Re: [Xen-devel] Re: Fix for get_s_time()

[Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>]

Keir,

Well, I'm glad you didn't take the patch as my overnight test was
not successful. I'm afraid that I was overly confident.
I'll re-submit when I get to the bottom of this.

thanks,
Dave

Keir Fraser wrote:

> Is the issue that there is a variable delay in waiting to acquire the
> platform_timer_lock? If so, you can fix that by, for example, making
> read_platform_stime() the *first* thing you do inside the
> local_irq_disable/enable() block in local-time_calibration(). i.e., *before*
> doing the rdtsc() and get_s_time(). This avoids a variable delay between the
> TSC-based time estimates and the platform-timer-based time estimate.
>
> If that fixes your issue I'll apply that in preference.
>
> -- Keir
>
> On 21/4/08 23:55, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:
>
>  
>
> > What's the race? All accesses to platform time are already done under the
> > platform_timer_lock as far as I can see.
> >
> > -- Keir
> >
> > On 21/4/08 21:32, "Dave Winchell" <dwinchell@xxxxxxxxxxxxxxx> wrote:
> >
> >    
> >
> > > Keir,
> > >
> > > In my work on layering hpet on get_s_time, I found a problem
> > > in get_s_time and related code. Because of the problem I was getting
> > > large jumps in the offset between local time and ntp time.
> > > These jumps were on the order of many seconds.
> > >
> > > The issue is the race between local_time_calibration() executing
> > > on one processor and platform_time_calibration() on another.
> > >
> > > I have included a patch which addresses the race in
> > > local_time_calibration(), cpu_frequency_change(), and
> > > init_percpu_time().
> > >
> > > I'm giving you this ahead of the hpet work as it affects all users
> > > of get_s_time().
> > >
> > > I'm confident of the fix in local_time_calibration() as I had failures there
> > > before the fix and no failures after. The other two I'm less confident
> > > in, so check
> > > my work closely there.
> > >
> > > On the hpet over get_s_time() front, this fix allows me to get .0014% error.
> > > This is very close to the error going to the hardware hpet each time.
> > >
> > > Regards,
> > > Dave
> > >
> > >
> > > diff -r a38a41de0800 xen/arch/x86/time.c
> > > --- a/xen/arch/x86/time.c Wed Apr 16 16:42:47 2008 +0100
> > > +++ b/xen/arch/x86/time.c Mon Apr 21 15:19:37 2008 -0400
> > > @@ -530,6 +530,16 @@ static s_time_t read_platform_stime(void
> > >
> > >     return stime;
> > > }
> > > +static s_time_t read_platform_stime_locked(void)
> > > +{
> > > +    u64 count;
> > > +    s_time_t stime;
> > > +
> > > +    count = plt_count64 + ((plt_src.read_counter() - plt_count) & plt_mask);
> > > +    stime = __read_platform_stime(count);
> > > +
> > > +    return stime;
> > > +}
> > >
> > > static void platform_time_calibration(void)
> > > {
> > > @@ -749,6 +759,7 @@ int cpu_frequency_change(u64 freq)
> > > {
> > >     struct cpu_time *t = &this_cpu(cpu_time);
> > >     u64 curr_tsc;
> > > +    unsigned long flags;
> > >
> > >     /* Sanity check: CPU frequency allegedly dropping below 1MHz? */
> > >     if ( freq < 1000000u )
> > > @@ -758,15 +769,15 @@ int cpu_frequency_change(u64 freq)
> > >         return -EINVAL;
> > >     }
> > >
> > > -    local_irq_disable();
> > > +    spin_lock_irqsave(&platform_timer_lock, flags);
> > >     rdtscll(curr_tsc);
> > >     t->local_tsc_stamp = curr_tsc;
> > > -    t->stime_master_stamp = read_platform_stime();
> > > +    t->stime_master_stamp = read_platform_stime_locked();
> > >     /* TSC-extrapolated time may be bogus after frequency change. */
> > >     /*t->stime_local_stamp = get_s_time();*/
> > >     t->stime_local_stamp = t->stime_master_stamp;
> > >     set_time_scale(&t->tsc_scale, freq);
> > > -    local_irq_enable();
> > > +    spin_unlock_irqrestore(&platform_timer_lock, flags);
> > >
> > >     /* A full epoch should pass before we check for deviation. */
> > >     set_timer(&t->calibration_timer, NOW() + EPOCH);
> > > @@ -830,16 +841,18 @@ static void local_time_calibration(void
> > >     /* The overall calibration scale multiplier. */
> > >     u32 calibration_mul_frac;
> > >
> > > +    unsigned long flags;
> > > +
> > >     prev_tsc          = t->local_tsc_stamp;
> > >     prev_local_stime  = t->stime_local_stamp;
> > >     prev_master_stime = t->stime_master_stamp;
> > >
> > >     /* Disable IRQs to get 'instantaneous' current timestamps. */
> > > -    local_irq_disable();
> > > +    spin_lock_irqsave(&platform_timer_lock, flags);
> > >     rdtscll(curr_tsc);
> > >     curr_local_stime  = get_s_time();
> > > -    curr_master_stime = read_platform_stime();
> > > -    local_irq_enable();
> > > +    curr_master_stime = read_platform_stime_locked();
> > > +    spin_unlock_irqrestore(&platform_timer_lock, flags);
> > >
> > > #if 0
> > >     printk("PRE%d: tsc=%"PRIu64" stime=%"PRIu64" master=%"PRIu64"\n",
> > > @@ -944,10 +957,10 @@ void init_percpu_time(void)
> > >     unsigned long flags;
> > >     s_time_t now;
> > >
> > > -    local_irq_save(flags);
> > > +    spin_lock_irqsave(&platform_timer_lock, flags);
> > >     rdtscll(t->local_tsc_stamp);
> > > -    now = !plt_src.read_counter ? 0 : read_platform_stime();
> > > -    local_irq_restore(flags);
> > > +    now = !plt_src.read_counter ? 0 : read_platform_stime_locked();
> > > +    spin_unlock_irqrestore(&platform_timer_lock, flags);
> > >
> > >     t->stime_master_stamp = now;
> > >     t->stime_local_stamp  = now;
> > >      
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >    
>  
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