Re: [External] Re: [PATCH] sched/fair: fix broken bandwidth control with nohz_full

[Chengming Zhou]

Hi,

On 2022/3/31 03:14, Phil Auld wrote:
> Hi Peter,
> 
> On Wed, Mar 30, 2022 at 08:23:27PM +0200 Peter Zijlstra wrote:
>> On Mon, Mar 28, 2022 at 12:44:54PM -0400, Steven Rostedt wrote:
>>> On Mon, 28 Mar 2022 17:56:07 +0200
>>> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>>
>>>>> echo $$ > test/cgroup.procs
>>>>> taskset -c 1 bash -c "while true; do let i++; done"  --> will be throttled  
>>>>
>>>> Ofcourse.. I'm arguing that bandiwdth control and NOHZ_FULL are somewhat
>>>> mutually exclusive, use-case wise. So I really don't get why you'd want
>>>> them both.
>>>
>>> Is it?
>>>
>>> One use case I can see for having both is for having a deadline task that
>>> needs to get something done in a tight deadline. NOHZ_FULL means "do not
>>> interrupt this task when it is the top priority task on the CPU and is
>>> running in user space".
>>
>> This is absolute batshit.. It means no such thing. We'll happily wake
>> another task to this CPU and re-enable the tick any instant.
>>
>> Worse; the use-case at hand pertains to cfs bandwidth control, which
>> pretty much guarantees there *will* be an interrupt.
> 
> The problem is (at least in some cases) that container orchestration userspace
> code allocates a whole CPU by setting quota == period.  Or 3 cpus as 3*period etc.
> 
> In cases where an isolated task is expected to run uninterrupted (only task in 
> the system affined to that cpu, nohz_full, nocbs etc) you can end up with it
> getting throttled even though it theoritically has enough bandwidth for the full
> cpu and therefore should never get throttled. 
> 
> There are radio network setups where the packet processing is isolated
> like this but the system as a whole is managed by container orchestration so 
> everything has cfs bandwidth quotas set.
> 
> I don't think generally the bandwidth controls in these cases are used for 
> CPU sharing (quota < period). I agree that doesn't make much sense with NOHZ_FULL
> and won't work right. 
> 
> It's doled out as full cpu(s) in these cases.
> 
> Thats not a VM case so is likely different from the one that started this thread
> but I thought I should mention it.

Yes, it's a different use-case from ours. Thanks for sharing with us. I should
put these in the patch log and send an updated version.

Thanks.

> 
> 
> Cheers,
> Phil
> 
>>
>>> Why is it mutually exclusive to have a deadline task that does not want to
>>> be interrupted by timer interrupts?
>>
>> This has absolutely nothing to do with deadline tasks, nada, noppes.
>>
>>> Just because the biggest pushers of NOHZ_FULL is for those that are running
>>> RT tasks completely in user space and event want to fault if it ever goes
>>> into the kernel, doesn't mean that's the only use case.
>>
>> Because there's costs associated with the whole thing. system entry/exit
>> get far more expensive. It just doesn't make much sense to use NOHZ_FULL
>> if you're not absoultely limiting system entry.
>>
>>> Chengming brought up VMs. That's a case to want to control the bandwidth,
>>> but also not interrupt them with timer interrupts when they are running as
>>> the top priority task on a CPU.
>>
>> It's CFS, there is nothing top priority about that.
>>
>