linux-kernel - RE: [PATCH] KVM: x86: use TPAUSE to replace PAUSE in halt polling

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Message-ID: <PH0PR11MB4824511867EAD25089612535CD799@PH0PR11MB4824.namprd11.prod.outlook.com>
Date:   Tue, 30 Aug 2022 05:56:09 +0000
From:   "Mi, Dapeng1" <dapeng1.mi@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "rafael@...nel.org" <rafael@...nel.org>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "zhenyuw@...ux.intel.com" <zhenyuw@...ux.intel.com>
Subject: RE: [PATCH] KVM: x86: use TPAUSE to replace PAUSE in halt polling

> From: Paolo Bonzini <pbonzini@...hat.com>
> Sent: Friday, August 26, 2022 6:14 PM
> To: Mi, Dapeng1 <dapeng1.mi@...el.com>; Christopherson,, Sean
> <seanjc@...gle.com>
> Cc: rafael@...nel.org; daniel.lezcano@...aro.org; linux-pm@...r.kernel.org;
> linux-kernel@...r.kernel.org; kvm@...r.kernel.org;
> zhenyuw@...ux.intel.com
> Subject: Re: [PATCH] KVM: x86: use TPAUSE to replace PAUSE in halt polling
> 
> On 8/25/22 13:31, Mi, Dapeng1 wrote:
> >> I say "if", because I think this needs to come with performance
> >> numbers to show the impact on guest latency so that KVM and its users
> >> can make an informed decision.
> >> And if it's unlikely that anyone will ever want to enable TPAUSE for
> >> halt polling, then it's not worth the extra complexity in KVM.
> > I ever run two scheduling related benchmarks, hackbench and schbench, I
> didn't see  there are obvious performance impact.
> >
> > Here are the hackbench and schbench data on Intel ADL platform.
> 
> Can you confirm (using debugfs for example) that halt polling is used while
> hackbench is running, and not used while it is not running?

Sorry, I may not describe the test case clearly. The hackbench and schbench are run on Host 
rather than a VM. When the hackbench or schbench is run on Host, there is a FIO workload running
in a VM in the background and the FIO would trigger a large number of HLT VM-exits and eventually
invoke halt polling. 

In this test, I want to check whether potential polling time extending would increase the scheduling
latency on host. But it looks the impact for scheduling latency is quite minimal.

> 
> In particular, I think you need to run the server and client on different VMs,
> for example using netperf's UDP_RR test.  With hackbench the ping-pong is
> simply between two tasks on the same CPU, and the hypervisor is not
> exercised at all.
> 

Here are the netperf's UDP_RR test result between two VMs locate on two different physical machines.

Netperf 			Vanilla (Avg.)		TPAUSE (Avg.)		%Delta
UDP_RR (Trans. Rate/s)		503.8			503.9			0.02%

It looks there is no obvious difference with TPAUSE change on UDP RR test.


> Paolo