linux-kernel - Re: [tip:sched/core] sched/rt: Simplify the IPI based RT balancing logic

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Message-ID: <20180119154430.GC14011@codeaurora.org>
Date:   Fri, 19 Jan 2018 21:14:30 +0530
From:   Pavan Kondeti <pkondeti@...eaurora.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     williams@...hat.com, Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, bristot@...hat.com,
        jkacur@...hat.com, efault@....de, hpa@...or.com,
        torvalds@...ux-foundation.org, swood@...hat.com,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched/rt: Simplify the IPI based RT balancing
 logic

Hi Steven,

On Fri, Jan 19, 2018 at 10:03:53AM -0500, Steven Rostedt wrote:
> On Fri, 19 Jan 2018 14:53:05 +0530
> Pavan Kondeti <pkondeti@...eaurora.org> wrote:
> 
> > I am seeing "spinlock already unlocked" BUG for rd->rto_lock on a 4.9
> > stable kernel based system. This issue is observed only after
> > inclusion of this patch. It appears to me that rq->rd can change
> > between spinlock is acquired and released in rto_push_irq_work_func()
> > IRQ work if hotplug is in progress. It was only reported couple of
> > times during long stress testing. The issue can be easily reproduced
> > if an artificial delay is introduced between lock and unlock of
> > rto_lock. The rq->rd is changed under rq->lock, so we can protect this
> > race with rq->lock. The below patch solved the problem. we are taking
> > rq->lock in pull_rt_task()->tell_cpu_to_push(), so I extended the same
> > here. Please let me know your thoughts on this.
> 
> As so rq->rd can change. Interesting.
> 
> > 
> > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > index d863d39..478192b 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -2284,6 +2284,7 @@ void rto_push_irq_work_func(struct irq_work *work)
> >                 raw_spin_unlock(&rq->lock);
> >         }
> > 
> > +       raw_spin_lock(&rq->lock);
> 
> 
> What about just saving the rd then?
> 
> 	struct root_domain *rd;
> 
> 	rd = READ_ONCE(rq->rd);
> 
> then use that. Then we don't need to worry about it changing.
> 

Yeah, it should work. I will give it a try and send the patch
for review.

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