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@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. ;-)
>
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
> >
