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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Thu, 29 Oct 2020 22:42:29 -0400
Message-ID: <CAEXW_YTnJWANAZNeR9b=5xUeHu1CAPq9vgYaH8WSHgApJZ21Rw@mail.gmail.com>
Subject: Re: [PATCH v8 -tip 08/26] sched/fair: Snapshot the min_vruntime of
 CPUs on force idle
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
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        Aubrey Li <aubrey.intel@gmail.com>,
        Thomas Glexiner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
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On Thu, Oct 29, 2020 at 10:36 PM Joel Fernandes <joel@joelfernandes.org> wrote:

> > > > +bool cfs_prio_less(struct task_struct *a, struct task_struct *b)
> > > > +{
> > > > +   struct rq *rq = task_rq(a);
> > > > +   struct sched_entity *sea = &a->se;
> > > > +   struct sched_entity *seb = &b->se;
> > > > +   struct cfs_rq *cfs_rqa;
> > > > +   struct cfs_rq *cfs_rqb;
> > > > +   s64 delta;
> > > > +
> > > > +   SCHED_WARN_ON(task_rq(b)->core != rq->core);
> > > > +
> > > > +   while (sea->cfs_rq->tg != seb->cfs_rq->tg) {
> > > > +           int sea_depth = sea->depth;
> > > > +           int seb_depth = seb->depth;
> > > > +
> > > > +           if (sea_depth >= seb_depth)
> > > > +                   sea = parent_entity(sea);
> > > > +           if (sea_depth <= seb_depth)
> > > > +                   seb = parent_entity(seb);
> > > > +   }
> > > > +
> > > > +   if (rq->core->core_forceidle) {
> > > > +           se_fi_update(sea, rq->core->core_forceidle_seq, true);
> > > > +           se_fi_update(seb, rq->core->core_forceidle_seq, true);
> > > > +   }
> > >
> > > As we chatted on IRC you mentioned the reason for the sync here is:
> > >
> > >  say we have 2 cgroups (a,b) under root, and we go force-idle in a, then we
> > >  update a and root. Then we pick and end up in b, but b hasn't been updated
> > >  yet.
> > >
> > > One thing I was wondering about that was, if the pick of 'b' happens much
> > > later than 'a', then the snapshot might be happening too late right?
> >
> > No, since this is the first pick in b since fi, it cannot have advanced.
> > So by updating to fi_seq before picking, we guarantee it is unchanged
> > since we went fi.
>
> Makes complete sense.
>
> I got it to a point where the latencies are much lower, but still not
> at a point where it's as good as the initial patch I posted.
>
> There could be more bugs. At the moment, the only one I corrected in
> your patch is making the truth table do !(!fib && fi). But there is
> still something else going on.

Forgot to ask, do you also need to do the task_vruntime_update() for
the unconstrained pick?

That's in line with what you mentioned: That you still need to do the
update if fi_before == false and fi_now == false.

So something like this?
@@ -4209,6 +4209,10 @@ pick_next_task(struct rq *rq, struct
task_struct *prev, struct rq_flags *rf)
                                next = p;
                                trace_printk("unconstrained pick: %s/%d %lx\n",
                                             next->comm, next->pid,
next->core_cookie);
+
+                               WARN_ON_ONCE(fi_before);
+                               task_vruntime_update(rq_i, p);
+
                                goto done;
                        }

Quoting the truth table:

> >         fib     fi      X
> >
> >         0       0       1
> >         0       1       0
> >         1       0       1
> >         1       1       1
> >

thanks,

 - Joel