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

[Juergen Gross]

On 23.09.22 09:05, Peter Zijlstra wrote:
> 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;
>   

With my Xen hat on I agree with this approach.


Juergen

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