Re: [PATCH 2/2] sched/ext: Add BPF functions for uclamp inc and dec

[Hongyan Xia <hongyan.xia2@arm.com>]

On 03/07/2024 19:15, Tejun Heo wrote:
> Hello.
> 
> On Wed, Jul 03, 2024 at 11:07:48AM +0100, Hongyan Xia wrote:
>> +__bpf_kfunc void scx_bpf_uclamp_rq_inc(s32 cpu, struct task_struct *p)
>> +{
>> +	uclamp_rq_inc(cpu_rq(cpu), p);
>> +}
>> +
>> +__bpf_kfunc void scx_bpf_uclamp_rq_dec(s32 cpu, struct task_struct *p)
>> +{
>> +	uclamp_rq_dec(cpu_rq(cpu), p);
>> +}
> 
> So, I don't think we can expose these functions directly to the BPF
> scheduler. The BPF schedulers shouldn't be able to break system integrity no
> matter what they do and with the above it'd be trivial to get the bucket
> counters unbalanced, right?

You are right.

Actually, avoiding double enqueue or dequeue is easy and might be just a 
one-line change. The real concern is when the BPF scheduler somehow 
still has tasks on uclamp buckets when it's unloaded. Then, unloading 
the scheduler needs to do uclamp_dec().

I'll see what I can do.