+Vincent
On 22/02/21 09:12, syzbot wrote: > syzbot has found a reproducer for the following issue on: > > HEAD commit: 31caf8b2 Merge branch 'linus' of git://git.kernel.org/pub/.. > git tree: upstream > console output: https://syzkaller.appspot.com/x/log.txt?x=16ab2682d00000 > kernel config: https://syzkaller.appspot.com/x/.config?x=b81388f0b32761d4 > dashboard link: https://syzkaller.appspot.com/bug?extid=d7581744d5fd27c9fbe1 > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1277457f500000 > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > Reported-by: syzbot+d7581744d5fd27c9f...@syzkaller.appspotmail.com > > ================================================================================ > UBSAN: shift-out-of-bounds in kernel/sched/fair.c:7712:14 > shift exponent 149 is too large for 64-bit type 'long unsigned int' That 149 is surprising. sd->cache_nice_tries is \in {1, 2}, and sd->nr_balanced_failed should be in the same ballpark. A successful load_balance() resets it to 0; a failed one increments it. Once it gets to sd->cache_nice_tries + 3, this should trigger an active balance, which will either set it to sd->cache_nice_tries+1 or reset it to 0. There is this one condition that could let it creep up uncontrollably: /* * Don't kick the active_load_balance_cpu_stop, * if the curr task on busiest CPU can't be * moved to this_cpu: */ if (!cpumask_test_cpu(this_cpu, busiest->curr->cpus_ptr)) { raw_spin_unlock_irqrestore(&busiest->lock, flags); goto out_one_pinned; } So despite the resulting sd->balance_interval increase, repeatedly hitting this might yield the above. Would we then want something like this? --- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 8a8bd7b13634..b65c24b5ae91 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7422,6 +7422,11 @@ struct lb_env { struct list_head tasks; }; +static inline unsigned int sd_balance_failed_cap(struct sched_domain *sd) +{ + return sd->cache_nice_tries + 3; +} + /* * Is this task likely cache-hot: */ @@ -9493,7 +9498,7 @@ imbalanced_active_balance(struct lb_env *env) * threads on a system with spare capacity */ if ((env->migration_type == migrate_task) && - (sd->nr_balance_failed > sd->cache_nice_tries+2)) + (sd->nr_balance_failed >= sd_balance_failed_cap(sd))) return 1; return 0; @@ -9737,8 +9742,10 @@ static int load_balance(int this_cpu, struct rq *this_rq, * frequent, pollute the failure counter causing * excessive cache_hot migrations and active balances. */ - if (idle != CPU_NEWLY_IDLE) - sd->nr_balance_failed++; + if (idle != CPU_NEWLY_IDLE) { + sd->nr_balance_failed = min(sd->nr_balance_failed + 1, + sd_balance_failed_cap(sd)); + } if (need_active_balance(&env)) { unsigned long flags;
