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

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Date: Mon, 5 Feb 2024 12:03:33 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
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, 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.  ;-)

							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
>