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 --- v1: https://lore.kernel.org/lkml/165843627080.142207.12667479241667142176.stgit@xxxxxxxxxxxxx/ 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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
