netdev - Re: [PATCH net-next v3] net: sched: sfq: add detailed drop reasons for monitoring

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Message-ID: <78899cbc-3e1f-4f84-83a1-a2a5e6301c05@redhat.com>
Date: Thu, 29 Jan 2026 11:36:24 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Jesper Dangaard Brouer <hawk@...nel.org>, netdev@...r.kernel.org,
 Eric Dumazet <eric.dumazet@...il.com>, "David S. Miller"
 <davem@...emloft.net>
Cc: bpf@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
 Toke Høiland-Jørgensen <toke@...e.dk>,
 horms@...nel.org, jiri@...nulli.us, edumazet@...gle.com,
 xiyou.wangcong@...il.com, jhs@...atatu.com, carges@...udflare.com,
 kernel-team@...udflare.com
Subject: Re: [PATCH net-next v3] net: sched: sfq: add detailed drop reasons
 for monitoring

On 1/23/26 1:18 PM, Jesper Dangaard Brouer wrote:
> Add specific drop reasons to SFQ qdisc to improve packet drop
> observability and monitoring capabilities. This change replaces
> generic qdisc_drop() calls with qdisc_drop_reason() to provide
> granular metrics about different drop scenarios in production
> environments.
> 
> Two new drop reasons are introduced:
> 
> - SKB_DROP_REASON_QDISC_SFQ_MAXFLOWS: Used when a new flow cannot
>   be created because the maximum number of flows (flows parameter)
>   has been reached and no free flow slots are available.
> 
> - SKB_DROP_REASON_QDISC_SFQ_MAXDEPTH: Used when a flow's queue
>   length exceeds the per-flow depth limit (depth parameter),
>   triggering either tail drop or head drop depending on headdrop
>   configuration.
> 
> The existing SKB_DROP_REASON_QDISC_OVERLIMIT is used in sfq_drop()
> when the overall qdisc limit is exceeded and packets are dropped
> from the longest queue.
> 
> The naming uses qdisc-specific drop reasons tied to SFQ tunables,
> following the pattern established by Eric's FQ commit 5765c7f6e317
> ("net_sched: sch_fq: add three drop_reason") which introduced
> FQ_BAND_LIMIT, FQ_HORIZON_LIMIT, and FQ_FLOW_LIMIT.
> 
> The new drop reasons are inserted in the middle of the enum after
> SKB_DROP_REASON_QDISC_CONGESTED to group all qdisc-related reasons
> together. While drop reason enum values are not UAPI, the enum
> names implicitly become UAPI as userspace tools rely on BTF to
> resolve names to values. This makes middle-insertion safe as long
> as names remain stable.
> 
> These detailed drop reasons enable production monitoring systems
> to distinguish between different SFQ drop scenarios and generate
> specific metrics for:
> - Flow table exhaustion (flows exceeded)
> - Per-flow congestion (depth limit exceeded)
> - Global qdisc congestion (overall limit exceeded)
> 
> This granular visibility allows operators to identify capacity
> planning needs, detect traffic patterns, and optimize SFQ
> configuration based on real-world drop patterns.
> 
> Signed-off-by: Jesper Dangaard Brouer <hawk@...nel.org>

Given the lack of endorsing from any 3rd party, the possible BPF
alternative and the previous controversial conversation, I'm also not
going to apply this change, sorry.

Paolo