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

[Wen Yang <wen.yang@linux.dev>]

On 10/19/26 21:20, Clark Williams wrote:
> 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?  :)
> 

Hi,

Thanks for pushing the rc, and for giving it time to soak before release.
On your question about the v6.1 series - you're right, it is larger:
15 patches. That is mostly because 6.1.y does not carry the ten 
NAPI/RPS/ defer_list prerequisites from v6.4-v6.9 that v6.6 already has, 
so they have to come along:
     10  prerequisite NAPI/RPS/defer_list patches
      4  the backlog-NAPI changes (the same set as the v6.6 series)
      1  revert of the RT defer_work workaround

Only one of the 14 upstream patches needed a change beyond context: 
patch 06 keeps raise_softirq_irqoff() instead of 
__raise_softirq_irqoff(), since 6.1-rt already carries 533824e6a0fb and 
the 2023 upstream text predates it.
The other 13 are line-for-line identical, and patch 13's five 
hand-placed hunks are context only.

The code is unchanged since I posted it, so there is nothing new to 
send; please feel free to apply and test the series as posted.

https://lore.kernel.org/all/cover.1791912669.git.wen.yang@linux.dev/


Thanks,
Wen