Re: [RFC PATCH v4 00/19] Core scheduling v4

From: Aubrey Li <aubrey.intel@gmail.com>
To: Phil Auld <pauld@redhat.com>
Cc: "Aaron Lu" <aaron.lwe@gmail.com>,
	"Vineeth Remanan Pillai" <vpillai@digitalocean.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>,
	"Linux List Kernel Mailing" <linux-kernel@vger.kernel.org>,
	"Dario Faggioli" <dfaggioli@suse.com>,
	"Frédéric Weisbecker" <fweisbec@gmail.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Greg Kerr" <kerrnel@google.com>,
	"Valentin Schneider" <valentin.schneider@arm.com>,
	"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
Date: Thu, 27 Feb 2020 22:37:20 +0800	[thread overview]
Message-ID: <CAERHkrtyAaEQqqMpV6HMKyHa47HNFwxs5peq4LQJem2z=DO1hg@mail.gmail.com> (raw)
In-Reply-To: <20200227141032.GA30178@pauld.bos.csb>

On Thu, Feb 27, 2020 at 10:10 PM Phil Auld <pauld@redhat.com> wrote:
>
> 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.

Can this be replicated?

>
> > 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.
>
Especially, if the load of cgA task + cgB task > cpu capacity, grouping cgA
tasks can avoid forced idle completely. Maintaining core level balance seems
not the best result. I guess that's why with core scheduler enabled we saw
10-20% improvement in some cases against the default core scheduler disabled.

Thanks,
-Aubrey