Re: [PATCH v2] sched: async unthrottling for cfs bandwidth

[Peter Zijlstra]

On Mon, Oct 31, 2022 at 02:56:13PM -0700, Benjamin Segall wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> 
> > On Wed, Oct 26, 2022 at 03:44:49PM -0700, Josh Don wrote:
> >> CFS bandwidth currently distributes new runtime and unthrottles cfs_rq's
> >> inline in an hrtimer callback. Runtime distribution is a per-cpu
> >> operation, and unthrottling is a per-cgroup operation, since a tg walk
> >> is required. On machines with a large number of cpus and large cgroup
> >> hierarchies, this cpus*cgroups work can be too much to do in a single
> >> hrtimer callback: since IRQ are disabled, hard lockups may easily occur.
> >> Specifically, we've found this scalability issue on configurations with
> >> 256 cpus, O(1000) cgroups in the hierarchy being throttled, and high
> >> memory bandwidth usage.
> >> 
> >> To fix this, we can instead unthrottle cfs_rq's asynchronously via a
> >> CSD. Each cpu is responsible for unthrottling itself, thus sharding the
> >> total work more fairly across the system, and avoiding hard lockups.
> >
> > So, TJ has been complaining about us throttling in kernel-space, causing
> > grief when we also happen to hold a mutex or some other resource and has
> > been prodding us to only throttle at the return-to-user boundary.
> >
> > Would this be an opportune moment to do this? That is, what if we
> > replace this CSD with a task_work that's ran on the return-to-user path
> > instead?
> 
> This is unthrottle, not throttle, but it would probably be

Duh..