if this a concern for users running purely software tc.

Unfortunately, I don't have access to any multi-CPU machines, so I haven't been
able to test the impact of that.

Alternatively I guess I could also maintain a static key in the counter update logic.


>> +
>> static inline struct Qdisc *tcf_block_q(struct tcf_block *block)
>> {
>> WARN_ON(tcf_block_shared(block));
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index d8dd293a7a27..7cd014e5066e 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -3910,6 +3910,9 @@ static int tc_run(struct tcx_entry *entry, struct sk_buff *skb,
>> if (!miniq)
>> return ret;
>>
>> + if (tcf_block_has_skip_sw_only(miniq->block))
>> + return ret;
>> +
>> tc_skb_cb(skb)->mru = 0;
>> tc_skb_cb(skb)->post_ct = false;
>> tcf_set_drop_reason(skb, *drop_reason);
>

--
Best regards
Asbjørn Sloth Tønnesen
Network Engineer
Fiberby - AS42541

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