Re: [PATCH v2] x86/hotplug: Do not put offline vCPUs in mwait idle state

[Juergen Gross <jgross@xxxxxxxx>]

On 16.01.23 07:01, Srivatsa S. Bhat wrote:
> From: "Srivatsa S. Bhat (VMware)" <srivatsa@xxxxxxxxxxxxx>
>
> Under hypervisors that support mwait passthrough, a vCPU in mwait
> CPU-idle state remains in guest context (instead of yielding to the
> hypervisor via VMEXIT), which helps speed up wakeups from idle.
>
> However, this runs into problems with CPU hotplug, because the Linux
> CPU offline path prefers to put the vCPU-to-be-offlined in mwait
> state, whenever mwait is available. As a result, since a vCPU in mwait
> remains in guest context and does not yield to the hypervisor, an
> offline vCPU *appears* to be 100% busy as viewed from the host, which
> prevents the hypervisor from running other vCPUs or workloads on the
> corresponding pCPU. [ Note that such a vCPU is not actually busy
> spinning though; it remains in mwait idle state in the guest ].
>
> Fix this by preventing the use of mwait idle state in the vCPU offline
> play_dead() path for any hypervisor, even if mwait support is
> available.
>
> Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@xxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Wyes Karny <wyes.karny@xxxxxxx>
> Cc: Lewis Caroll <lewis.carroll@xxxxxxx>
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: Alexey Makhalov <amakhalov@xxxxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: VMware PV-Drivers Reviewers <pv-drivers@xxxxxxxxxx>
> Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>


Juergen

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