Re: [PATCH v3 5/5] softirq: Avoid unnecessary wakeup of ksoftirqd when a call to do_sofirq() is pending

[K Prateek Nayak]

Hello Sebastian,

Thank you for reviewing the series!

On 10/25/2024 10:33 PM, Sebastian Andrzej Siewior wrote:
> On 2024-10-14 09:03:39 [+0000], K Prateek Nayak wrote:
> > Since commit b2a02fc43a1f4 ("smp: Optimize
> > send_call_function_single_ipi()"), sending an actual interrupt to an
> > idle CPU in TIF_POLLING_NRFLAG mode can be avoided by queuing the SMP
> > call function on the call function queue of the CPU and setting the
> > TIF_NEED_RESCHED bit in idle task's thread info. The call function is
> > handled in the idle exit path when do_idle() calls
> > flush_smp_call_function_queue().
> >
> > However, since flush_smp_call_function_queue() is executed in idle
> > thread's context, in_interrupt() check within a call function will
> > return false. raise_softirq() uses this check to decide whether to wake
> > ksoftirqd, since, a softirq raised from an interrupt context will be
> > handled at irq exit. In all other cases, raise_softirq() wakes up
> > ksoftirqd to handle the softirq on !PREEMPT_RT kernel.
>
> Stupid question. You talk about the invocation from nohz_csd_func(),
> right?.
> Given that this is an IPI and always invoked from an IRQ then the
> softirq is invoked on IRQ-exit.

Yes, there is no issues in that case.

> If it is flushed from
> flush_smp_call_function_queue() then the softirq is handled via
> do_softirq_post_smp_call_flush(). In that case couldn't you just tell
> nohz_csd_func() to use __raise_softirq_irqoff(SCHED_SOFTIRQ) ? This
> should solve this, right?

I cannot think of any reason why it wouldn't work. Let me check real
quick and update the series if it works. Thanks a ton for the
suggestion!

> > diff --git a/kernel/softirq.c b/kernel/softirq.c
> > index 0730c2b43ae4..3a6b3e67ea24 100644
> > --- a/kernel/softirq.c
> > +++ b/kernel/softirq.c
> > @@ -99,6 +99,10 @@ EXPORT_PER_CPU_SYMBOL_GPL(hardirq_context);
> >    *
> >    * The per CPU counter prevents pointless wakeups of ksoftirqd in case that
> >    * the task which is in a softirq disabled section is preempted or blocks.
> > + *
> > + * The bottom bits of softirq_ctrl::cnt is used to indicate an impending call
> > + * to do_softirq() to prevent pointless wakeups of ksoftirqd since the CPU
> > + * promises to handle softirqs soon.
> >    */
>
> The comment that you are extending and the comment regarding
> SOFTIRQ_OFFSET were nearby. I don't like that those two are now far
> apart.

Noted. If the above suggestion doesn't work, I'll rearrange this bit and
refresh the series.

> >   struct softirq_ctrl {
> >   	local_lock_t	lock;
> > @@ -109,6 +113,16 @@ static DEFINE_PER_CPU_ALIGNED(struct softirq_ctrl, softirq_ctrl) = {
> >   	.lock	= INIT_LOCAL_LOCK(softirq_ctrl.lock),
> >   };
> >   
> > +inline void set_do_softirq_pending(void)
> > +{
> > +	__this_cpu_inc(softirq_ctrl.cnt);
> > +}
> > +
> > +inline void clr_do_softirq_pending(void)
>
> there should be no inline here.

Ack. Will fix in the subsequent version if the alternate approach
doesn't work.

> > +{
> > +	__this_cpu_dec(softirq_ctrl.cnt);
> > +}
> > +
> >   static inline bool should_wake_ksoftirqd(void)
> >   {
> >   	return !this_cpu_read(softirq_ctrl.cnt);
>
> Sebastian

--
Thanks and Regards,
Prateek