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

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Date:   Thu, 10 Nov 2016 13:15:58 +0100
From:   luca abeni <luca.abeni@...tn.it>
To:     Juri Lelli <juri.lelli@....com>
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 Thu, 10 Nov 2016 11:56:10 +0000
Juri Lelli <juri.lelli@....com> wrote:

> On 10/11/16 10:04, Juri Lelli wrote:
> > 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? 
> 
> Mmm, wait again.
> 
> Cannot the following happen?
> 
>  - inactive_timer fires and does sub_running_bw (as the task is not
>    RUNNING)
>  - another cpu does try_to_wake_up and blocks on pi_lock
>  - inactive timer releases both pi and rq locks (but is still
> executing, let's say it is doing put_task_struct())
>  - try_to_wake_up goes ahead and calls select_task_rq_dl
>    + it finds inactive_timer active
>    + sub_running_bw again :(
Uhm... Right; this can happen :(

Ok; I'll think about some possible solution for this race... If I do
not find any simple way to solve it, I'll add a "contending" flag,
which allows to know if the inactive timer handler already executed or
not.

BTW, talking about sched_dl_entity flags: I see there are three
different int fields "dl_throttled, "dl_boosted" and "dl_yielded"; any
reason for doing this instead of having a "dl_flags" field and setting
its different bits when the entity is throttled, boosted or yielded? In
other words: if I need this "contending" flag, should I add a new
"dl_contending" field?



			Thanks,
				Luca