I was trying different values for time granularity but did not get the results I expected,
which I think is because of the function below:
/**
* timespec_trunc - Truncate timespec to a granularity
* @t: Timespec
* @gran: Granularity in ns.
*
* Truncate a timespec to a granularity. gran must be smaller than a second.
* Always rounds down.
*
* This function should be only used for timestamps returned by
* current_kernel_time() or CURRENT_TIME, not with do_gettimeofday() because
* it doesn't handle the better resolution of the latter.
*/
struct timespec timespec_trunc(struct timespec t, unsigned gran)
{
/*
* Division is pretty slow so avoid it for common cases.
* Currently current_kernel_time() never returns better than
* jiffies resolution. Exploit that.
*/
if (gran <= jiffies_to_usecs(1) * 1000) {
/* nothing */
} else if (gran == 1000000000) {
t.tv_nsec = 0;
} else {
t.tv_nsec -= t.tv_nsec % gran;
}
return t;
}
EXPORT_SYMBOL(timespec_trunc);
The first condition gives no rounding, but although current_kernel_time()
is only updated every jiffy, it is not rounded to that granularity.
With HZ=250 and gran=1000000, I was expecting to see just 3 decimal places
of time. But instead I get:
# touch a
# stat a
File: `a'
Size: 0 Blocks: 0 IO Block: 4096 regular empty file
Device: fc01h/64513d Inode: 65 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2008-04-09 17:05:38.032726306 +0300
Modify: 2008-04-09 17:05:38.032726306 +0300
Change: 2008-04-09 17:05:38.032726306 +0300
If I change gran to 10000000, it works:
# touch a
# stat a
File: `a'
Size: 0 Blocks: 0 IO Block: 4096 regular empty file
Device: fc01h/64513d Inode: 65 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2008-04-09 17:34:25.180000000 +0300
Modify: 2008-04-09 17:34:25.180000000 +0300
Change: 2008-04-09 17:34:25.180000000 +0300
Is this how it is meant to be?
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