Re: [PATCH RFC v2 tsc] Check for sockets instead of CPUs to make code match comment

[Feng Tang <feng.tang@intel.com>]

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/

Thanks,
Feng