Re: [PATCH v4 3/3] sched/core: Prevent wakeup of ksoftirqd during idle load balance

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-10-30 07:15:57 [+0000], K Prateek Nayak wrote:
> Scheduler raises a SCHED_SOFTIRQ to trigger a load balancing event on
> from the IPI handler on the idle CPU. Since the softirq can be raised
> from flush_smp_call_function_queue(), it can end up waking up ksoftirqd,
> which can give an illusion of the idle CPU being busy when doing an idle
> load balancing.
> 
> Adding a trace_printk() in nohz_csd_func() at the spot of raising
> SCHED_SOFTIRQ and enabling trace events for sched_switch, sched_wakeup,
> and softirq_entry (for SCHED_SOFTIRQ vector alone) helps observing the
> current behavior:
> 
>        <idle>-0   [000] dN.1.:  nohz_csd_func: Raising SCHED_SOFTIRQ from nohz_csd_func
>        <idle>-0   [000] dN.4.:  sched_wakeup: comm=ksoftirqd/0 pid=16 prio=120 target_cpu=000
>        <idle>-0   [000] .Ns1.:  softirq_entry: vec=7 [action=SCHED]
>        <idle>-0   [000] .Ns1.:  softirq_exit: vec=7  [action=SCHED]
>        <idle>-0   [000] d..2.:  sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=ksoftirqd/0 next_pid=16 next_prio=120
>   ksoftirqd/0-16  [000] d..2.:  sched_switch: prev_comm=ksoftirqd/0 prev_pid=16 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
>        ...
> 
> ksoftirqd is woken up before the idle thread calls
> do_softirq_post_smp_call_flush() which can make the runqueue appear
> busy and prevent the idle load balancer from pulling task from an
> overloaded runqueue towards itself[1].
> 
> Since the softirq raised is guranteed to be serviced in irq_exit() or
> via do_softirq_post_smp_call_flush(), set SCHED_SOFTIRQ without checking
> the need to wakeup ksoftirq for idle load balancing.
> 
> Following are the observations with the changes when enabling the same
> set of events:
> 
>        <idle>-0       [000] dN.1.: nohz_csd_func: Raising SCHED_SOFTIRQ for nohz_idle_balance
>        <idle>-0       [000] dN.1.: softirq_raise: vec=7 [action=SCHED]
>        <idle>-0       [000] .Ns1.: softirq_entry: vec=7 [action=SCHED]
> 
> No unnecessary ksoftirqd wakeups are seen from idle task's context to
> service the softirq.

| Use __raise_softirq_irqoff() to raise the softirq. The SMP function call
| is always invoked on the requested CPU in an interrupt handler. It is
| guaranteed that soft interrupts are handled at the end.

You could extend it

| If the SMP function is invoked from an idle CPU via
| flush_smp_call_function_queue() then the HARD-IRQ flag is not set and
| raise_softirq_irqoff() wakes needlessly ksoftirqd because soft
| interrupts are handled before ksoftirqd get on the CPU.

This on its own is a reasonable optimisation. A different question would
be if flush_smp_call_function_queue() should pretend to be in-IRQ like a
regular IPI but…

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

> Fixes: b2a02fc43a1f ("smp: Optimize send_call_function_single_ipi()")
> Reported-by: Julia Lawall <julia.lawall@inria.fr>
> Closes: https://lore.kernel.org/lkml/fcf823f-195e-6c9a-eac3-25f870cb35ac@inria.fr/ [1]
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
> v3..v4:
> 
> o New patch based on Sebastian's suggestion.
> ---
>  kernel/sched/core.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index aaf99c0bcb49..2ee3621d6e7e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1244,7 +1244,18 @@ static void nohz_csd_func(void *info)
>  	rq->idle_balance = idle_cpu(cpu);
>  	if (rq->idle_balance) {
>  		rq->nohz_idle_balance = flags;
> -		raise_softirq_irqoff(SCHED_SOFTIRQ);
> +
> +		/*
> +		 * Don't wakeup ksoftirqd when raising SCHED_SOFTIRQ
> +		 * since the idle load balancer may mistake wakeup of
> +		 * ksoftirqd as a genuine task wakeup and bail out from
> +		 * load balancing early. Since it is guaranteed that
> +		 * pending softirqs will be handled soon, either on
> +		 * irq_exit() or via do_softirq_post_smp_call_flush(),
> +		 * raise SCHED_SOFTIRQ without checking the need to
> +		 * wakeup ksoftirqd.
> +		 */

/*
 * This is always invoked from an interrupt handler, simply raise the
 * softirq.
 */

should be enough IMHO. But *I* would even skip that, since it is
obvious.

> +		__raise_softirq_irqoff(SCHED_SOFTIRQ);
>  	}
>  }

Sebastian