Received-SPF: pass (google.com: domain of linux-kernel+bounces-151221-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Precedence: bulk
MIME-Version: 1.0
References: <20240226082349.302363-1-yu.c.chen@intel.com> <758ebf4e-ee84-414b-99ec-182537bcc168@bytedance.com>
 <20240408115833.GF21904@noisy.programming.kicks-ass.net> <ZhPtCyRmPxa0DpMe@chenyu5-mobl2>
 <20240409092104.GA2665@noisy.programming.kicks-ass.net> <ZiAWTU5xb/JMn/Hs@chenyu5-mobl2>
 <CAB8ipk-fejQ41Jgk6z52+T6CP+impwbaOAfhA9vG_-FB9BeRyw@mail.gmail.com>
 <ZiEaKOQwiNEglYtS@chenyu5-mobl2> <20240419082440.GB6345@noisy.programming.kicks-ass.net>
In-Reply-To: <20240419082440.GB6345@noisy.programming.kicks-ass.net>
From: Xuewen Yan <xuewen.yan94@gmail.com>
Date: Fri, 19 Apr 2024 17:17:17 +0800
Message-ID: <CAB8ipk_B7zsQt2wa5pda4ehN6f0yHi_mV=unVThnOYTKQz=5aw@mail.gmail.com>
Subject: Re: [RFC PATCH] sched/eevdf: Return leftmost entity in pick_eevdf()
 if no eligible entity is found
To: Peter Zijlstra <peterz@infradead.org>
Cc: Chen Yu <yu.c.chen@intel.com>, Abel Wu <wuyun.abel@bytedance.com>, 
	Ingo Molnar <mingo@redhat.com>, Vincent Guittot <vincent.guittot@linaro.org>, 
	Juri Lelli <juri.lelli@redhat.com>, Tim Chen <tim.c.chen@intel.com>, 
	Tiwei Bie <tiwei.btw@antgroup.com>, Honglei Wang <wanghonglei@didichuxing.com>, 
	Aaron Lu <aaron.lu@intel.com>, Chen Yu <yu.chen.surf@gmail.com>, 
	Yujie Liu <yujie.liu@intel.com>, linux-kernel@vger.kernel.org, 
	kernel test robot <oliver.sang@intel.com>, Tianchen Ding <dtcccc@linux.alibaba.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Fri, Apr 19, 2024 at 4:24=E2=80=AFPM Peter Zijlstra <peterz@infradead.or=
g> wrote:
>
> On Thu, Apr 18, 2024 at 09:03:36PM +0800, Chen Yu wrote:
>
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 31bca05c3612..9f203012e8f5 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -696,15 +696,23 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
> >   *
> >   * XXX could add max_slice to the augmented data to track this.
> >   */
> > +
> > +static s64 limit_entity_lag(struct sched_entity *se, s64 lag)
> > +{
> > +     s64 limit;
> > +
> > +     limit =3D calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se)=
;
> > +     return clamp(lag, -limit, limit);
> > +}
>
> Right, helper makes sense.
>
> > @@ -3721,6 +3729,7 @@ static void reweight_eevdf(struct cfs_rq *cfs_rq,=
 struct sched_entity *se,
> >       if (avruntime !=3D se->vruntime) {
> >               vlag =3D (s64)(avruntime - se->vruntime);
> >               vlag =3D div_s64(vlag * old_weight, weight);
> > +             vlag =3D limit_entity_lag(se, vlag);
> >               se->vruntime =3D avruntime - vlag;
>
> So the !on_rq case has clamping in update_entity_lag() which is before
> scaling. And that makes more sense to me, because putting a limit on
> vlag before the multiplication *should* ensure the multiplication itself
> doesn't overflow.
>
> But now you allow it to compute garbage and then clip the garbage.
>
> >       }
> >
> > @@ -3768,6 +3777,9 @@ static void reweight_entity(struct cfs_rq *cfs_rq=
, struct sched_entity *se,
> >
> >       update_load_set(&se->load, weight);
> >
> > +     if (!se->on_rq)
> > +             se->vlag =3D limit_entity_lag(se, se->vlag);
> > +
>
> Except you now add clamping after scaling too, but in a really weird
> place. Should this not go right after the div_s64() that scales?

Yes, it's better to put it  after the div_s64().

>
> Unlike the reweight_eevdf() case, there might be an argument for doing
> it after scaling in this case. Namely, you can have multiple reweights
> stacking their scale ops.
>
>
> Also, could you put a few words in on how often these clips are hit? I
> suspect it's fairly rare (but crucial when it does).

I think this probability may be not small, because the user space
often modifies the task's nice value while the task is sleeping.
I also would add logs to check it.

BR