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

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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: eab03c23c2 ("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
This commit is contained in:
Xuewen Yan 2024-04-22 16:22:38 +08:00 committed by Peter Zijlstra
parent afae8002b4
commit 1560d1f6eb
1 changed files with 12 additions and 6 deletions
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18
kernel/sched/fair.c
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@ -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 s64 entity_lag(u64 avruntime, struct sched_entity *se)
{
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)
{
s64 lag, limit;
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;
}