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Message-ID: <6f365e5b-220c-4b2e-8a56-a5135b216e62@arm.com>
Date: Fri, 13 Sep 2024 19:36:27 +0200
From: Pierre Gondois <pierre.gondois@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org,
Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH] sched/fair: Fix integer underflow
Hello Vincent,
On 9/13/24 18:14, Vincent Guittot wrote:
> Hi Pierre
>
> On Fri, 13 Sept 2024 at 10:58, Pierre Gondois <pierre.gondois@....com> wrote:
>>
>> (struct sg_lb_stats).idle_cpus is of type 'unsigned int'.
>> (local->idle_cpus - busiest->idle_cpus) can underflow to UINT_MAX
>> for instance, and max_t(long, 0, UINT_MAX) will output UINT_MAX.
>>
>> Use lsub_positive() instead of max_t().
>
> Have you faced the problem or this patch is based on code review ?
>
> we have the below in sched_balance_find_src_group() that should ensure
> that we have local->idle_cpus > busiest->idle_cpus
>
> if (busiest->group_weight > 1 &&
> local->idle_cpus <= (busiest->idle_cpus + 1)) {
> /*
> * If the busiest group is not overloaded
> * and there is no imbalance between this and busiest
> * group wrt idle CPUs, it is balanced. The imbalance
> * becomes significant if the diff is greater than 1
> * otherwise we might end up to just move the imbalance
> * on another group. Of course this applies only if
> * there is more than 1 CPU per group.
> */
> goto out_balanced;
> }
It was with this setup:
- busiest->group_type == group_overloaded
so it might not have checked the value. I can check the exact path if desired,
Regards,
Pierre
>
>>
>> Fixes: 0b0695f2b34a ("sched/fair: Rework load_balance()")
>> cc: stable@...r.kernel.org
>> Signed-off-by: Pierre Gondois <pierre.gondois@....com>
>> ---
>> kernel/sched/fair.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 9057584ec06d..6d9124499f52 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -10775,8 +10775,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>> * idle CPUs.
>> */
>> env->migration_type = migrate_task;
>> - env->imbalance = max_t(long, 0,
>> - (local->idle_cpus - busiest->idle_cpus));
>> + env->imbalance = local->idle_cpus;
>> + lsub_positive(&env->imbalance, busiest->idle_cpus);
>> }
>>
>> #ifdef CONFIG_NUMA
>> --
>> 2.25.1
>>
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