linux-kernel - Re: [PATCH 1/2][v2] cpuidle-haltpoll: Replace default_idle with arch_cpu

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Date:   Thu, 8 Dec 2022 12:39:24 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Li,Rongqing" <lirongqing@...du.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "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>,
        "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 1/2][v2] cpuidle-haltpoll: Replace default_idle with arch_cpu_idle

On Thu, Dec 8, 2022 at 2:48 AM Li,Rongqing <lirongqing@...du.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Rafael J. Wysocki <rafael@...nel.org>
> > Sent: Wednesday, December 7, 2022 10:38 PM
> > To: Li,Rongqing <lirongqing@...du.com>
> > Cc: tglx@...utronix.de; mingo@...hat.com; bp@...en8.de;
> > dave.hansen@...ux.intel.com; x86@...nel.org; rafael@...nel.org;
> > daniel.lezcano@...aro.org; peterz@...radead.org; akpm@...ux-foundation.org;
> > tony.luck@...el.com; jpoimboe@...nel.org; linux-kernel@...r.kernel.org;
> > linux-pm@...r.kernel.org
> > Subject: Re: [PATCH 1/2][v2] cpuidle-haltpoll: Replace default_idle with
> > arch_cpu_idle
> >
> > On Wed, Dec 7, 2022 at 1:42 PM <lirongqing@...du.com> wrote:
> > >
> > > From: Li RongQing <lirongqing@...du.com>
> > >
> > > When KVM guest has MWAIT and mwait_idle is used as default idle
> > > function, but once cpuidle-haltpoll is loaded, default_idle in
> > > default_enter_idle is used, which is using HLT, and cause a
> > > performance regression. As the commit aebef63cf7ff ("x86: Remove
> > > vendor checks from prefer_mwait_c1_over_halt") explains that mwait is
> > > preferred
> > >
> > > so replace default_idle with arch_cpu_idle which can using MWAIT
> > > optimization.
> > >
> > > latency of sockperf ping-pong localhost test is reduced by more 20%
> > > unixbench has 5% performance improvements for single core
> > >
> > > Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> > > Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > Signed-off-by: Li RongQing <lirongqing@...du.com>
> > > ---
> > >  arch/x86/kernel/process.c          | 1 +
> > >  drivers/cpuidle/cpuidle-haltpoll.c | 2 +-
> > >  2 files changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > > index c21b734..303afad 100644
> > > --- a/arch/x86/kernel/process.c
> > > +++ b/arch/x86/kernel/process.c
> > > @@ -721,6 +721,7 @@ void arch_cpu_idle(void)  {
> > >         x86_idle();
> > >  }
> > > +EXPORT_SYMBOL(arch_cpu_idle);
> >
> > Why do you need this export at all?
> >
>
> When cpuidle-haltpoll is built as module,

But it isn't now.

> if arch_cpu_idle is not export, cpuidle-haltpoll will complain "arch_cpu_idle" undefined

So no, this won't happen.