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

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Message-ID: <bafc1688-02ea-77a4-fb1c-2fe6afa8a7cc@oracle.com>
Date:   Mon, 4 Nov 2019 11:10:25 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     Marcelo Tosatti <mtosatti@...hat.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 2019/11/2 5:26, Marcelo Tosatti wrote:
> 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?

Maybe I'm too strict here. My understanding is: if block_ns = guest_halt_poll_ns,
dev->poll_limit_ns will grow to guest_halt_poll_ns, then block_ns = dev->poll_limit_ns,
there is not a margin to ensure poll time is enough to cover the equal block time.
In this case, shrinking may be a better choice?

>
>> @@ -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.

See above explanation.

Zhenzhong