* Mathieu Desnoyers ([email protected]) wrote:
> * Andi Kleen ([email protected]) wrote:
> >
> > > I noticed the same thing about interrupts off when going through the
> > > code.
> >
> > That's only on a slow path during cpu frequency changing while the TSC is instable.
> > Shouldn't be that common.
> >
> > -Andi
>
> Hrm, I don't see why you can get away without disabling interrupts in
> the fast path:
>
> +unsigned long long tsc_sched_clock(void)
> +{
> + unsigned long long r;
> + struct sc_data *sc = &get_cpu_var(sc_data);
> +
> + if (unlikely(sc->unstable)) {
> + r = (jiffies_64 - sc->sync_base) * (1000000000 / HZ);
> + r += sc->ns_base;
> + /*
> + * last_val is used to avoid non monotonity on a
> + * stable->unstable transition. Make sure the time
> + * never goes to before the last value returned by the
> + * TSC clock.
> + */
> + while (r <= sc->last_val) {
> + rmb();
> + r = sc->last_val + 1;
> + rmb();
> + }
> + sc->last_val = r;
>
> Here, slow path, we update last_val (64 bits value). Must be protected.
>
> + } else {
> + rdtscll(r);
> + r = __cycles_2_ns(sc, r);
> + sc->last_val = r;
>
> Here, fast path, we update last_val too so it is ready to be read when
> the tsc will become unstable.
>
> If we don't disable interrupts around its update, we could have: (LSB vs
> MSB update order is arbitrary)
>
> update sc->last_val 32MSB
> interrupt comes
> update sc->last_val 32MSB
> update sc->last_val 32LSB
> iret
> update sc->last_val 32LSB
>
> So if, after this, we run tsc_sched_clock() with an unstable TSC, we
> read a last_val containing the interrupt's MSB and the last_val LSB. It
> can particularity hurt if we are around a 32 bits overflow, because time
> could "jump" forward of about 1.43 seconds on a 3 GHz system.
>
> So I guess we need synchronization on the fast path, and therefore using
> cmpxchg_local on x86_64 and cmpxchg64_local on i386 makes sense.
>
The case above explained the issue for i386. For x86_64, the race goes
like this:
read tsc
interrupt
read tsc
update sc->last_val
iret
update sc->last_val
Here, last_val is not at its highest value anymore. This is why a
cmpxchg is useful on x86_64.
Mathieu
> Mathieu
>
> + }
> +
> + put_cpu_var(sc_data);
> +
> + return r;
> +}
>
>
>
> --
> Mathieu Desnoyers
> Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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