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

["Srivatsa S. Bhat" <srivatsa@csail.mit.edu>]

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
---

v1: https://lore.kernel.org/lkml/165843627080.142207.12667479241667142176.stgit@csail.mit.edu/

 arch/x86/kernel/smpboot.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 55cad72715d9..125a5d4bfded 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1763,6 +1763,15 @@ static inline void mwait_play_dead(void)
 		return;
 	if (!this_cpu_has(X86_FEATURE_CLFLUSH))
 		return;
+
+	/*
+	 * Do not use mwait in CPU offline play_dead if running under
+	 * any hypervisor, to make sure that the offline vCPU actually
+	 * yields to the hypervisor (which may not happen otherwise if
+	 * the hypervisor supports mwait passthrough).
+	 */
+	if (this_cpu_has(X86_FEATURE_HYPERVISOR))
+		return;
 	if (__this_cpu_read(cpu_info.cpuid_level) < CPUID_MWAIT_LEAF)
 		return;
 
-- 
2.25.1