RE: [PATCH v3] cpuidle: poll_state: Add time limit to poll_idle()

Doug Smythies
2018-03-26T07:13:55Z

On 2018.03.25 23:00 Doug Smythies wrote:
> On 2018.03.25 14:25 Rik van Riel wrote:
> On Sun, 2018-03-25 at 23:34 +0200, Rafael J. Wysocki wrote:
>> On Sunday, March 25, 2018 10:15:52 PM CEST Rik van Riel wrote:

...[snip]...

>>>>
>>>> OK, I am still seeing a performance
>>>> degradation with the above, though
>>>> not throughout the entire workload.
>>>>
>>>> It appears that making the idle loop
>>>> do anything besides cpu_relax() for
>>>> a significant amount of time slows
>>>> things down.
>>>
>>> I see.
>
> I have no proof, but I do not see that as
> the problem.
>
> I think the issue is the overall exiting
> and then re-entering idle state 0 much
> more often, and the related overheads, where
> interrupts are disabled for short periods.
>
> My jury rigged way of trying to create similar
> conditions seems to always have the ISR return with
> the need_resched() flag set, so there is no difference
> in idle state 0 entries per unit time between kernel
> 4.16-rc6 and one with the poll fixes added.
>
> i.e. the difference between these numbers over some time:
>
> cat /sys/devices/system/cpu/cpu*/cpuidle/state0/usage
>
> Rik, I wonder if you see a difference with your real
> workflow?

Using iperf, I was able to show a difference on my computer.
Another computer was used as the server, and my test computer
was the client. (the other way around didn't show a difference)

With Kernel 4.16-rc6 I got about ~2000 idle state 0 entries
per minute and ~155 seconds residency. ~32 watts package power.

With the poll stuff included I got ~46000 idle state 0 entries
per minute and ~53 seconds residency. ~20 watts package power.

... Doug