Re: [PATCH 2/2] sched/fair: Avoid re-syncing runtime_enabled on root domain changes

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Nov 12, 2026 at 06:10:55AM -0500, Qiliang Yuan wrote:
> On Thu, Nov 12, 2026 at 11:56:24AM +0100, Peter Zijlstra wrote:
> > How do you end up with 4k cgroups ?!? That's wild.
> 
> It is common on hosts running AI agent harnesses: every agent sandbox
> gets its own container, and each pod and container has a cgroup with
> the cpu controller enabled, so a single kernel easily carries over a
> thousand task groups. 4000 is the upper end of that, picked to show the
> scaling.

*groan*

> > Also, why do you care about the rebuild performance? I'm not entirely
> > sure it makes sense to add data for something that 'never' happens. Do
> > we really need a new member variable for this?
> 
> On those setups it is not rare. Agent harnesses and KubeVirt both
> hot-add and remove vCPUs at run time to follow the load, and every
> hotplug rebuilds the sched domains of the guest.

So CPU hotplug is a slow path; there's nothing that's going to 'fix'
that. If it hurts, its one of those things you need to stop doing.

> A rebuild runs rq_attach_root() on every active CPU, and with the rq
> lock held and interrupts disabled it walks all task groups twice. 

Right, that is a bit daft. It should be relatively straight forward to
make that one loop.

> With 4000 cpu cgroups on a 32-CPU VM, one rq_attach_root() call takes
> 188 us at the median and up to 938 us, against 2 us with both patches
> applied (20 partition switches, 1280 calls). So every rebuild leaves
> each CPU with interrupts off for close to a millisecond in the worst
> case.

Not ideal, but again, you did do hotplug so you get to keep the pieces.

> The member is there because rq_online_fair() can't tell a rebuild from
> a CPU coming up: the CPU is active in both cases, but only the latter
> can have runtime_enabled out of date after unthrottle_offline_cfs_rqs().

So the actual hotplug code obviously knows. It should be fairly simple
to add a boolean argument to both functions and set the right value in
rq_attach_root() and sched_set_rq_{on,off}line() respectively.