linux-kernel - RE: [PATCH] x86/smpboot: Don't do mwait_play

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Date:   Tue, 3 Apr 2018 14:47:46 +0000
From:   "Ghannam, Yazen" <Yazen.Ghannam@....com>
To:     Ingo Molnar <mingo@...nel.org>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bp@...e.de" <bp@...e.de>
Subject: RE: [PATCH] x86/smpboot: Don't do mwait_play_dead() on AMD systems

> -----Original Message-----
> From: Ingo Molnar <mingo.kernel.org@...il.com> On Behalf Of Ingo Molnar
> Sent: Tuesday, April 3, 2018 10:41 AM
> To: Ghannam, Yazen <Yazen.Ghannam@....com>
> Cc: x86@...nel.org; linux-kernel@...r.kernel.org; bp@...e.de
> Subject: Re: [PATCH] x86/smpboot: Don't do mwait_play_dead() on AMD
> systems
> 
> 
> * Ghannam, Yazen <Yazen.Ghannam@....com> wrote:
> 
> > > -----Original Message-----
> > > From: Ingo Molnar <mingo.kernel.org@...il.com> On Behalf Of Ingo
> Molnar
> > > Sent: Tuesday, April 3, 2018 7:04 AM
> > > To: Ghannam, Yazen <Yazen.Ghannam@....com>
> > > Cc: x86@...nel.org; linux-kernel@...r.kernel.org; bp@...e.de
> > > Subject: Re: [PATCH] x86/smpboot: Don't do mwait_play_dead() on AMD
> > > systems
> > >
> > >
> > > * Yazen Ghannam <Yazen.Ghannam@....com> wrote:
> > >
> > > > From: Yazen Ghannam <yazen.ghannam@....com>
> > > >
> > > > Recent AMD systems support using MWAIT for C1 state. However, MWAIT
> will
> > > > not allow deeper cstates than C1 on current systems.
> > > >
> > > > With play_dead() we expect the OS to use the deepest state available.
> > > > The deepest state available on AMD systems is reached through SystemIO
> > > > or HALT. If MWAIT is available, we use it instead of the other methods,
> > > > so we never reach the deepest state.
> > > >
> > > > Don't try to use MWAIT to play_dead() on AMD systems. Instead, we'll use
> > > > CPUIDLE to enter the deepest state advertised by firmware. If CPUIDLE is
> > > > not available then we fallback to HALT.
> > > >
> > > > Signed-off-by: Yazen Ghannam <yazen.ghannam@....com>
> > > > ---
> > > >  arch/x86/kernel/smpboot.c | 3 +++
> > > >  1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > > > index ff99e2b6fc54..67cf00b25f83 100644
> > > > --- a/arch/x86/kernel/smpboot.c
> > > > +++ b/arch/x86/kernel/smpboot.c
> > > > @@ -1536,6 +1536,9 @@ static inline void mwait_play_dead(void)
> > > >  	void *mwait_ptr;
> > > >  	int i;
> > > >
> > > > +	/* Don't try native MWAIT on AMD. Stick to CPUIDLE and HALT. */
> > > > +	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
> > > > +		return;
> > >
> > > The comment should mainly explain the 'why is this done', not the 'what is
> done'
> > > which is pretty obvious from the code ...
> > >
> >
> > Yes, I'll drop that comment since the commit message has the explanation.
> 
> Or rather, explain the 'why' in the comment, because otherwise this is a pretty
> obscure condition that is not self-documenting?
> 

Okay, how about this?

"MWAIT only provides shallow Cstates on AMD systems. Fallback to CPUIDLE and
HALT to have access to deeper Cstates."

Thanks,
Yazen