sched/eevdf: Prevent vlag from going out of bounds in reweight_eevdf()

It was possible to have pick_eevdf() return NULL, which then causes a
NULL-deref. This turned out to be due to entity_eligible() returning
falsely negative because of a s64 multiplcation overflow.

Specifically, reweight_eevdf() computes the vlag without considering
the limit placed upon vlag as update_entity_lag() does, and then the
scaling multiplication (remember that weight is 20bit fixed point) can
overflow. This then leads to the new vruntime being weird which then
causes the above entity_eligible() to go side-ways and claim nothing
is eligible.

Thus limit the range of vlag accordingly.

All this was quite rare, but fatal when it does happen.

Closes: https://lore.kernel.org/all/ZhuYyrh3mweP_Kd8@nz.home/
Closes: https://lore.kernel.org/all/CA+9S74ih+45M_2TPUY_mPPVDhNvyYfy1J1ftSix+KjiTVxg8nw@mail.gmail.com/
Closes: https://lore.kernel.org/lkml/202401301012.2ed95df0-oliver.sang@intel.com/
Fixes: eab03c23 ("sched/eevdf: Fix vruntime adjustment on reweight")
Reported-by: Sergei Trofimovich <slyich@gmail.com>
Reported-by: Igor Raits <igor@gooddata.com>
Reported-by: Breno Leitao <leitao@debian.org>
Reported-by: kernel test robot <oliver.sang@intel.com>
Reported-by: Yujie Liu <yujie.liu@intel.com>
Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com>
Reviewed-and-tested-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240422082238.5784-1-xuewen.yan@unisoc.com

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);
 }
 
 /*
@@ -3760,7 +3766,7 @@ static void reweight_eevdf(struct sched_entity *se, u64 avruntime,
 	 *	   = 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;
 	}