linux-kernel - Re: [PATCH 5/5] cpuidle-haltpoll: fix up the branch check

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Message-ID: <20191101212613.GB20672@amt.cnet>
Date:   Fri, 1 Nov 2019 19:26:15 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Zhenzhong Duan <zhenzhong.duan@...cle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        joao.m.martins@...cle.com, rafael.j.wysocki@...el.com,
        rkrcmar@...hat.com, pbonzini@...hat.com
Subject: Re: [PATCH 5/5] cpuidle-haltpoll: fix up the branch check

On Sat, Oct 26, 2019 at 11:23:59AM +0800, Zhenzhong Duan wrote:
> Ensure pool time is longer than block_ns, so there is a margin to
> avoid vCPU get into block state unnecessorily.
> 
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@...cle.com>
> ---
>  drivers/cpuidle/governors/haltpoll.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cpuidle/governors/haltpoll.c b/drivers/cpuidle/governors/haltpoll.c
> index 4b00d7a..59eadaf 100644
> --- a/drivers/cpuidle/governors/haltpoll.c
> +++ b/drivers/cpuidle/governors/haltpoll.c
> @@ -81,9 +81,9 @@ static void adjust_poll_limit(struct cpuidle_device *dev, unsigned int block_us)
>  	u64 block_ns = block_us*NSEC_PER_USEC;
>  
>  	/* Grow cpu_halt_poll_us if
> -	 * cpu_halt_poll_us < block_ns < guest_halt_poll_us
> +	 * cpu_halt_poll_us <= block_ns < guest_halt_poll_us
>  	 */
> -	if (block_ns > dev->poll_limit_ns && block_ns <= guest_halt_poll_ns) {
> +	if (block_ns >= dev->poll_limit_ns && block_ns < guest_halt_poll_ns) {
					      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If block_ns == guest_halt_poll_ns, you won't allow dev->poll_limit_ns to
grow. Why is that?

> @@ -101,7 +101,7 @@ static void adjust_poll_limit(struct cpuidle_device *dev, unsigned int block_us)
>  			val = guest_halt_poll_ns;
>  
>  		dev->poll_limit_ns = val;
> -	} else if (block_ns > guest_halt_poll_ns &&
> +	} else if (block_ns >= guest_halt_poll_ns &&
>  		   guest_halt_poll_allow_shrink) {
>  		unsigned int shrink = guest_halt_poll_shrink;

And here you shrink if block_ns == guest_halt_poll_ns. Not sure
why that makes sense either.