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

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Message-ID: <1375c16c-1a81-f845-f9e6-d698148c4ffc@suse.com>
Date:   Mon, 16 Jan 2023 09:38:25 +0100
From:   Juergen Gross <jgross@...e.com>
To:     "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>,
        linux-kernel@...r.kernel.org
Cc:     amakhalov@...are.com, ganb@...are.com, ankitja@...are.com,
        bordoloih@...are.com, keerthanak@...are.com, blamoreaux@...are.com,
        namit@...are.com, Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Wyes Karny <wyes.karny@....com>,
        Lewis Caroll <lewis.carroll@....com>,
        Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org,
        VMware PV-Drivers Reviewers <pv-drivers@...are.com>,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v2] x86/hotplug: Do not put offline vCPUs in mwait idle
 state

On 16.01.23 07:01, Srivatsa S. Bhat wrote:
> From: "Srivatsa S. Bhat (VMware)" <srivatsa@...il.mit.edu>
> 
> 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@...radead.org>
> Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@...il.mit.edu>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
> Cc: "Paul E. McKenney" <paulmck@...nel.org>
> Cc: Wyes Karny <wyes.karny@....com>
> Cc: Lewis Caroll <lewis.carroll@....com>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Alexey Makhalov <amakhalov@...are.com>
> Cc: Juergen Gross <jgross@...e.com>
> Cc: x86@...nel.org
> Cc: VMware PV-Drivers Reviewers <pv-drivers@...are.com>
> Cc: virtualization@...ts.linux-foundation.org
> Cc: kvm@...r.kernel.org
> Cc: xen-devel@...ts.xenproject.org

Reviewed-by: Juergen Gross <jgross@...e.com>


Juergen


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