linux-kernel - Re: [RFC PATCH 41/86] sched: handle resched policy in resched

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Date:   Wed, 8 Nov 2023 11:46:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, paulmck@...nel.org,
        linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...lab.com, richard@....at, mjguzik@...il.com
Subject: Re: [RFC PATCH 41/86] sched: handle resched policy in resched_curr()

On Wed, Nov 08, 2023 at 02:26:37AM -0800, Ankur Arora wrote:
> 
> Peter Zijlstra <peterz@...radead.org> writes:
> 
> > On Tue, Nov 07, 2023 at 01:57:27PM -0800, Ankur Arora wrote:
> >
> >> --- a/kernel/sched/core.c
> >> +++ b/kernel/sched/core.c
> >> @@ -1027,13 +1027,13 @@ void wake_up_q(struct wake_q_head *head)
> >>  }
> >>
> >>  /*
> >> - * resched_curr - mark rq's current task 'to be rescheduled now'.
> >> + * __resched_curr - mark rq's current task 'to be rescheduled'.
> >>   *
> >> - * On UP this means the setting of the need_resched flag, on SMP it
> >> - * might also involve a cross-CPU call to trigger the scheduler on
> >> - * the target CPU.
> >> + * On UP this means the setting of the need_resched flag, on SMP, for
> >> + * eager resched it might also involve a cross-CPU call to trigger
> >> + * the scheduler on the target CPU.
> >>   */
> >> -void resched_curr(struct rq *rq)
> >> +void __resched_curr(struct rq *rq, resched_t rs)
> >>  {
> >>  	struct task_struct *curr = rq->curr;
> >>  	int cpu;
> >> @@ -1046,17 +1046,77 @@ void resched_curr(struct rq *rq)
> >>  	cpu = cpu_of(rq);
> >>
> >>  	if (cpu == smp_processor_id()) {
> >> -		set_tsk_need_resched(curr, RESCHED_eager);
> >> -		set_preempt_need_resched();
> >> +		set_tsk_need_resched(curr, rs);
> >> +		if (rs == RESCHED_eager)
> >> +			set_preempt_need_resched();
> >>  		return;
> >>  	}
> >>
> >> -	if (set_nr_and_not_polling(curr, RESCHED_eager))
> >> -		smp_send_reschedule(cpu);
> >> -	else
> >> +	if (set_nr_and_not_polling(curr, rs)) {
> >> +		if (rs == RESCHED_eager)
> >> +			smp_send_reschedule(cpu);
> >
> > I think you just broke things.
> >
> > Not all idle threads have POLLING support, in which case you need that
> > IPI to wake them up, even if it's LAZY.
> 
> Yes, I was concerned about that too. But doesn't this check against the
> idle_sched_class in resched_curr() cover that?

I that's what that was. Hmm, maybe.

I mean, we have idle-injection too, those don't as FIFO, but as such,
they can only get preempted from RT/DL, and those will already force
preempt anyway.

The way you've split and structured the code makes it very hard to
follow. Something like:

	if (set_nr_and_not_polling(curr, rs) &&
	    (rs == RESCHED_force || is_idle_task(curr)))
		smp_send_reschedule();

is *far* clearer, no?