Re: [PATCH] softirq: allow raising NET_RX_SOFTIRQ from SMP call on RT

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

On 12/22/25 14:03, K Prateek Nayak wrote:
> Hello Wen,
> 
> On 12/19/2025 11:28 PM, wen.yang@linux.dev wrote:
>> From: Wen Yang <wen.yang@linux.dev>
>>
>> RPS distributes network processing load across CPUs by enqueuing
>> packets on a remote CPU's backlog and raising NET_RX_SOFTIRQ to
>> process them. This is done via an smp_call_function.
> 
> I'm assuming net_rps_send_ipi() is the function responsible for IPIs
> and the only two callers - net_rps_action_and_irq_enable() and
> dev_cpu_dead() - first check for "!use_backlog_threads()" before
> sending the IPI which is always false in case of PREEMPT_RT.
> 
> What is the path where you end up sending this IPI on PREEMPT_RT?
> Can you please trace the path of this problematic IPI on the latest
> upstream and prevent that instead?
> 

Thank you for your suggestion.
Our production environment uses 6.1 and 6.6 kernels, and after porting 
commit dad6b9770263 ("net: Allow to use SMP threads for backlog NAPI.") 
and its dependent patches, this issue has indeed been fixed.

--
Best wishes,
Wen

>>
>> On PREEMPT_RT kernels, raising a softirq from an SMP call function
>> can increase preemption-off latencies. A WARN_ON_ONCE() in
>> do_softirq_post_smp_call_flush() enforces this.
>>
>> A previous commit 6675ce20046d ("softirq: Allow raising SCHED_SOFTIRQ from SMP-call-function on RT kernel")
> 
> There is also a good justification in 6675ce20046d that idle load
> balancing periodically checks need_resched() and will bail out
> immediately if it detects a wakeup but the same cannot be expected of
> all NET_RX_SOFTIRQ handlers so ignoring the warning isn't a solution.
>