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

["Srivatsa S. Bhat" <srivatsa@csail.mit.edu>]

On 9/23/22 3:45 AM, Borislav Petkov wrote:
> + 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@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@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.
>>

That would be better indeed, thank you!

>> 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.
> 

I'll send out a v2 after trying out these changes.

Thank you!

Regards,
Srivatsa
VMware Photon OS
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