Re: [PATCH] smp/hotplug, x86/vmware: Put offline vCPUs in halt instead of mwait

[Peter Zijlstra]

On Thu, Jul 21, 2022 at 01:44:33PM -0700, Srivatsa S. Bhat wrote:
> From: Srivatsa S. Bhat (VMware) <srivatsa@xxxxxxxxxxxxx>
> 
> 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) <srivatsa@xxxxxxxxxxxxx>
> ---

> +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;