linux-kernel - Re: [PATCH v2] sched/eevdf: Prevent vlag from going out of bounds when reweight

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Date: Mon, 22 Apr 2024 16:33:37 +0800
From: Xuewen Yan <xuewen.yan94@...il.com>
To: Xuewen Yan <xuewen.yan@...soc.com>
Cc: peterz@...radead.org, mingo@...hat.com, juri.lelli@...hat.com, 
	vincent.guittot@...aro.org, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com, 
	yu.c.chen@...el.com, ke.wang@...soc.com, di.shen@...soc.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/eevdf: Prevent vlag from going out of bounds
 when reweight_eevdf

Hi Peter,

Because the issue may be urgent for many people, I sent the patch first.
However, when I test on the Android system, I find there still are
vlags which exceed the limit.
On the Android system, the nice value of a task will change very
frequently. The limit can also be exceeded.
Maybe the !on_rq case is still necessary.
So I'm planning to propose another patch for !on_rq case later after
careful testing locally.

BR

On Mon, Apr 22, 2024 at 4:23 PM Xuewen Yan <xuewen.yan@...soc.com> wrote:
>
> kernel encounters the following error when running workload:
>
> BUG: kernel NULL pointer dereference, address: 0000002c
> EIP: set_next_entity (fair.c:?)
>
> which was caused by NULL pointer returned by pick_eevdf().
>
> Further investigation has shown that, the entity_eligible() has an
> false-negative issue when the entity's vruntime is far behind the
> cfs_rq.min_vruntime that, the (vruntime - cfs_rq->min_vruntime) * load
> caused a s64 overflow, thus every entity on the rb-tree is not
> eligible, which results in a NULL candidate.
>
> The reason why entity's vruntime is far behind the cfs_rq.min_vruntime
> is because during a on_rq task group's update_cfs_group()->reweight_eevdf(),
> there is no limit on the new entity's vlag. If the new weight is much
> smaller than the old one,
>
> vlag = div_s64(vlag * old_weight, weight)
>
> generates a huge vlag, and results in very small(negative) vruntime.
>
> Thus limit the range of vlag accordingly.
>
> Reported-by: Sergei Trofimovich <slyich@...il.com>
> Closes: https://lore.kernel.org/all/ZhuYyrh3mweP_Kd8@nz.home/
> Reported-by: Igor Raits <igor@...ddata.com>
> Closes: https://lore.kernel.org/all/CA+9S74ih+45M_2TPUY_mPPVDhNvyYfy1J1ftSix+KjiTVxg8nw@mail.gmail.com/
> Reported-by: Breno Leitao <leitao@...ian.org>
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/lkml/202401301012.2ed95df0-oliver.sang@intel.com/
> Reported-by: Yujie Liu <yujie.liu@...el.com>
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
> changes of v2:
> -add reported-by (suggested by <yu.c.chen@...el.com>)
> -remork the changelog (<yu.c.chen@...el.com>)
> -remove the judgement of fork (Peter)
> -remove the !on_rq case. (Peter)
> ---
> Previous discussion link:
> https://lore.kernel.org/lkml/20240226082349.302363-1-yu.c.chen@intel.com/
> https://lore.kernel.org/all/20240130080643.1828-1-xuewen.yan@unisoc.com/
> ---
> ---
>  kernel/sched/fair.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 03be0d1330a6..64826f406d6d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -696,15 +696,21 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
>   *
>   * XXX could add max_slice to the augmented data to track this.
>   */
> -static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +static s64 entity_lag(u64 avruntime, struct sched_entity *se)
>  {
> -       s64 lag, limit;
> +       s64 vlag, limit;
> +
> +       vlag = avruntime - se->vruntime;
> +       limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se);
> +
> +       return clamp(vlag, -limit, limit);
> +}
>
> +static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +{
>         SCHED_WARN_ON(!se->on_rq);
> -       lag = avg_vruntime(cfs_rq) - se->vruntime;
>
> -       limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se);
> -       se->vlag = clamp(lag, -limit, limit);
> +       se->vlag = entity_lag(avg_vruntime(cfs_rq), se);
>  }
>
>  /*
> @@ -3761,7 +3767,7 @@ static void reweight_eevdf(struct cfs_rq *cfs_rq, struct sched_entity *se,
>          *         = V  - vl'
>          */
>         if (avruntime != se->vruntime) {
> -               vlag = (s64)(avruntime - se->vruntime);
> +               vlag = entity_lag(avruntime, se);
>                 vlag = div_s64(vlag * old_weight, weight);
>                 se->vruntime = avruntime - vlag;
>         }
> --
> 2.25.1
>
>