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

[Zhenzhong Duan]

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@xxxxxxxxxx>
> > ---
> >   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