From: Zhenzhong Duan <zhenzhong.duan@oracle.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
joao.m.martins@oracle.com, rafael.j.wysocki@intel.com,
rkrcmar@redhat.com, pbonzini@redhat.com
Subject: Re: [PATCH 5/5] cpuidle-haltpoll: fix up the branch check
Date: Tue, 5 Nov 2019 14:49:18 +0800 [thread overview]
Message-ID: <b7c33ed5-799b-51a0-b35b-86d979a7ad6c@oracle.com> (raw)
In-Reply-To: <20191104150103.GA14887@amt.cnet>
On 2019/11/4 23:01, Marcelo Tosatti wrote:
> On Mon, Nov 04, 2019 at 11:10:25AM +0800, Zhenzhong Duan wrote:
>> 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@oracle.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,
> OK.
>
>> then block_ns = dev->poll_limit_ns,
> block_ns = dev->poll_limit_ns = guest_halt_poll_ns. OK.
>
>> 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?
> Ok, so you are considering _on the next_ halt instance, if block_ns =
> guest_halt_poll_ns again?
Yes, I realized it's rarely to happen in nanosecond granularity.
>
> Then without the suggested modification: we don't shrink, poll for
> guest_halt_poll_ns again.
>
> With your modification: we shrink, because block_ns ==
> guest_halt_poll_ns.
>
> IMO what really clarifies things here is either the real sleep pattern
> or a synthetic sleep pattern similar to the real thing.
Agree
>
> Do you have a scenario where the current algorithm is maintaining
> a low dev->poll_limit_ns and performance is hurt?
>
> If you could come up with examples, such as the client/server pair at
> https://lore.kernel.org/lkml/20190514135022.GD4392@amt.cnet/T/
>
> or just a sequence of delays:
> block_ns, block_ns, block_ns-1,...
>
> It would be easier to visualize this.
Looks hard to generate a sequence of delays of same value in nanoseconds
which is also CPU cycle granularity.
I think this patch doesn't help much for a real scenario, so pls ignore it.
Thanks
Zhenzhong
prev parent reply other threads:[~2019-11-05 6:52 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-26 3:23 [PATCH 0/5] misc fixes on halt-poll code both KVM and guest Zhenzhong Duan
2019-10-26 3:23 ` [PATCH 1/5] KVM: simplify branch check in host poll code Zhenzhong Duan
2019-11-01 21:03 ` Marcelo Tosatti
2019-11-04 3:49 ` Zhenzhong Duan
2019-10-26 3:23 ` [PATCH 2/5] KVM: add a check to ensure grow start value is nonzero Zhenzhong Duan
2019-11-11 13:49 ` Paolo Bonzini
2019-10-26 3:23 ` [PATCH 3/5] KVM: ensure pool time is longer than block_ns Zhenzhong Duan
2019-11-01 21:16 ` Marcelo Tosatti
2019-11-11 13:53 ` Paolo Bonzini
2019-11-12 12:14 ` Zhenzhong Duan
2019-10-26 3:23 ` [PATCH 4/5] cpuidle-haltpoll: add a check to ensure grow start value is nonzero Zhenzhong Duan
2019-11-01 21:19 ` Marcelo Tosatti
2019-11-04 2:56 ` Zhenzhong Duan
2019-11-11 13:54 ` Paolo Bonzini
2019-10-26 3:23 ` [PATCH 5/5] cpuidle-haltpoll: fix up the branch check Zhenzhong Duan
2019-11-01 21:26 ` Marcelo Tosatti
2019-11-04 3:10 ` Zhenzhong Duan
2019-11-04 15:01 ` Marcelo Tosatti
2019-11-05 6:49 ` Zhenzhong Duan [this message]
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