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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Mar 28, 2022 at 07:07:51PM +0800, Chengming Zhou wrote:
> With nohz_full enabled on cpu, the scheduler_tick() will be stopped
> when only one CFS task left on rq.
> 
> scheduler_tick()
>   task_tick_fair()
>     entity_tick()
>       update_curr()
>         account_cfs_rq_runtime(cfs_rq, delta_exec) --> stopped
> 
> So that running task can't account its runtime periodically, but
> the cfs_bandwidth hrtimer still __refill_cfs_bandwidth_runtime()
> periodically. Later in one period, the task would account very
> big delta_exec, which cause the cfs_rq to be throttled for a
> long time.
> 
> There are two solutions for the problem, the first is that we
> can check in sched_can_stop_tick() if current task's cfs_rq
> have runtime_enabled, in which case we don't stop tick. But
> it will make nohz_full almost useless in cloud environment
> that every container has the cpu bandwidth control setting.

How is NOHZ_FULL useful in that environment to begin with? If you set
bandwidth crap, the expectation is that there is overcommit, which more
or less assumes lots of scheduling, presumably VMs or somesuch crud.

So how does NOHZ_FULL make sense?