linux-kernel - Re: [PATCH v2] cpuidle: poll_state: Add time limit to poll

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Date:   Fri, 23 Mar 2018 10:07:31 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Doug Smythies <dsmythies@...us.net>
Cc:     Rik van Riel <riel@...riel.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@...dresden.de>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] cpuidle: poll_state: Add time limit to poll_idle()

On Fri, Mar 23, 2018 at 9:57 AM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> On Fri, Mar 23, 2018 at 4:19 AM, Doug Smythies <dsmythies@...us.net> wrote:
>> On 2018.03.22 12:12 Doug Smythies wrote:
>>>On 2018.03.22 09:32 Rik van Riel wrote:
>>>> On Wed, 2018-03-14 at 13:04 +0100, Peter Zijlstra wrote:
>>>>
>>>>> On x86 we don't have to use that time_check_counter thing,
>>>>> sched_clock()
>>>>> is really cheap, not sure if it makes sense on other platforms.
>>>>
>>>> Are you sure? I saw a 5-10% increase in CPU use,
>>>> for a constant query rate to a memcache style
>>>> workload, with v3 of this patch.
>>>
>>> I would very much like to be able to repeat your test results.
>>> However, I am not sure what you mean by "memcache style workload".
>>> Is there a test you can point me to? Say a Phoronix type test, for example.
>>>
>>> All of my tests with the V3 of this patch have been fine.
>>
>> What is the difference between sched_clock() talked about herein,
>> and local_clock() used in the patch?
>
> It is almost the same (modulo offset) unless sched_clock_stable()
> returns 'false'.
>
>> I'm not sure how good it is but I made a test. I didn't believe
>> the results, so I did it 3 times.
>>
>> V7.3 is as from the git branch.
>> V7.3p is plus the patch adding the counter loop to poll_state.c
>>
>> The test is a tight loop (about 19600 loops per second) running
>> on all 8 CPUs. I can not seem to get my system to use Idle State
>> 0, so I disabled Idle States 1 and 2 to force use of Idle State 0.
>>
>> V7.3 uses a processor package power of 62.5 Watts
>> V7.3p uses a processor package power of 53.4 Watts, or 14.6% less power.
>>
>> The loop times do not change.
>> The Idle state 0 residency per unit time does not change.
>
> OK, so this means that the results should improve for Rik with this
> patch too. :-)

BTW, can you possibly check how much of a difference it makes to
reduce POLL_IDLE_COUNT in the patch to, say, 500 or even more?

The lower it is, the less noise it will introduce AFAICS.