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Bhat" , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Thomas Gleixner , Ingo Molnar , Dave Hansen , "H. Peter Anvin" , Alexey Makhalov , Juergen Gross , x86@kernel.org, VMware PV-Drivers Reviewers , ganb@vmware.com, sturlapati@vmware.com, bordoloih@vmware.com, ankitja@vmware.com, keerthanak@vmware.com, namit@vmware.com, srivatsab@vmware.com, kvm ML Subject: Re: [PATCH] smp/hotplug, x86/vmware: Put offline vCPUs in halt instead of mwait Message-ID: References: <165843627080.142207.12667479241667142176.stgit@csail.mit.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org + kvm ML and leaving the whole mail quoted in for them. On Fri, Sep 23, 2022 at 09:05:26AM +0200, Peter Zijlstra wrote: > On Thu, Jul 21, 2022 at 01:44:33PM -0700, Srivatsa S. Bhat wrote: > > From: Srivatsa S. Bhat (VMware) > > > > VMware ESXi allows enabling a passthru mwait CPU-idle state in the > > guest using the following VMX option: > > > > monitor_control.mwait_in_guest = "TRUE" > > > > This lets a vCPU in mwait to remain in guest context (instead of > > yielding to the hypervisor via a 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 ESXi, which > > prevents the hypervisor from running other vCPUs or workloads on the > > corresponding pCPU (particularly when vCPU - pCPU mappings are > > statically defined by the user). > > I would hope vCPU pinning is a mandatory thing when MWAIT passthrough it > set? > > > [ Note that such a vCPU is not > > actually busy spinning though; it remains in mwait idle state in the > > guest ]. > > > > Fix this by overriding the CPU offline play_dead() callback for VMware > > hypervisor, by putting the CPU in halt state (which actually yields to > > the hypervisor), even if mwait support is available. > > > > Signed-off-by: Srivatsa S. Bhat (VMware) > > --- > > > +static void vmware_play_dead(void) > > +{ > > + play_dead_common(); > > + tboot_shutdown(TB_SHUTDOWN_WFS); > > + > > + /* > > + * Put the vCPU going offline in halt instead of mwait (even > > + * if mwait support is available), to make sure that the > > + * offline vCPU yields to the hypervisor (which may not happen > > + * with mwait, for example, if the guest's VMX is configured > > + * to retain the vCPU in guest context upon mwait). > > + */ > > + hlt_play_dead(); > > +} > > #endif > > > > static __init int activate_jump_labels(void) > > @@ -349,6 +365,7 @@ static void __init vmware_paravirt_ops_setup(void) > > #ifdef CONFIG_SMP > > smp_ops.smp_prepare_boot_cpu = > > vmware_smp_prepare_boot_cpu; > > + smp_ops.play_dead = vmware_play_dead; > > if (cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, > > "x86/vmware:online", > > vmware_cpu_online, > > No real objection here; but would not something like the below fix the > problem more generally? I'm thinking MWAIT passthrough for *any* > hypervisor doesn't want play_dead to use it. > > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c > index f24227bc3220..166cb3aaca8a 100644 > --- a/arch/x86/kernel/smpboot.c > +++ b/arch/x86/kernel/smpboot.c > @@ -1759,6 +1759,8 @@ static inline void mwait_play_dead(void) > return; > if (!this_cpu_has(X86_FEATURE_CLFLUSH)) > return; > + if (this_cpu_has(X86_FEATURE_HYPERVISOR)) > + return; > if (__this_cpu_read(cpu_info.cpuid_level) < CPUID_MWAIT_LEAF) > return; Yeah, it would be nice if we could get a consensus here from all relevant HVs. -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette