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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Thu, 8 Dec 2022 12:39:24 +0100
Message-ID: <CAJZ5v0jofKbdRbKzLSEMkjQjP1GTcbt7OYqO=0ZYvpt_HwTQTg@mail.gmail.com>
Subject: Re: [PATCH 1/2][v2] cpuidle-haltpoll: Replace default_idle with arch_cpu_idle
To:     "Li,Rongqing" <lirongqing@baidu.com>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
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        "x86@kernel.org" <x86@kernel.org>,
        "daniel.lezcano@linaro.org" <daniel.lezcano@linaro.org>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "tony.luck@intel.com" <tony.luck@intel.com>,
        "jpoimboe@kernel.org" <jpoimboe@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>
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On Thu, Dec 8, 2022 at 2:48 AM Li,Rongqing <lirongqing@baidu.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Rafael J. Wysocki <rafael@kernel.org>
> > Sent: Wednesday, December 7, 2022 10:38 PM
> > To: Li,Rongqing <lirongqing@baidu.com>
> > Cc: tglx@linutronix.de; mingo@redhat.com; bp@alien8.de;
> > dave.hansen@linux.intel.com; x86@kernel.org; rafael@kernel.org;
> > daniel.lezcano@linaro.org; peterz@infradead.org; akpm@linux-foundation.org;
> > tony.luck@intel.com; jpoimboe@kernel.org; linux-kernel@vger.kernel.org;
> > linux-pm@vger.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@baidu.com> wrote:
> > >
> > > From: Li RongQing <lirongqing@baidu.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@linutronix.de>
> > > Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > Signed-off-by: Li RongQing <lirongqing@baidu.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.