linux-kernel - RE: [PATCH] cpuidle-haltpoll: Disable kvm guest polling when mwait

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Message-ID: <875yenfnkz.ffs@tglx>
Date:   Wed, 07 Dec 2022 12:41:16 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Li,Rongqing" <lirongqing@...du.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "jpoimboe@...nel.org" <jpoimboe@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: RE: [PATCH] cpuidle-haltpoll: Disable kvm guest polling when
 mwait_idle is used

On Wed, Dec 07 2022 at 10:49, Rongqing Li wrote:
>> Before change, "sockperf ping-pong -i 127.0.0.1 -p 20001  --tcp " latency is:
>> sockperf: Summary: Latency is 6.245 usec
>> 
>> this patch, disable cpuidle-haltpoll
>> sockperf: Summary: Latency is 4.671 usec
>> 
>> 
>> using arch_cpu_idle() in default_enter_idle()
>> sockperf: Summary: Latency is 4.285 usec
>> 
>
> When I did upper tests, I taskset sockperf server and client to
> different cpus, so using arch_cpu_idle() in default_enter_idle() gets
> better result.
>
> I test unixbench also, find that Don't loading cpuidle-haltholl can
> get more performance on 8 cores (2 threads per core) intel cpu which
> cstate is disabled in host
>
> Don't load cpuidle-haltpoll:
> System Benchmarks Index Score                                        1995.8
> System Benchmarks Index Score                                        8482.8
>
> Replace default_idle with arch_cpu_idle, and load cpuidle-haltpoll, code like below:
> System Benchmarks Index Score                                        1971.4
> System Benchmarks Index Score                                        8467.7
>
> So I think we should not load cpuidle-haltpoll driver when guest has mwait

So in the above you got:

Driver loaded not modified:     6.245
Driver not loaded:              4.671           ~25%
Driver loaded modified:         4.285           ~30%

Now with unixbench:

Driver not loaded:              8482.8
Driver loaded modified:         8467.7          ~0.2%

So because of 0.2% delta you justify to throw away a 5% win?

If you really care about the 0.2%, then blacklist the module for your
use case.

Thanks,

        tglx