Re: [PATCH] i386: fix TSC clock source calibration error

[Chuck Ebbert]

On 10/16/2007 10:50 AM, Dave Johnson wrote:
> From: Dave Johnson <djohnson@xxxxxxxxxxxxxxxxxxxxxx>
> 
> I ran into this problem on a system that was unable to obtain NTP sync
> because the clock was running very slow (over 10000ppm slow). ntpd had
> declared all of its peers 'reject' with 'peer_dist' reason.
> 
> On investigation, the tsc_khz variable was significantly incorrect
> causing xtime to run slow.  After a reboot tsc_khz was correct so I
> did a reboot test to see how often the problem occurred:
> 
> Test was done on a 2000 Mhz Xeon system.  Of 689 reboots, 8 of them
> had unacceptable tsc_khz values (>500ppm):
> 
>  range of tsc_khz  # of boots  % of boots
> -----------------  ----------  ----------
>         < 1999750           0      0.000%
> 1999750 - 1999800          21      3.048%
> 1999800 - 1999850         166     24.128%
> 1999850 - 1999900         241     35.029%
> 1999900 - 1999950         211     30.669%
> 1999950 - 2000000          42      6.105%
> 2000000 - 2000000           0      0.000%
> 2000050 - 2000100           0      0.000%
>                    [...]
> 2000100 - 2015000           1      0.145%  << BAD
> 2015000 - 2030000           6      0.872%  << BAD
> 2030000 - 2045000           1      0.145%  << BAD
> 2045000 <                   0      0.000%
> 
> The worst boot was 2032.577 Mhz, over 1.5% off!
> 
> It appears that on rare occasions, mach_countup() is taking longer to
> complete than necessary.
> 
> I suspect that this is caused by the CPU taking a periodic SMI
> interrupt right at the end of the 30ms calibration loop.  This would
> cause the loop to delay while the SMI BIOS hander runs. The resulting
> TSC value is beyond what it actually should be resulting in a higher
> tsc_khz.
> 
> The below patch makes native_calculate_cpu_khz() take the best
> (shortest duration, lowest khz) run of it's 3 calibration loops.  If a
> SMI goes off causing a bad result (long duration, higher khz) it will
> be discarded.
> 
> With the patch applied, 300 boots of the same system produce good
> results:
> 
>  range of tsc_khz  # of boots  % of boots
> -----------------  ----------  ----------
>         < 1999750           0      0.000%
> 1999750 - 1999800          30     10.000%
> 1999800 - 1999850         166     55.333%
> 1999850 - 1999900          89     29.667%
> 1999900 - 1999950          15      5.000%
> 1999950 <                   0      0.000%
> 
> Problem was found and tested against 2.6.18.  Patch is against 2.6.22.
> 
> Signed-off-by: Dave Johnson <djohnson@xxxxxxxxxxxxxxxxxxxxxx>
> 
> ===== arch/i386/kernel/tsc.c 1.27 vs edited =====
> --- 1.27/arch/i386/kernel/tsc.c	2007-05-02 13:27:18 -04:00
> +++ edited/arch/i386/kernel/tsc.c	2007-10-15 16:31:04 -04:00
> @@ -122,7 +122,7 @@
>  {
>  	unsigned long long start, end;
>  	unsigned long count;
> -	u64 delta64;
> +	u64 delta64 = (u64)ULLONG_MAX;
>  	int i;
>  	unsigned long flags;
>  
> @@ -134,6 +134,7 @@
>  		rdtscll(start);
>  		mach_countup(&count);
>  		rdtscll(end);
> +		delta64 = min(delta64, (end - start));
>  	}
>  	/*
>  	 * Error: ECTCNEVERSET
> @@ -143,8 +144,6 @@
>  	 */
>  	if (count <= 1)
>  		goto err;
> -
> -	delta64 = end - start;
>  
>  	/* cpu freq too fast: */
>  	if (delta64 > (1ULL<<32))
> 

(added cc:)
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