linux-kernel - Re: [RFC 00/60] Coscheduling for Linux

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Date:   Mon, 17 Sep 2018 17:33:42 -0700
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     "Jan H. Schönherr" <jschoenh@...zon.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC 00/60] Coscheduling for Linux



On 09/07/2018 02:39 PM, Jan H. Schönherr wrote:
> This patch series extends CFS with support for coscheduling. The
> implementation is versatile enough to cover many different coscheduling
> use-cases, while at the same time being non-intrusive, so that behavior of
> legacy workloads does not change.
>
> Peter Zijlstra once called coscheduling a "scalability nightmare waiting to
> happen". Well, with this patch series, coscheduling certainly happened.
> However, I disagree on the scalability nightmare. :)
>
> In the remainder of this email, you will find:
>
> A) Quickstart guide for the impatient.
> B) Why would I want this?
> C) How does it work?
> D) What can I *not* do with this?
> E) What's the overhead?
> F) High-level overview of the patches in this series.
>
> Regards
> Jan
>
>
> A) Quickstart guide for the impatient.
> --------------------------------------
>
> Here is a quickstart guide to set up coscheduling at core-level for
> selected tasks on an SMT-capable system:
>
> 1. Apply the patch series to v4.19-rc2.
> 2. Compile with "CONFIG_COSCHEDULING=y".
> 3. Boot into the newly built kernel with an additional kernel command line
>     argument "cosched_max_level=1" to enable coscheduling up to core-level.
> 4. Create one or more cgroups and set their "cpu.scheduled" to "1".
> 5. Put tasks into the created cgroups and set their affinity explicitly.
> 6. Enjoy tasks of the same group and on the same core executing
>     simultaneously, whenever they are executed.
>
> You are not restricted to coscheduling at core-level. Just select higher
> numbers in steps 3 and 4. See also further below for more information, esp.
> when you want to try higher numbers on larger systems.
>
> Setting affinity explicitly for tasks within coscheduled cgroups is
> currently necessary, as the load balancing portion is still missing in this
> series.
>
I don't get the affinity part. If I create two cgroups by giving them only
cpu shares (no cpuset) and set their cpu.scheduled=1, will this ensure
co-scheduling of each group on core level for all cores in the system?

Thanks,
Subhra