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

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Date:   Thu, 22 Mar 2018 20:19:24 -0700
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Rik van Riel'" <riel@...riel.com>,
        "'Rafael J. Wysocki'" <rjw@...ysocki.net>
Cc:     "'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>,
        "Doug Smythies" <dsmythies@...us.net>
Subject: RE: [PATCH v2] cpuidle: poll_state: Add time limit to poll_idle()

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?

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.

... Doug