Re: [PATCH 6.6-rt 0/5] net: Backport backlog-NAPI threads to resolve PREEMPT_RT softirq warning in SMP flush

[Clark Williams <clrkwllms@kernel.org>]

On Mon, Oct 19, 2026 at 02:15:22AM +0800, wen.yang@linux.dev wrote:
> From: Wen Yang <wen.yang@linux.dev>
> 
> Backport backlog-NAPI threads to v6.6-rt-next first, as requested by
> Sebastian and Clark, before following up with the v6.1-rt backport.
> The base is 4c4970859022 (Linux 6.6.151-rt78-rc1).
> 
> The existing RT workaround, 1f7514c9c75d ("net: Avoid the IPI to free the"),
> redirects deferred skb cleanup to a worker but leaves RPS remote-backlog
> scheduling on the function-call IPI path. In a fresh RT guest this still
> triggers do_softirq_post_smp_call_flush() from idle queue flushing.
> 
> The ten earlier NAPI/RPS prerequisites are already present in v6.6, so this
> series reverts the workaround and backports the four upstream changes (all
> in upstream v6.10) in dependency order. Backlog-NAPI threads replace the
> RPS/defer function-call notifications; ordinary scheduler wakeup IPI traffic
> can still occur. Two of the changes were previously backported to ordinary
> 6.6.y and reverted (507692c05636, 5a530c8ead06) only because PREEMPT_RT is
> absent there, not for a functional defect.
> 
> Backport notes
> --------------
> Only 3/5 needs context adaptations; they are listed in that patch's commit
> message. The other four apply without context edits. Unlike v6.1, v6.6 keeps
> WRITE_ONCE(napi->list_owner, ...) because the field already exists here.
> 
> Verification
> ------------
> Each of the five stages was replayed against the base snapshot and compiled
> for net/core/dev.o and net/core/skbuff.o. The baseline and final RT/RPS
> bzImages were built with byte-identical normalized configs using GCC 9.4.0,
> with no compiler warnings or errors.
> 
> Tests use virtme-ng 1.38, x86_64 QEMU/KVM, four vCPUs and 2 GiB memory:
> 
>   vng --run <bzImage> --cpus 4 --memory 2G \
>       --qemu-opts='-machine accel=kvm' --exec 'bash <script>'
> 
> The config enables PREEMPT_RT, SMP, SYSFS, NET_NS, VETH, RPS,
> IRQ_TIME_ACCOUNTING and KPROBES, with NR_CPUS=4. Run the inline script below
> in a fresh guest for each image; it prints one column of the table, and the
> columns are joined outside the guest. It covers both mechanisms: the RPS/IPI
> path (ping flood with RPS steered to CPU1) and the defer_list purge path
> (cross-CPU TCP frees traced with kprobes). See its header for details.
> 

Wen,

I just pushed v6.6.159-rt82-rc1 up to the linux-stable-rt repository in
v6.6-rt-next, waiting for kernelci.org to pick it up and test it. I'll
let it soak for a bit so people can test it, then we can release it
later this week.

I'm betting the v6.1-rt series is going to be a bit bigger?  :)

Clark

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