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

From: Juergen Gross <jgross@suse.com>
To: "Srivatsa S. Bhat" <srivatsa@csail.mit.edu>,
	linux-kernel@vger.kernel.org
Cc: amakhalov@vmware.com, ganb@vmware.com, ankitja@vmware.com,
	bordoloih@vmware.com, keerthanak@vmware.com,
	blamoreaux@vmware.com, namit@vmware.com,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Wyes Karny <wyes.karny@amd.com>,
	Lewis Caroll <lewis.carroll@amd.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	x86@kernel.org,
	VMware PV-Drivers Reviewers <pv-drivers@vmware.com>,
	virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH v2] x86/hotplug: Do not put offline vCPUs in mwait idle state
Date: Mon, 16 Jan 2023 09:38:25 +0100	[thread overview]
Message-ID: <1375c16c-1a81-f845-f9e6-d698148c4ffc@suse.com> (raw)
In-Reply-To: <20230116060134.80259-1-srivatsa@csail.mit.edu>


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On 16.01.23 07:01, Srivatsa S. Bhat wrote:
> From: "Srivatsa S. Bhat (VMware)" <srivatsa@csail.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@infradead.org>
> Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Wyes Karny <wyes.karny@amd.com>
> Cc: Lewis Caroll <lewis.carroll@amd.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Alexey Makhalov <amakhalov@vmware.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: x86@kernel.org
> Cc: VMware PV-Drivers Reviewers <pv-drivers@vmware.com>
> Cc: virtualization@lists.linux-foundation.org
> Cc: kvm@vger.kernel.org
> Cc: xen-devel@lists.xenproject.org

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


Juergen


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