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From: Xuewen Yan <xuewen.yan94@gmail.com>
Date: Thu, 18 Apr 2024 10:57:22 +0800
Message-ID: <CAB8ipk-fejQ41Jgk6z52+T6CP+impwbaOAfhA9vG_-FB9BeRyw@mail.gmail.com>
Subject: Re: [RFC PATCH] sched/eevdf: Return leftmost entity in pick_eevdf()
 if no eligible entity is found
To: Chen Yu <yu.c.chen@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>, 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>
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Hi Yu

On Thu, Apr 18, 2024 at 2:35=E2=80=AFAM Chen Yu <yu.c.chen@intel.com> wrote=
:
>
> On 2024-04-09 at 11:21:04 +0200, Peter Zijlstra wrote:
> > On Mon, Apr 08, 2024 at 09:11:39PM +0800, Chen Yu wrote:
> > > On 2024-04-08 at 13:58:33 +0200, Peter Zijlstra wrote:
> > > > On Thu, Feb 29, 2024 at 05:00:18PM +0800, Abel Wu wrote:
> > > >
> > > > > > According to the log, vruntime is 18435852013561943404, the
> > > > > > cfs_rq->min_vruntime is 763383370431, the load is 629 + 2048 =
=3D 2677,
> > > > > > thus:
> > > > > > s64 delta =3D (s64)(18435852013561943404 - 763383370431) =3D -1=
0892823530978643
> > > > > >      delta * 2677 =3D 7733399554989275921
> > > > > > that is to say, the multiply result overflow the s64, which tur=
ns the
> > > > > > negative value into a positive value, thus eligible check fails=
.
> > > > >
> > > > > Indeed.
> > > >
> > > > From the data presented it looks like min_vruntime is wrong and nee=
ds
> > > > update. If you can readily reproduce this, dump the vruntime of all
> > > > tasks on the runqueue and see if min_vruntime is indeed correct.
> > > >
> > >
> > > This was the dump of all the entities on the tree, from left to right=
,
> >
> > Oh, my bad, I thought it was the pick path.
> >
> > > and also from top down in middle order traverse, when this issue happ=
ens:
> > >
> > > [  514.461242][ T8390] cfs_rq avg_vruntime:386638640128 avg_load:2048=
 cfs_rq->min_vruntime:763383370431
> > > [  514.535935][ T8390] current on_rq se 0xc5851400, deadline:18435852=
013562231446
> > >                     min_vruntime:18437121115753667698 vruntime:184358=
52013561943404, load:629
> > >
> > >
> > > [  514.536772][ T8390] Traverse rb-tree from left to right
> > > [  514.537138][ T8390]  se 0xec1234e0 deadline:763384870431 min_vrunt=
ime:763383370431 vruntime:763383370431 non-eligible  <-- leftmost se
> > > [  514.537835][ T8390]  se 0xec4fcf20 deadline:763762447228 min_vrunt=
ime:763760947228 vruntime:763760947228 non-eligible
> > >
> > > [  514.538539][ T8390] Traverse rb-tree from topdown
> > > [  514.538877][ T8390]  middle se 0xec1234e0 deadline:763384870431 mi=
n_vruntime:763383370431 vruntime:763383370431 non-eligible   <-- root se
> > > [  514.539605][ T8390]  middle se 0xec4fcf20 deadline:763762447228 mi=
n_vruntime:763760947228 vruntime:763760947228 non-eligible
> > >
> > > The tree looks like:
> > >
> > >           se (0xec1234e0)
> > >                   |
> > >                   |
> > >                   ----> se (0xec4fcf20)
> > >
> > >
> > > The root se 0xec1234e0 is also the leftmost se, its min_vruntime and
> > > vruntime are both 763383370431, which is aligned with
> > > cfs_rq->min_vruntime. It seems that the cfs_rq's min_vruntime gets
> > > updated correctly, because it is monotonic increasing.
> >
> > Right.
> >
> > > My guess is that, for some reason, one newly forked se in a newly
> > > created task group, in the rb-tree has not been picked for a long
> > > time(maybe not eligible). Its vruntime stopped at the negative
> > > value(near (unsigned long)(-(1LL << 20)) for a long time, its vruntim=
e
> > > is long behind the cfs_rq->vruntime, thus the overflow happens.
> >
> > I'll have to do the math again, but that's something in the order of no=
t
> > picking a task in about a day, that would be 'bad' :-)
> >
> > Is there any sane way to reproduce this, and how often does it happen?
>
> After adding some ftrace in place_entity() and pick_eevdf(), with the
> help from Yujie in lkp, the issue was reproduced today. The reason why se=
's vruntime
> is very small seems to be related to task group's reweight_entity():
>
> vlag =3D (s64)(avruntime - se->vruntime);
> vlag =3D div_s64(vlag * old_weight, weight);
> se->vruntime =3D avruntime - vlag;
>
> The vlag above is not limited by neither 2*se->slice nor TICK_NSEC,
> if the new weight is very small, which is very likely, then the vlag
> could be very large, results in a very small vruntime.
>
>
> The followings are the details why I think above could bring problems:
>
> Here is the debug log printed by place_entity():
>
>
> [  397.597268]cfs_rq:0xe75f7100
>               cfs_rq.avg_vruntime:-1111846207333767
>               cfs_rq.min_vruntime:810640668779
>               avg_vruntime():686982466017
>               curr(0xc59f4f20 rb_producer weight:15 vruntime:144777319665=
4 sum_exec_ns:187707021870 ctx(0 73)
>               leftmost(0xeacb6e00 vruntime:332464705486 sum_exec_ns:78776=
125437 load:677)
> ..
>
> [  397.877251]cfs_rq:0xe75f7100
>               cfs_rq.avg_vruntime:-759390883821798
>               cfs_rq.min_vruntime:810640668779
>               avg_vruntime(): 689577229374
>               curr(0xc59f4f20 rb_producer weight:15 vruntime:145364090799=
8 sum_ns:187792974673 ctx(0 73)
>               leftmost(0xeacb6e00 vruntime:-59752941080010 sum_ns:7877612=
5437 load:4)
>
>
> The leftmost se is a task group, its vruntime reduces from 332464705486 t=
o
> -59752941080010, because its load reduced from 677 to 4 due to update_cfs=
_group()
> on the tree entities.
>
> Back to reweight_entity():
> vlag =3D avruntime - se->vruntime =3D 689577229374 - 332464705486 =3D 357=
112523888;
> vlag =3D vlag * old_weight / weight =3D 357112523888 * 677 / 4 =3D 604412=
94668044;
> se->vruntime =3D avruntime - vlag =3D -59751717438670;
>
> the new se vruntime -59751717438670 is close to what we printed -59752941=
080010,
> consider that the avg_vruntime() vary.
>
> Then later this leftmost se has changed its load back and forth, and when=
 the load is 2,
> the vuntime has reached a dangerous threshold to trigger the s64 overflow=
 in
> eligible check:
>
> [  398.011991]cfs_rq:0xe75f7100
>               cfs_rq.avg_vruntime:-11875977385353427
>               cfs_rq.min_vruntime:810640668779
>               cfs_rq.avg_load:96985
>               leftmost(0xeacb6e00 vruntime:18446623907344963655 load:2)
>
> vruntime_eligible()
> {
>
>    key =3D se.vruntime - cfs_rq.min_vruntime =3D -120977005256740;
>    key * avg_load overflow s64...
> }
>
> As a result the leftmost one can not be picked, and NULL is returned.
>
> One workaround patch I'm thinking of, if this analysis is in the
> right direction, maybe I can have a test later:
>
> thanks,
> Chenyu
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6e0968fb9ba8..7ab26cdc3487 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3965,8 +3965,13 @@ static void reweight_eevdf(struct cfs_rq *cfs_rq, =
struct sched_entity *se,
>          *         =3D V  - vl'
>          */
>         if (avruntime !=3D se->vruntime) {
> +               s64 limit;
> +
>                 vlag =3D (s64)(avruntime - se->vruntime);
>                 vlag =3D div_s64(vlag * old_weight, weight);
> +               /* TBD: using old weight or new weight? */
> +               limit =3D calc_delta_fair(max_t(u64, 2*se->slice, TICK_NS=
EC), se);
> +               vlag =3D clamp(lag, -limit, limit);
>                 se->vruntime =3D avruntime - vlag;
>         }
>

According to previous discussion:
https://lore.kernel.org/all/CAB8ipk9N9verfQp6U9s8+TQgNbA5J0DWkOB1dShf20n0xb=
x94w@mail.gmail.com/

Could this patch avoid this problem?

BR

> --
> 2.25.1
>
>
>