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

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

Hi Boris, Thomas, Peter,

On 7/21/22 1:44 PM, 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). [ 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>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Alexey Makhalov <amakhalov@vmware.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: x86@kernel.org
> Cc: VMware PV-Drivers Reviewers <pv-drivers@vmware.com>
> ---

Could you share your thoughts on this patch when you get a chance,
please? I verified that this patch still applies cleanly on current
mainline (6.0-rc6). I'm happy to resend the patch though, if it helps.

Thank you!

Regards,
Srivatsa

> 
>  arch/x86/kernel/cpu/vmware.c |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/vmware.c b/arch/x86/kernel/cpu/vmware.c
> index c04b933f48d3..420e359ed9bb 100644
> --- a/arch/x86/kernel/cpu/vmware.c
> +++ b/arch/x86/kernel/cpu/vmware.c
> @@ -27,6 +27,7 @@
>  #include <linux/clocksource.h>
>  #include <linux/cpu.h>
>  #include <linux/reboot.h>
> +#include <linux/tboot.h>
>  #include <linux/static_call.h>
>  #include <asm/div64.h>
>  #include <asm/x86_init.h>
> @@ -312,6 +313,21 @@ static int vmware_cpu_down_prepare(unsigned int cpu)
>  	local_irq_enable();
>  	return 0;
>  }
> +
> +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,
> 
> 
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