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

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Message-ID: <20190806141617.GR2332@hirez.programming.kicks-ass.net>
Date:   Tue, 6 Aug 2019 16:16:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vineeth Remanan Pillai <vpillai@...italocean.com>
Cc:     Aaron Lu <aaron.lu@...ux.alibaba.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Subhra Mazumdar <subhra.mazumdar@...cle.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v3 00/16] Core scheduling v3

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.