thr3ads.net - Virtualization - [PATCH] smp/hotplug, x86/vmware: Put offline vCPUs in halt instead of mwait [Sep 2022]

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Peter Zijlstra

2022-Sep-23 07:05 UTC

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

On Thu, Jul 21, 2022 at 01:44:33PM -0700, Srivatsa S. Bhat
wrote:> From: Srivatsa S. Bhat (VMware) <srivatsa at csail.mit.edu>
> 
> 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 at csail.mit.edu>
> ---
> +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;

Borislav Petkov

2022-Sep-23 10:45 UTC

head link

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

+ 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) <srivatsa at csail.mit.edu>
> > 
> > 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 at
csail.mit.edu>
> > ---
> 
> > +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

Juergen Gross

2022-Sep-23 10:52 UTC

head link

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

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 at csail.mit.edu>
>>
>> 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 at
csail.mit.edu>
>> ---
> 
>> +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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Virtualization - Sep 2022 - [PATCH] smp/hotplug, x86/vmware: Put offline vCPUs in halt instead of mwait

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

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

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