Re: svn commit: r238755 - head/sys/x86/x86

On Wed, 25 Jul 2012, Andriy Gapon wrote:

> on 25/07/2012 13:21 Konstantin Belousov said the following:
>> ...
>> diff --git a/sys/x86/x86/tsc.c b/sys/x86/x86/tsc.c
>> index 085c339..229b351 100644
>> --- a/sys/x86/x86/tsc.c
>> +++ b/sys/x86/x86/tsc.c
>> @@ -594,6 +594,7 @@ static u_int
>>  tsc_get_timecount(struct timecounter *tc __unused)
>>  {
>>
>> +	rmb();
>>  	return (rdtsc32());
>>  }
>
> This makes sense to me.  We probably want correctness over performance here.
> [BTW, I originally thought that the change was here; brain malfunction]

And I liked the original change because it wasn't here :-).

>> @@ -602,8 +603,9 @@ tsc_get_timecount_low(struct timecounter *tc)
>>  {
>>  	uint32_t rv;
>>
>> +	rmb();
>>  	__asm __volatile("rdtsc; shrd %%cl, %%edx, %0"
>> -	: "=a" (rv) : "c" ((int)(intptr_t)tc->tc_priv) : "edx");
>> +	    : "=a" (rv) : "c" ((int)(intptr_t)tc->tc_priv) : "edx");
>>  	return (rv);
>>  }
>>
>
> It would correct here too, but not sure if it would make any difference given that
> some lower bits are discarded anyway.  Probably depends on exact CPU.

It is needed to pessimize this too. :-)

As I have complained before, the loss of resolution from the shift is
easy to see by reading the time from userland, even with syscall overhead
taking 10-20 times longer than the read.  On core2 with TSC-low, a clock-
checking utility gives:

% min 481, max 12031, mean 530.589452, std 51.633626
% 1th: 550 (1296487 observations)
% 2th: 481 (448425 observations)
% 3th: 482 (142650 observations)
% 4th: 549 (61945 observations)
% 5th: 551 (47619 observations)

The numbers are diffences in nanoseconds measured by clock_gettime().
The jump from 481 to 549 is 68.  From this I can tell that the clock
frequency is 1.86 Ghz and the shift is 128, or the clock frequency is
3.72 Ghz and the shift is 256.

On AthlonXP with TSC:

% min 273, max 29075, mean 274.412811, std 80.425963
% 1th: 273 (853962 observations)
% 2th: 274 (745606 observations)
% 3th: 275 (400212 observations)
% 4th: 276 (20 observations)
% 5th: 280 (10 observations)

Now the numbers cluster about the mean.  Although syscalls take much longer
than the loss of resolution with TSC-low, and even the core2 TSC takes
almost as long to read as the loss, it is still possible to see things
happening at the limits of the resolution (~0.5 nsec).

> And, oh hmm, I read AMD Software Optimization Guide for AMD Family 10h Processors
> and they suggest using cpuid (with a note that it may be intercepted in
> virtualized environments) or _mfence_ in the discussed role (Appendix F of the
> document).
> Googling for 'rdtsc mfence lfence' yields some interesting results.

The second hit was for the shrd pessimization/loss of resolution and
a memory access hack in lkml in 2011.  I now seem to remember jkim
mentioning the memory access hack.  rmb() on i386 has a related memory
access hack, but now with a lock prefix that defeats the point of the
2011 hack (it wanted to save 5 nsec by removing fences).  rmb() on
amd64 uses lfence.

Some of the other hits are a bit old.  The 8th one was by me in the
thread about kib@ implementing gettimeofday() in userland.

Bruce
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