linux-kernel - Re: [PATCH V4 1/2] sched/fair: Fix load

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Date:   Mon, 5 Jun 2017 12:33:48 -0600
From:   Jeffrey Hugo <jhugo@...eaurora.org>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Austin Christ <austinwc@...eaurora.org>,
        Tyler Baicar <tbaicar@...eaurora.org>,
        Timur Tabi <timur@...eaurora.org>
Subject: Re: [PATCH V4 1/2] sched/fair: Fix load_balance() affinity redo path

On 6/5/2017 11:23 AM, Jeffrey Hugo wrote:
> On 6/2/2017 4:27 PM, Jeffrey Hugo wrote:
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index d711093..84255ab 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6737,10 +6737,10 @@ int can_migrate_task(struct task_struct *p, 
>> struct lb_env *env)
>>            * our sched_group. We may want to revisit it if we couldn't
>>            * meet load balance goals by pulling other tasks on src_cpu.
>>            *
>> -         * Also avoid computing new_dst_cpu if we have already computed
>> -         * one in current iteration.
>> +         * Avoid computing new_dst_cpu for NEWLY_IDLE or if we have
>> +         * already computed one in current iteration.
>>            */
>> -        if (!env->dst_grpmask || (env->flags & LBF_DST_PINNED))
>> +        if (env->idle == CPU_NEWLY_IDLE || (env->flags & 
>> LBF_DST_PINNED))
>>               return 0;
> 
> Self NACK.  This breaks active_load_balance_cpu_stop().  Looks like 
> env->idle == CPU_IDLE, but env->dst_grpmask is uninitialized, so it can 
> be NULL, which causes a null pointer dereference a few lines later.
> 
> I'm still having a look to see what makes sense to address the issue.
> 
> 

As far as I can see, there appears to be two options to resolve the issue -

1. Update active_load_balance_cpu_stop() to initialize dst_grpmask to a 
sane value

2. Undo the proposed changes in load_balance() to "ensure" dst_grpmask 
is valid, and calculate the value on demand when checking to see if the 
redo path needs to be done.

The downside to #1 is that dst_grpmask is not needed in the 
active_load_balance_cpu_stop() path, and the loop to calculate a new 
dst_cpu will be used.  Extra code is evaluated, but there appears to be 
no side effects.

The downside to #2 is that dst_grpmask is valid the majority of the time 
in load_balance(), so calculating it on demand is redundant most of the 
time, but again there appears to be no side effects.

It somewhat feels like a choice of which option is less bad.

Peter/Dietmar, any preferences?

-- 
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.