On Wed, Jan 29, 2025 at 04:32:26PM +0100, Florian Bezdeka wrote:
> From: K Prateek Nayak <kprateek.nayak@xxxxxxx>
>
> commit 6675ce20046d149e1e1ffe7e9577947dee17aad5 upstream.
>
> do_softirq_post_smp_call_flush() on PREEMPT_RT kernels carries a
> WARN_ON_ONCE() for any SOFTIRQ being raised from an SMP-call-function.
> Since do_softirq_post_smp_call_flush() is called with preempt disabled,
> raising a SOFTIRQ during flush_smp_call_function_queue() can lead to
> longer preempt disabled sections.
>
> Since commit b2a02fc43a1f ("smp: Optimize
> send_call_function_single_ipi()") IPIs to an idle CPU in
> TIF_POLLING_NRFLAG mode can be optimized out by instead setting
> TIF_NEED_RESCHED bit in idle task's thread_info and relying on the
> flush_smp_call_function_queue() in the idle-exit path to run the
> SMP-call-function.
>
> To trigger an idle load balancing, the scheduler queues
> nohz_csd_function() responsible for triggering an idle load balancing on
> a target nohz idle CPU and sends an IPI. Only now, this IPI is optimized
> out and the SMP-call-function is executed from
> flush_smp_call_function_queue() in do_idle() which can raise a
> SCHED_SOFTIRQ to trigger the balancing.
>
> So far, this went undetected since, the need_resched() check in
> nohz_csd_function() would make it bail out of idle load balancing early
> as the idle thread does not clear TIF_POLLING_NRFLAG before calling
> flush_smp_call_function_queue(). The need_resched() check was added with
> the intent to catch a new task wakeup, however, it has recently
> discovered to be unnecessary and will be removed in the subsequent
> commit after which nohz_csd_function() can raise a SCHED_SOFTIRQ from
> flush_smp_call_function_queue() to trigger an idle load balance on an
> idle target in TIF_POLLING_NRFLAG mode.
>
> nohz_csd_function() bails out early if "idle_cpu()" check for the
> target CPU, and does not lock the target CPU's rq until the very end,
> once it has found tasks to run on the CPU and will not inhibit the
> wakeup of, or running of a newly woken up higher priority task. Account
> for this and prevent a WARN_ON_ONCE() when SCHED_SOFTIRQ is raised from
> flush_smp_call_function_queue().
>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20241119054432.6405-2-kprateek.nayak@xxxxxxx
> Tested-by: Felix Moessbauer <felix.moessbauer@xxxxxxxxxxx>
> Signed-off-by: Florian Bezdeka <florian.bezdeka@xxxxxxxxxxx>
> ---
>
> Newer stable branches (6.12, 6.6) got this already, 5.10 and lower are
> not affected.
>
> The warning triggered for SCHED_SOFTIRQ under high network load while
> testing.
But RT is not in the 6.1.y tree, right? Or is it? Why was it only
backported to 6.6.y and 6.12.y?
thanks,
greg k-h
