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Date: Tue, 2 Dec 2025 12:30:09 +0100
From: Greg KH <gregkh@linuxfoundation.org>
To: zyc zyc <zyc199902@zohomail.cn>
Cc: stable <stable@vger.kernel.org>
Subject: Re: =?utf-8?B?5Zue5aSNOjYuMTIuNTAgcmVncmVz?=
 =?utf-8?Q?sion=3A_netem?= =?utf-8?Q?=3A?= cannot mix duplicating netems with
 other netems in tree.
Message-ID: <2025120248-operation-explain-1991@gregkh>
References: <19ace674022.114eb26e714992.3171091003233609170@zohomail.cn>
 <19adda5a1e2.12410b78222774.9191120410578703463@zohomail.cn>
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In-Reply-To: <19adda5a1e2.12410b78222774.9191120410578703463@zohomail.cn>

On Tue, Dec 02, 2025 at 06:39:00PM +0800, zyc zyc wrote:
> Hello,
> 
> Resend my last email without HTML.
> 
> ---- zyc zyc <zyc199902@zohomail.cn> 在 Sat, 2025-11-29 18:57:01 写到：---
> 
>  > Hello, maintainer
>  > 
>  > I would like to report what appears to be a regression in 6.12.50 kernel release related to netem.
>  > It rejects our configuration with the message:
>  > Error: netem: cannot mix duplicating netems with other netems in tree.
>  > 
>  > This breaks setups that previously worked correctly for many years.
>  > 
>  > 
>  > Our team uses multiple netem qdiscs in the same HTB branch, arranged in a parallel fashion using a prio fan-out. Each branch of the prio qdisc has its own distinct netem instance with different duplication characteristics.
>  > 
>  > This is used to emulate our production conditions where a single logical path fans out into two downstream segments, for example:
>  > 
>  > two ECMP next hops with different misbehaviour characteristics, or
>  > 
>  > 
>  > an HA firewall cluster where only one node is replaying frames, or
>  > 
>  > 
>  > two LAG / ToR paths where one path intermittently duplicates packets.
>  > 
>  > 
>  > In our environments, only a subset of flows are affected, and different downstream devices may cause different styles of duplication.
>  > This regression breaks existing automated tests, training environments, and network simulation pipelines.
>  > 
>  > I would be happy to provide our reproducer if needed.
>  > 
>  > Thank you for your time and for maintaining Linux kernel.

Can you use 'git bisect' to find the offending commit?

thanks,

greg k-h