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Date:   Tue, 5 Oct 2021 09:49:43 -0400
From:   Nitesh Lal <nilal@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Wanpeng Li <kernellwp@...il.com>
Subject: Re: [PATCH v1] KVM: isolation: retain initial mask for kthread VM worker

On Tue, Oct 5, 2021 at 9:22 AM Marcelo Tosatti <mtosatti@...hat.com> wrote:
>
> On Tue, Oct 05, 2021 at 01:25:52PM +0200, Paolo Bonzini wrote:
> > On 05/10/21 12:58, Marcelo Tosatti wrote:
> > > > There are other effects of cgroups (e.g. memory accounting) than just the
> > > > cpumask;
> > >
> > > Is kvm-nx-hpage using significant amounts of memory?
> >
> > No, that was just an example (and not a good one indeed, because
> > kvm-nx-hpage is not using a substantial amount of either memory or CPU).
> > But for example vhost also uses cgroup_attach_task_all, so it should have
> > the same issue with SCHED_FIFO?
>
> Yes. Would need to fix vhost as well.
>
> >
> > > > Why doesn't the scheduler move the task to a CPU that is not being hogged by
> > > > vCPU SCHED_FIFO tasks?
> > > Because cpuset placement is enforced:
> >
> > Yes, but I would expect the parent cgroup to include both isolated CPUs (for
> > the vCPU threads) and non-isolated housekeeping vCPUs (for the QEMU I/O
> > thread).
>
> Yes, the parent, but why would that matter? If you are in a child
> cpuset, you are restricted to the child cpuset mask (and not the
> parents).

Yes, and at the time of cpuset_attach, the task is attached to any one of
the CPUs that are in the effective cpumask.

>
> > The QEMU I/O thread is not hogging the CPU 100% of the time, and
> > therefore the nx-recovery thread should be able to run on that CPU.
>
> Yes, but:
>
> 1) The cpumask of the parent thread is not inherited
>
>         set_cpus_allowed_ptr(task, housekeeping_cpumask(HK_FLAG_KTHREAD));
>
> On __kthread_create_on_node should fail (because its cgroup, the one
> inherited from QEMU, contains only isolated CPUs).
>

Just to confirm, do you mean fail only for unbounded kthreads?


-- 
Thanks
Nitesh

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