linux-kernel - Re: [PATCH 5/8] tsc: Check for sockets instead of CPUs to make code match comment

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Message-ID: <ZcFObO-fcvq1_xj7@tardis>
Date: Mon, 5 Feb 2024 13:09:00 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
	Zhengxu Chen <zhxchen17@...a.com>,
	Danielle Costantino <dcostantino@...a.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Feng Tang <feng.tang@...el.com>, Waiman Long <longman@...hat.com>,
	John Stultz <jstultz@...gle.com>, x86@...nel.org
Subject: Re: [PATCH 5/8] tsc: Check for sockets instead of CPUs to make code
 match comment

On Mon, Feb 05, 2024 at 12:03:33PM -0800, Paul E. McKenney wrote:
> On Mon, Jan 29, 2024 at 03:56:38PM -0800, Boqun Feng wrote:
> > From: "Paul E. McKenney" <paulmck@...nel.org>
> > 
> > The unsynchronized_tsc() eventually checks num_possible_cpus(), and
> > if the system is non-Intel and the number of possible CPUs is greater
> > than one, assumes that TSCs are unsynchronized.  This despite the
> > comment saying "assume multi socket systems are not synchronized",
> > that is, socket rather than CPU.  This behavior was preserved by
> > commit 8fbbc4b45ce3 ("x86: merge tsc_init and clocksource code") and
> > by the previous relevant commit 7e69f2b1ead2 ("clocksource: Remove the
> > update callback").
> > 
> > The clocksource drivers were added by commit 5d0cf410e94b ("Time: i386
> > Clocksource Drivers") back in 2006, and the comment still said "socket"
> > rather than "CPU".
> > 
> > Therefore, bravely (and perhaps foolishly) make the code match the
> > comment.
> > 
> > Note that it is possible to bypass both code and comment by booting
> > with tsc=reliable, but this also disables the clocksource watchdog,
> > which is undesirable when trust in the TSC is strictly limited.
> > 
> > Reported-by: Zhengxu Chen <zhxchen17@...a.com>
> > Reported-by: Danielle Costantino <dcostantino@...a.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Feng Tang <feng.tang@...el.com>
> > Cc: Waiman Long <longman@...hat.com>
> > Cc: John Stultz <jstultz@...gle.com>
> > Cc: <x86@...nel.org>
> 
> Hello, Boqun,
> 
> Please drop this one, as I never got an ack from the maintainers, and
> quite possibly for good reason.  ;-)
> 

Got it I will drop it in the PR, the topic branch has been updated:

	git://git.kernel.org/pub/scm/linux/kernel/git/boqun/linux.git rcu-misc.2024.02.05a

Regards,
Boqun

> 							Thanx, Paul
> 
> > ---
> >  arch/x86/kernel/tsc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> > index 15f97c0abc9d..d45084c6a15e 100644
> > --- a/arch/x86/kernel/tsc.c
> > +++ b/arch/x86/kernel/tsc.c
> > @@ -1287,7 +1287,7 @@ int unsynchronized_tsc(void)
> >  	 */
> >  	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL) {
> >  		/* assume multi socket systems are not synchronized: */
> > -		if (num_possible_cpus() > 1)
> > +		if (nr_online_nodes > 1)
> >  			return 1;
> >  	}
> >  
> > -- 
> > 2.43.0
> >