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

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Date:   Mon, 4 Nov 2019 13:01:06 -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 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@...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, 

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?

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.

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.

> >>@@ -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