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Date: Thu, 4 Apr 2024 10:39:00 -0700
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Feng Tang <feng.tang@intel.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	Zhengxu Chen <zhxchen17@meta.com>,
	Danielle Costantino <dcostantino@meta.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Waiman Long <longman@redhat.com>, John Stultz <jstultz@google.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Subject: Re: [PATCH RFC v2 tsc] Check for sockets instead of CPUs to make
 code match comment
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 <Zg416EXHJa9CdI9L@feng-clx.sh.intel.com>
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On Thu, Apr 04, 2024 at 01:08:56PM +0800, Feng Tang wrote:
> On Wed, Apr 03, 2024 at 12:10:41PM -0700, Paul E. McKenney wrote:
> > 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.
> > 
> > Changes since v1:
> > 
> > o	Forward-port to v6.9-rc1.
> > 
> > Reported-by: Zhengxu Chen <zhxchen17@meta.com>
> > Reported-by: Danielle Costantino <dcostantino@meta.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Feng Tang <feng.tang@intel.com>
> > Cc: Waiman Long <longman@redhat.com>
> > Cc: John Stultz <jstultz@google.com>
> > Cc: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
> > Cc: <x86@kernel.org>
> > 
> > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> > index 5a69a49acc963..e938b990bea19 100644
> > --- a/arch/x86/kernel/tsc.c
> > +++ b/arch/x86/kernel/tsc.c
> > @@ -1289,7 +1289,7 @@ 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)
> 
> Regarding package/socket number, Thomas' topology refactoring patchset
> (merged in 6.9-rc1) makes topology_max_packages() more accurate than
> nr_online_nodes(), more details in https://lore.kernel.org/lkml/20240327025105.2861341-1-feng.tang@intel.com/

Thank you, and updated with attribution.

							Thanx, Paul