linux-kernel - Re: [RFC v3 2/6] Improve the tracking of active utilisation

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Message-ID: <20161110100428.GH16920@e106622-lin>
Date:   Thu, 10 Nov 2016 10:04:28 +0000
From:   Juri Lelli <juri.lelli@....com>
To:     luca abeni <luca.abeni@...tn.it>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC v3 2/6] Improve the tracking of active utilisation

On 02/11/16 03:35, Luca Abeni wrote:
> On Tue, 1 Nov 2016 22:46:33 +0100
> luca abeni <luca.abeni@...tn.it> wrote:
> [...]
> > > > @@ -1074,6 +1161,14 @@ select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
> > > >  	}
> > > >  	rcu_read_unlock();
> > > >  
> > > > +	rq = task_rq(p);
> > > > +	raw_spin_lock(&rq->lock);
> > > > +	if (hrtimer_active(&p->dl.inactive_timer)) {
> > > > +		sub_running_bw(&p->dl, &rq->dl);
> > > > +		hrtimer_try_to_cancel(&p->dl.inactive_timer);    
> > > 
> > > Can't we subtract twice if it happens that after we grabbed rq_lock the timer
> > > fired, so it's now waiting for that lock and it goes ahead and sub_running_bw
> > > again after we release the lock?  
> > Uhm... I somehow convinced myself that this could not happen, but I do not
> > remember the details, sorry :(
> I think I remember the answer now: pi_lock is acquired before invoking select_task_rq
> and is released after invoking enqueue_task... So, if there is a pending inactive
> timer, its handler will be executed after the task is enqueued... It will see the task
> as RUNNING, and will not decrease the active utilisation.
> 

Oh, because we do task_rq_lock() inactive_task_timer(). So, that should
save us from the double subtract. Would you mind adding something along
the line of what you said above as a comment for next version?

Thanks,

- Juri