linux-kernel - linux-next: new scheduler messages span: 0-15 (max cpu

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Message-Id: <e399c212-50f2-56ec-d3e6-27d3742b4917@de.ibm.com>
Date:   Mon, 19 Sep 2016 15:19:11 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...nel.org>, Tejun Heo <tj@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: linux-next: new scheduler messages span: 0-15 (max cpu_capacity =
 589) when starting KVM guests

Dietmar, Ingo, Tejun,

since commit cd92bfd3b8cb0ec2ee825e55a3aee704cd55aea9
   sched/core: Store maximum per-CPU capacity in root domain

I get tons of messages from the scheduler like
[..]
span: 0-15 (max cpu_capacity = 589)
span: 0-15 (max cpu_capacity = 589)
span: 0-15 (max cpu_capacity = 589)
span: 0-15 (max cpu_capacity = 589)
[..]

whenever I start kvm guests with libvirt.

The reason seems to be that libvirt via systemd/machined tries to move all
guest vcpus into its cpuset and for whatever reasons, the way it is done
will always call rebuild_sched_domains from the cgroup code.

While the message alone is somewhat of a nuisance, I think rebuilding
the scheduling domains for moving kvm vcpus is really expensive.

Tejun, do you have an idea whats going on here? Is libvirt using
the cgroup interface wrong (e.g. also d a memory migrate or whatever)

Christian