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

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Date:   Fri, 31 May 2019 15:44:57 +0800
From:   Aaron Lu <aaron.lu@...ux.alibaba.com>
To:     Aubrey Li <aubrey.intel@...il.com>
Cc:     Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.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>,
        Subhra Mazumdar <subhra.mazumdar@...cle.com>,
        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 Fri, May 31, 2019 at 02:53:21PM +0800, Aubrey Li wrote:
> On Fri, May 31, 2019 at 2:09 PM Aaron Lu <aaron.lu@...ux.alibaba.com> wrote:
> >
> > On 2019/5/31 13:12, Aubrey Li wrote:
> > > On Fri, May 31, 2019 at 11:01 AM Aaron Lu <aaron.lu@...ux.alibaba.com> wrote:
> > >>
> > >> This feels like "date" failed to schedule on some CPU
> > >> on time.
> > >>
> > >> My first reaction is: when shell wakes up from sleep, it will
> > >> fork date. If the script is untagged and those workloads are
> > >> tagged and all available cores are already running workload
> > >> threads, the forked date can lose to the running workload
> > >> threads due to __prio_less() can't properly do vruntime comparison
> > >> for tasks on different CPUs. So those idle siblings can't run
> > >> date and are idled instead. See my previous post on this:
> > >> https://lore.kernel.org/lkml/20190429033620.GA128241@aaronlu/
> > >> (Now that I re-read my post, I see that I didn't make it clear
> > >> that se_bash and se_hog are assigned different tags(e.g. hog is
> > >> tagged and bash is untagged).
> > >
> > > Yes, script is untagged. This looks like exactly the problem in you
> > > previous post. I didn't follow that, does that discussion lead to a solution?
> >
> > No immediate solution yet.
> >
> > >>
> > >> Siblings being forced idle is expected due to the nature of core
> > >> scheduling, but when two tasks belonging to two siblings are
> > >> fighting for schedule, we should let the higher priority one win.
> > >>
> > >> It used to work on v2 is probably due to we mistakenly
> > >> allow different tagged tasks to schedule on the same core at
> > >> the same time, but that is fixed in v3.
> > >
> > > I have 64 threads running on a 104-CPU server, that is, when the
> >
> > 104-CPU means 52 cores I guess.
> > 64 threads may(should?) spread on all the 52 cores and that is enough
> > to make 'date' suffer.
> 
> 64 threads should spread onto all the 52 cores, but why they can get
> scheduled while untagged "date" can not? Is it because in the current

If 'date' didn't get scheduled, there will be no output at all unless
all those workload threads finished :-)

I guess the workload you used is not entirely CPU intensive, or 'date'
can be totally blocked due to START_DEBIT. But note that START_DEBIT
isn't the problem here, cross CPU vruntime comparison is.

> implementation the task with cookie always has higher priority than the
> task without a cookie?

No.