linux-kernel - Re: [PATCH MANUALSEL 5.4 2/4] KVM: do not shrink halt_poll_ns below grow

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Date:   Wed, 6 Oct 2021 13:23:35 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.4 2/4] KVM: do not shrink halt_poll_ns below
 grow_start

On 06/10/21 13:12, Sasha Levin wrote:
> From: Sergey Senozhatsky <senozhatsky@...omium.org>
> 
> [ Upstream commit ae232ea460888dc5a8b37e840c553b02521fbf18 ]
> 
> grow_halt_poll_ns() ignores values between 0 and
> halt_poll_ns_grow_start (10000 by default). However,
> when we shrink halt_poll_ns we may fall way below
> halt_poll_ns_grow_start and endup with halt_poll_ns
> values that don't make a lot of sense: like 1 or 9,
> or 19.
> 
> VCPU1 trace (halt_poll_ns_shrink equals 2):
> 
> VCPU1 grow 10000
> VCPU1 shrink 5000
> VCPU1 shrink 2500
> VCPU1 shrink 1250
> VCPU1 shrink 625
> VCPU1 shrink 312
> VCPU1 shrink 156
> VCPU1 shrink 78
> VCPU1 shrink 39
> VCPU1 shrink 19
> VCPU1 shrink 9
> VCPU1 shrink 4
> 
> Mirror what grow_halt_poll_ns() does and set halt_poll_ns
> to 0 as soon as new shrink-ed halt_poll_ns value falls
> below halt_poll_ns_grow_start.
> 
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Message-Id: <20210902031100.252080-1-senozhatsky@...omium.org>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   virt/kvm/kvm_main.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 77f84cbca740..f31976010622 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2470,15 +2470,19 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
>   
>   static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
>   {
> -	unsigned int old, val, shrink;
> +	unsigned int old, val, shrink, grow_start;
>   
>   	old = val = vcpu->halt_poll_ns;
>   	shrink = READ_ONCE(halt_poll_ns_shrink);
> +	grow_start = READ_ONCE(halt_poll_ns_grow_start);
>   	if (shrink == 0)
>   		val = 0;
>   	else
>   		val /= shrink;
>   
> +	if (val < grow_start)
> +		val = 0;
> +
>   	vcpu->halt_poll_ns = val;
>   	trace_kvm_halt_poll_ns_shrink(vcpu->vcpu_id, val, old);
>   }
> 

Acked-by: Paolo Bonzini <pbonzini@...hat.com>