Re: [RFC PATCH v3 00/16] Core scheduling v3

[Peter Zijlstra]

On Tue, Aug 06, 2019 at 08:24:17AM -0400, Vineeth Remanan Pillai wrote:
> Peter's rebalance logic actually takes care of most of the runq
> imbalance caused
> due to cookie tagging. What we have found from our testing is, fairness issue is
> caused mostly due to a Hyperthread going idle and not waking up. Aaron's 3rd
> patch works around that. As Julien mentioned, we are working on a per thread
> coresched idle thread concept. The problem that we found was, idle thread causes
> accounting issues and wakeup issues as it was not designed to be used in this
> context. So if we can have a low priority thread which looks like any other task
> to the scheduler, things becomes easy for the scheduler and we achieve security
> as well. Please share your thoughts on this idea.

What accounting in particular is upset? Is it things like
select_idle_sibling() that thinks the thread is idle and tries to place
tasks there?

It should be possible to change idle_cpu() to not report a forced-idle
CPU as idle.

(also; it should be possible to optimize select_idle_sibling() for the
core-sched case specifically)

> The results are encouraging, but we do not yet have the coresched idle
> to not spin 100%. We will soon post the patch once it is a bit more
> stable for running the tests that we all have done so far.

There's play_idle(), which is the entry point for idle injection.

In general, I don't particularly like 'fake' idle threads, please be
very specific in describing what issues it works around such that we can
look at alternatives.