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Date:   Thu, 27 Feb 2020 09:10:33 -0500
From:   Phil Auld <pauld@redhat.com>
To:     Aaron Lu <aaron.lwe@gmail.com>
Cc:     Vineeth Remanan Pillai <vpillai@digitalocean.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Turner <pjt@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Greg Kerr <kerrnel@google.com>,
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        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC PATCH v4 00/19] Core scheduling v4
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Hi Aaron,

On Thu, Feb 27, 2020 at 10:04:32AM +0800 Aaron Lu wrote:
> On Tue, Feb 25, 2020 at 03:51:37PM -0500, Vineeth Remanan Pillai wrote:
> > On a 2sockets/16cores/32threads VM, I grouped 8 sysbench(cpu mode)
> > > threads into one cgroup(cgA) and another 16 sysbench(cpu mode) threads
> > > into another cgroup(cgB). cgA and cgB's cpusets are set to the same
> > > socket's 8 cores/16 CPUs and cgA's cpu.shares is set to 10240 while cgB's
> > > cpu.shares is set to 2(so consider cgB as noise workload and cgA as
> > > the real workload).
> > >
> > > I had expected cgA to occupy 8 cpus(with each cpu on a different core)
> > 
> > The expected behaviour could also be that 8 processes share 4 cores and
> > 8 hw threads right? This is what we are seeing mostly
> 
> I expect the 8 cgA tasks to spread on each core, instead of occupying
> 4 cores/8 hw threads. If they stay on 4 cores/8 hw threads, than on the
> core level, these cores' load would be much higher than other cores
> which are running cgB's tasks, this doesn't look right to me.
> 

I don't think that's a valid assumption, at least since the load balancer rework.

The scheduler will be looking much more at the number of running task versus
the group weight. So in this case 2 running tasks, 2 siblings at the core level
will look fine. There will be no reason to migrate. 

> I think the end result should be: each core has two tasks queued, one
> cgA task and one cgB task(to maintain load balance on the core level).
> The two tasks are queued on different hw thread, with cgA's task runs
> most of the time on one thread and cgB's task being forced idle most
> of the time on the other thread.
> 

With the core scheduler that does not seem to be a desired outcome. I think
grouping the 8 cgA tasks on the 8 cpus of 4 cores seems right. 


Cheers,
Phil

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