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

[Csdncannon <csdncannon@gmail.com>]

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Hi guys
       In fact my problem is gettimeofday cannot return right value
sometimes, and this will bring instability to our system software. You can
find a law from the log that there is a 17592 seconds' shift every time
error occurs.



2010/3/26 Csdncannon <csdncannon@gmail.com>

> Yes, the missing 64-bit conversion is the key problem, I will try removing
> isync later.
>
> Thanks for your support.
>
>
> 2010/3/26 Segher Boessenkool <segher@kernel.crashing.org>
>
>> > Yes indeed.  Could you post the relevant piece if disassembly from
>>
>> > your original binary (the one that has the problem)?  Or send me the
>> > binary (not to the mailing list), I'll do it then.
>>
>> Ah scratch that.  I compiled your original code (after fixing the
>> compile errors -- there is no such type as "bool" in C).
>>
>> The problem is that  (upper << 32) | lower  thing.  "upper" is a 32-bit
>> type, so shifting it by 32 or more bits is undefined.  GCC compiles this
>> to (shortened):
>>
>> 0: mftbu 9 ; mftbl 11 ; mftbu 0 ; cmpw 0,9 ; bne 0b  # so far so good
>>   slwi 0,0,0 ; or 4,0,11 ; li 3,0 ; blr
>>
>> so it shifts by 0, i.e. it does  upper | lower .
>>
>> Case closed, no hardware problem :-)
>>
>>
>> Segher
>>
>>
>

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[-- Attachment #2: gettime.c --]
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#include <iostream>

#include <stdlib.h>

#include <string.h>

#include <sys/time.h>

 

int main(void) 

{

    struct timeval now, timeout;

    gettimeofday( &now, (struct timezone *)NULL );

    int count = 0;

 

    while(1)

    {

        count ++;

        if(gettimeofday( &timeout, (struct timezone *)NULL ))
		{
			printf("gettimeofday failed1: tv_sec %d , tv_usec %d \n", timeout.tv_sec, timeout.tv_usec);
		}		

        usleep( 5 );

        if(gettimeofday( &now, (struct timezone *)NULL ))
		{
			printf("gettimeofday failed2: tv_sec %d , tv_usec %d \n", now.tv_sec, now.tv_usec);
		};

 

        if ((timeout.tv_sec - now.tv_sec) > 1)

        {

            printf("Serious : timeout %d , now %d \n", timeout.tv_sec, now.tv_sec);

        }
		else if((now.tv_sec - timeout.tv_sec) > 1)
		{
			printf("Failed : timeout %d , now %d \n", timeout.tv_sec, now.tv_sec);
		}

        if(count > 1000000)

        {

            count = 0;

            printf(" %d ,   %d \n", timeout.tv_sec, now.tv_sec);

        }

    }

 

}


[-- Attachment #3: gettime.log --]
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root@tlab8378:/root> /mnt/server/Gino/gettime
 1270540744 ,   1270540744
Failed : timeout 1270540749 , now 1270558341
Serious : timeout 1270558341 , now 1270540749
 1270540755 ,   1270540755
 1270540765 ,   1270540765
Failed : timeout 1270540774 , now 1270558366
Serious : timeout 1270558366 , now 1270540774
 1270540775 ,   1270540775
 1270540786 ,   1270540786
 1270540796 ,   1270540796
Failed : timeout 1270540802 , now 1270558394
Serious : timeout 1270558394 , now 1270540802
Failed : timeout 1270540803 , now 1270558395
Serious : timeout 1270558395 , now 1270540803
 1270540806 ,   1270540806
Failed : timeout 1270540814 , now 1270558406
Serious : timeout 1270558406 , now 1270540814
 1270540816 ,   1270540816
 1270540827 ,   1270540827
 1270540837 ,   1270540837
 1270540847 ,   1270540847
Failed : timeout 1270540855 , now 1270558447
Serious : timeout 1270558447 , now 1270540855
 1270540858 ,   1270540858