Re: Continual reading from the PowerPc time base register is not stable

[Csdncannon <csdncannon@gmail.com>]

[-- Attachment #1.1: Type: text/plain, Size: 1832 bytes --]

I am really sorry that the previously attached code is wrong, this one
"timebase.c" is the right one, and the "log_timebase" file is the right log.

We are using FreeScale PowerPc 8378, kernel 2.6.28 and compiled as 32-bit.


Thanks
Gino

2010/3/25 Arnd Bergmann <arnd@arndb.de>

> On Thursday 25 March 2010, Benjamin Herrenschmidt wrote:
> > On Thu, 2010-03-25 at 10:41 +0800, Csdncannon wrote:
> > >          In my program, the value of the 64-bit time base register is
> > > read out, and you will find the later value is even smaller than the
> > > earlier value from the log “log_timebase”. While the kernel depends on
> > > the accuracy of the timebase for the compensation of the lost PIT
> > > interrupt, the negative value between two continual timebase reading
> > > will bring to the jump of the jiffies. And this timebase problem will
> > > bring to the instability of the gettimeofday system call.
> > >
> > >          Do you have any idea about this problem, thanks for your any
> > > advice. Attached is the code and log.
> >
> > This is a concern, it should definitely not happen. What machine is
> > that ? is the code compiled 32-bit or 64-bit ? What kernel version ?
> >
> > Arnd, any chance that could relate to the bug you've been chasing on
> > Cell ?
>
> We're still busy with the problem analysis on Cell, waiting for a time
> slot to run the next test kernel. So far it seems like the timebase
> is actually synchronized at a significant accuracy on QS22 to never
> cause this problem with correct code, however it is possible to
> observe incorrect timebase values on Cell whenever the mftb instruction
> is not serialized with memory accesses, e.g. by using an isync in front
> of the mftb. On Power6 and other CPUs, that problem will not happen.
>
>        Arnd
>

[-- Attachment #1.2: Type: text/html, Size: 2319 bytes --]

[-- Attachment #2: timebase.c --]
[-- Type: application/octet-stream, Size: 1604 bytes --]

/* TSC sync test
 *		by: john stultz (johnstul@us.ibm.com)
 *		(C) Copyright IBM 2003, 2005
 *		Licensed under the GPL
 */


#include <stdio.h>
#include <sys/time.h>
#include <stdlib.h>

#define CALLS_PER_LOOP 64

volatile unsigned long long getTimeBase()
{
	unsigned long upper,lower,upper2;
	do {
		asm volatile("sync; isync":::"memory");
		asm volatile("mftbu %0" : "=r" (upper));
		asm volatile("sync; isync":::"memory");
		asm volatile("mftbl %0" : "=r" (lower));
		asm volatile("sync; isync":::"memory");
		asm volatile("mftbu %0" : "=r" (upper2));
		asm volatile("sync; isync":::"memory");
	}while(upper2!=upper);

	return (upper<<32)|lower;
}

int main(int argc, char *argv[])
{
	volatile unsigned long long list[CALLS_PER_LOOP];
	bool bad[CALLS_PER_LOOP];
	int i, inconsistent;


	/* timestamp start of test */
	system("date");
	while(1){
		inconsistent = 0;

		/* Fill list */
		for(i=0; i < CALLS_PER_LOOP; i++)
			list[i] = getTimeBase();
		
		/* Check for inconsistencies */
		for(i=0; i < CALLS_PER_LOOP-1; i++)
		{
			if(list[i] > list[i+1])
			{
				inconsistent = i+1;
				bad[i] = true;
			}
			else{
				bad[i] = false;
			}
		}

		/* display inconsistency */
		if(inconsistent){
			inconsistent--;
			for(i=0; i < CALLS_PER_LOOP; i++){
				if(bad[i] == true)
					printf("--------------------\n");
				printf("0x%llx\n",list[i]);
				if(bad[i-1] == true && bad[i] == false )
					printf("--------------------\n");
			}
			fflush(0);
			/* timestamp inconsistency*/
			system("date");	
		}

	}
	return 0;
}