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

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Date:   Fri, 31 May 2019 14:09:08 +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 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.

> system has ~40% idle time, and "date" is still failed to be picked
> up onto CPU on time. This may be the nature of core scheduling,
> but it seems to be far from fairness.

Exactly.

> Shouldn't we share the core between (sysbench+gemmbench)
> and (date)? I mean core level sharing instead of  "date" starvation?

We need to make core scheduling fair, but due to no
immediate solution to vruntime comparison cross CPUs, it's not
done yet.