Damien Zammit, le sam. 28 févr. 2026 06:14:08 +0000, a ecrit:
> There were problems resuming the lapic timer, causing lockups on -smp 2.
> We used to use the PIT on cpu0 for clock interrupts and lapic timer
> for periodic interrupts on other cpus. Now the PIT is only used to
> calibrate the lapic timer to unify the clock sources to be lapic based.
> Re-enable the lapic timer on all cpus for clock sources of periodic intrs.
Mmmm, but I'm seeing the clock being completely unreliable on the
boralus buildd, losing 2s every 10s!
I.e. the lapic timer doesn't seem reliable, probably just because the
calibration got faulty (this is a VM, so not completely surprising). In
a VM, we probably want to keep relying on the PIT, supposed to be
providing an accurate source of time.
Samuel
>
> TESTED:
> - Passes i386 CI
> - Passes x86_64 CI (except user32)
> - Boots to debian login using -smp 2 or 6
> - Interrupts are received periodically on all cpus
> - (printed all cpu numbers in hardclock)
>
> Reported by Brent Baccala (and -smp 2 issue also known for a while)
>
> ---
> i386/i386/mp_desc.c | 4 ++++
> i386/i386at/model_dep.c | 17 +++++++++++++----
> 2 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/i386/i386/mp_desc.c b/i386/i386/mp_desc.c
> index 1343861c..1d60c892 100644
> --- a/i386/i386/mp_desc.c
> +++ b/i386/i386/mp_desc.c
> @@ -340,5 +340,9 @@ start_other_cpus(void)
>
> /* Re-enable IOAPIC interrupts as per setup */
> lapic_enable();
> +
> + /* Re-enable lapic timer for clock interrupts on BSP.
> + * This is needed after lapic was disabled/enabled */
> + lapic_enable_timer();
> }
> #endif /* NCPUS > 1 */
> diff --git a/i386/i386at/model_dep.c b/i386/i386at/model_dep.c
> index eea98754..f10d3eac 100644
> --- a/i386/i386at/model_dep.c
> +++ b/i386/i386at/model_dep.c
> @@ -177,6 +177,11 @@ void machine_init(void)
> #if defined(APIC)
> ioapic_configure();
> #endif
> + /*
> + * Start PIT clock interrupts, this is used
> + * initially before lapic timer is calibrated
> + * for use as a periodic clock source.
> + */
> clkstart();
>
> /*
> @@ -631,11 +636,15 @@ void
> startrtclock(void)
> {
> #ifdef APIC
> - unmask_irq(timer_pin);
> - calibrate_lapic_timer();
> - if (cpu_number() != 0) {
> - lapic_enable_timer();
> + if (cpu_number() == 0) {
> + /* cpu0 calls this before other cpus do
> + * to calibrate the lapic timer once for all cpus. */
> + unmask_irq(timer_pin);
> + calibrate_lapic_timer();
> + mask_irq(timer_pin);
> + /* The PIT is no longer required past here */
> }
> + lapic_enable_timer();
> #else
> clkstart();
> #ifndef MACH_HYP
> --
> 2.51.0
>
>
>
--
Samuel
> Il [e2fsck] a bien démarré, mais il m'a rendu la main aussitot en me
> disant "houlala, c'est pas beau à voir votre truc, je préfèrerai que
> vous teniez vous même la tronçonneuse" (traduction libre)
NC in Guide du linuxien pervers : "Bien configurer sa tronçonneuse."
