On Mon, Jan 29, 2024 at 03:56:38PM -0800, Boqun Feng wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
>
> 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@xxxxxxxx>
> Reported-by: Danielle Costantino <dcostantino@xxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Feng Tang <feng.tang@xxxxxxxxx>
> Cc: Waiman Long <longman@xxxxxxxxxx>
> Cc: John Stultz <jstultz@xxxxxxxxxx>
> Cc: <x86@xxxxxxxxxx>
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
>
