On Wed, Mar 23, 2016 at 09:58:42AM +0200, Konstantin Belousov wrote:
> If TSC has the behaviour you described, i.e. suddenly jumping random
> steps on single CPU, from the point of view of kernel, then the system
> is seriosly misbehaving. The timekeeping stuff would be badly broken
> regardless of the ipi_wait(). I do not see why should we worry about
> that in ipi_wait().
>
> I proposed slightly different thing, i.e. using timekeep code to indirect
> to TSC if configured so. Below is the proof of concept patch, use of
> nanouptime() may be too naive, and binuptime() would cause tiny bit less
> overhead, but I do not want to think about arithmetic.
>
> diff --git a/sys/x86/x86/local_apic.c b/sys/x86/x86/local_apic.c
> index 7e5087b..fdc12a4 100644
> --- a/sys/x86/x86/local_apic.c
> +++ b/sys/x86/x86/local_apic.c
> @@ -1621,31 +1621,37 @@ SYSINIT(apic_setup_io, SI_SUB_INTR, SI_ORDER_THIRD,
> apic_setup_io, NULL);
> * private to the MD code. The public interface for the rest of the
> * kernel is defined in mp_machdep.c.
> */
> +
> +/*
> + * Wait delay microseconds for IPI to be sent. If delay is -1, we
> + * wait forever.
> + */
> static int
> native_lapic_ipi_wait(int delay)
> {
> - int x;
> + struct timespec end, x;
>
> /* LAPIC_ICR.APIC_DELSTAT_MASK is undefined in x2APIC mode */
> if (x2apic_mode)
> return (1);
>
> - /*
> - * Wait delay microseconds for IPI to be sent. If delay is
> - * -1, we wait forever.
> - */
> - if (delay == -1) {
> - while ((lapic_read_icr_lo() & APIC_DELSTAT_MASK) !=
> - APIC_DELSTAT_IDLE)
> - ia32_pause();
> - return (1);
> + if (delay != -1) {
> + KASSERT(delay > 0, ("wrong delay %d", delay));
> + x.tv_sec = delay / 1000000;
> + x.tv_nsec = (delay % 1000000) * 1000;
> + nanouptime(&end);
> + timespecadd(&end, &x);
> }
> -
> - for (x = 0; x < delay; x++) {
> + for (;;) {
> if ((lapic_read_icr_lo() & APIC_DELSTAT_MASK) ==
> APIC_DELSTAT_IDLE)
> return (1);
> - DELAY(1);
> + ia32_pause();
> + if (delay != -1) {
> + nanouptime(&x);
> + if (timespeccmp(&x, &end, >))
> + break;
> + }
> }
> return (0);
> }
Of course this cannot work if timecounter takes any locks, even spinlock.
Only TSC or similar lock-less mechanisms are allowed there. Meh.
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