Messages in this thread | | | Date | Mon, 16 Jan 2023 09:38:25 +0100 | Subject | Re: [PATCH v2] x86/hotplug: Do not put offline vCPUs in mwait idle state | From | Juergen Gross <> |
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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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