Re: [PATCH] [15/58] i386: Rewrite sched_clock

[Mathieu Desnoyers]

* Mathieu Desnoyers (mathieu.desnoyers@xxxxxxxxxx) wrote:
> * Andi Kleen (ak@xxxxxxx) 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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