Subject: Re: [PATCH 11/14] KVM: stats: Add stat to detect if vcpu is currently blocking

From: David Matlack <[email-masked]>
Date: Tue, 28 Sep 2021 22:04:47 +0000

On Fri, Sep 24, 2021 at 05:55:25PM -0700, Sean Christopherson wrote:
> From: Jing Zhang <[email-masked]>
> 
> Add a "blocking" stat that userspace can use to detect the case where a
> vCPU is not being run because of a vCPU/guest action, e.g. HLT or WFS on
> x86, WFI on arm64, etc...  Current guest/host/halt stats don't show this
> well, e.g. if a guest halts for a long period of time then the vCPU could
> appear pathologically blocked due to a host condition, when in reality the
> vCPU has been put into a not-runnable state by the guest.
> 
> Originally-by: Cannon Matthews <[email-masked]>
> Suggested-by: Sean Christopherson <[email-masked]>
> Signed-off-by: Jing Zhang <[email-masked]>
> [sean: renamed stat to "blocking", massaged changelog]
> Signed-off-by: Sean Christopherson <[email-masked]>

Reviewed-by: David Matlack <[email-masked]>
> ---
>  include/linux/kvm_host.h  | 3 ++-
>  include/linux/kvm_types.h | 1 +
>  virt/kvm/kvm_main.c       | 2 ++
>  3 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 655c2b24db2d..9bb1972e396a 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1453,7 +1453,8 @@ struct _kvm_stats_desc {
>  	STATS_DESC_LOGHIST_TIME_NSEC(VCPU_GENERIC, halt_poll_fail_hist,	       \
>  			HALT_POLL_HIST_COUNT),				       \
>  	STATS_DESC_LOGHIST_TIME_NSEC(VCPU_GENERIC, halt_wait_hist,	       \
> -			HALT_POLL_HIST_COUNT)
> +			HALT_POLL_HIST_COUNT),				       \
> +	STATS_DESC_ICOUNTER(VCPU_GENERIC, blocking)
>  
>  extern struct dentry *kvm_debugfs_dir;
>  
> diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> index 2237abb93ccd..c4f9257bf32d 100644
> --- a/include/linux/kvm_types.h
> +++ b/include/linux/kvm_types.h
> @@ -94,6 +94,7 @@ struct kvm_vcpu_stat_generic {
>  	u64 halt_poll_success_hist[HALT_POLL_HIST_COUNT];
>  	u64 halt_poll_fail_hist[HALT_POLL_HIST_COUNT];
>  	u64 halt_wait_hist[HALT_POLL_HIST_COUNT];
> +	u64 blocking;
>  };
>  
>  #define KVM_STATS_NAME_SIZE	48
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index fe34457530c2..2980d2b88559 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3208,6 +3208,7 @@ bool kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  {
>  	bool waited = false;
>  
> +	vcpu->stat.generic.blocking = 1;
>  	kvm_arch_vcpu_blocking(vcpu);
>  
>  	prepare_to_rcuwait(&vcpu->wait);
> @@ -3223,6 +3224,7 @@ bool kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  	finish_rcuwait(&vcpu->wait);
>  
>  	kvm_arch_vcpu_unblocking(vcpu);
> +	vcpu->stat.generic.blocking = 0;
>  
>  	return waited;
>  }
> -- 
> 2.33.0.685.g46640cef36-goog
> 

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