Re: [PATCH 0/7 v4] move update blocked load outside newidle_balance

[Peter Zijlstra]

On Wed, Feb 24, 2021 at 02:30:00PM +0100, Vincent Guittot wrote:
> Joel reported long preempt and irq off sequence in newidle_balance because
> of a large number of CPU cgroups in use and having to be updated. This
> patchset moves the update outside newidle_imblance. This enables to early
> abort during the updates in case of pending irq as an example.
> 
> Instead of kicking a normal ILB that will wakes up CPU which is already
> idle, patch 6 triggers the update of statistics in the idle thread of
> the CPU before selecting and entering an idle state.

I'm confused... update_blocked_averages(), which calls
__update_blocked_fair(), which is the one doing the cgroup iteration
thing, runs with rq->lock held, and thus will have IRQs disabled any
which way around we turn this thing.

Or is the problem that we called nohz_idle_balance(), which does
update_nohz_stats() -> update_blocked_averages() for evey NOHZ cpu from
newidle balance, such that we get NR_NOHZ_CPUS * NR_CGROUPS IRQ latency?
Which is now reduced to just NR_CGROUPS ?