linux-kernel - Re: [PATCH v3 08/57] sched: Simplify wake_up_if

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Message-ID: <20230612200229.GN4253@hirez.programming.kicks-ass.net>
Date:   Mon, 12 Jun 2023 22:02:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
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Subject: Re: [PATCH v3 08/57] sched: Simplify wake_up_if_idle()

On Mon, Jun 12, 2023 at 11:02:28AM -0700, Boqun Feng wrote:
> On Mon, Jun 12, 2023 at 11:07:21AM +0200, Peter Zijlstra wrote:
> > Use guards to reduce gotos and simplify control flow.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  kernel/sched/core.c  |   20 ++++++--------------
> >  kernel/sched/sched.h |   15 +++++++++++++++
> >  2 files changed, 21 insertions(+), 14 deletions(-)
> > 
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3872,21 +3872,13 @@ static void __ttwu_queue_wakelist(struct
> >  void wake_up_if_idle(int cpu)
> >  {
> >  	struct rq *rq = cpu_rq(cpu);
> > -	struct rq_flags rf;
> >  
> > -	rcu_read_lock();
> > -
> > -	if (!is_idle_task(rcu_dereference(rq->curr)))
> > -		goto out;
> > -
> > -	rq_lock_irqsave(rq, &rf);
> > -	if (is_idle_task(rq->curr))
> > -		resched_curr(rq);
> > -	/* Else CPU is not idle, do nothing here: */
> > -	rq_unlock_irqrestore(rq, &rf);
> > -
> > -out:
> > -	rcu_read_unlock();
> > +	guard(rcu)();
> > +	if (is_idle_task(rcu_dereference(rq->curr))) {
> > +		guard(rq_lock)(rq);
> 
> We assume that irq must be disabled when this function called?
> Otherwise, I don't understand why this is not
> 
> 	guard(rq_lock_irqsave)(rq);
> 

You're quite right, I messed that up.