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

[K Prateek Nayak <kprateek.nayak@amd.com>]

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?

> 
> 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.

-- 
Thanks and Regards,
Prateek