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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

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;
> 
> 
> 
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