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

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Date:   Fri, 13 Sep 2019 21:57:52 +0800
From:   Aaron Lu <aaron.lu@...ux.alibaba.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Julien Desfossez <jdesfossez@...italocean.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Subhra Mazumdar <subhra.mazumdar@...cle.com>,
        Vineeth Remanan Pillai <vpillai@...italocean.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, Sep 12, 2019 at 10:05:43AM -0700, Tim Chen wrote:
> On 9/12/19 5:04 AM, Aaron Lu wrote:
> 
> > Well, I have done following tests:
> > 1 Julien's test script: https://paste.debian.net/plainh/834cf45c
> > 2 start two tagged will-it-scale/page_fault1, see how each performs;
> > 3 Aubrey's mysql test: https://github.com/aubreyli/coresched_bench.git
> > 
> > They all show your patchset performs equally well...And consider what
> > the patch does, I think they are really doing the same thing in
> > different ways.
> > 
> 
> Aaron,
> 
> The new feature of my new patches attempt to load balance between cores,
> and remove imbalance of cgroup load on a core that causes forced idle.
> Whereas previous patches attempt for fairness of cgroup between sibling threads,
> so I think the goals are kind of orthogonal and complementary.
> 
> The premise is this, say cgroup1 is occupying 50% of cpu on cpu thread 1
> and 25% of cpu on cpu thread 2, that means we have a 25% cpu imbalance
> and cpu is force idled 25% of the time.  So ideally we need to remove
> 12.5% of cgroup 1 load from cpu thread 1 to sibling thread 2, so they
> both run at 37.5% on both thread for cgroup1 load without causing
> any force idled time.  Otherwise we will try to remove 25% of cgroup1
> load from cpu thread 1 to another core that has cgroup1 load to match.
> 
> This load balance is done in the regular load balance paths.
> 
> Previously for v3, only sched_core_balance made an attempt to pull a cookie task, and only
> in the idle balance path. So if the cpu is kept busy, the cgroup load imbalance
> between sibling threads could last a long time.  And the thread fairness
> patches for v3 don't help to balance load for such cases.
> 
> The new patches take into actual consideration of the amount of load imbalance
> of the same group between sibling threads when selecting task to pull, 
> and it also prevent task migration that creates
> more load imbalance. So hopefully this feature will help when we have
> more cores and need load balance across the cores.  This tries to help
> even cgroup workload between threads to minimize forced idle time, and also
> even out load across cores.

Will take a look at your new patches, thanks for the explanation.

> In your test, how many cores are on your machine and how many threads did
> each page_fault1 spawn off?

The test VM has 16 cores and 32 threads.
I created 2 tagged cgroups to run page_fault1 and each page_fault1 has
16 processes, like this:
$ ./src/will-it-scale/page_fault1_processes -t 16 -s 60