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

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Message-ID: <20191011073338.GA125778@aaronlu>
Date:   Fri, 11 Oct 2019 15:33:38 +0800
From:   Aaron Lu <aaron.lu@...ux.alibaba.com>
To:     Vineeth Remanan Pillai <vpillai@...italocean.com>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Thu, Oct 10, 2019 at 10:29:47AM -0400, Vineeth Remanan Pillai wrote:
> > I didn't see why we need do this.
> >
> > We only need to have the root level sched entities' vruntime become core
> > wide since we will compare vruntime for them across hyperthreads. For
> > sched entities on sub cfs_rqs, we never(at least, not now) compare their
> > vruntime outside their cfs_rqs.
> >
> The reason we need to do this is because, new tasks that gets created will
> have a vruntime based on the new min_vruntime and old tasks will have it
> based on the old min_vruntime

I think this is expected behaviour.

> and it can cause starvation based on how
> you set the min_vruntime.

Care to elaborate the starvation problem?

> With this new patch, we normalize the whole
> tree so that new tasks and old tasks compare with the same min_vruntime.

Again, what's the point of normalizing sched entities' vruntime in
sub-cfs_rqs? Their vruntime comparisons only happen inside their own
cfs_rq, we don't do cross CPU vruntime comparison for them.