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

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Date:   Wed, 24 Aug 2022 14:08:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Dapeng Mi' <dapeng1.mi@...el.com>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
CC:     "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: Dapeng Mi
> Sent: 24 August 2022 10:11
> 
> TPAUSE is a new instruction on Intel processors which can instruct
> processor enters a power/performance optimized state. Halt polling
> uses PAUSE instruction to wait vCPU is waked up. The polling time
> could be long and cause extra power consumption in some cases.
> 
> Use TPAUSE to replace the PAUSE instruction in halt polling to get
> a better power saving and performance.

What is the effect on wakeup latency?
Quite often that is far more important than a bit of power saving.

The automatic entry of sleep states is a PITA already.
Block 30 RT threads in cv_wait() and then do cv_broadcast().
Use ftrace to see just how long it takes the last thread
to wake up.

	David

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