On Thursday 13 January 2005 06:12, Blaisorblade wrote:
> Well, effectively your reasoning holds, apart for one point: the code
> should not hang because of that large INITIAL_JIFFIES. What we care about
> are only the value of jiffies relative to INITIAL_JIFFIES, indeed.
>
> In fact, that code works on 64-bit arch's. But INITIAL_JIFFIES is not 0 to
> make sure it wraps 5 minutes after the boot, to make sure the wrapping
> mechanism is debugged. And this, currently, does not happen likely.
>
> While on 32-bit archs that construct is like "(unsigned int) (-300*HZ)",
> on 64-bit archs to get a negative number we'd need something like:
>
> (unsigned long)(long)(-300*HZ)
>
> or
>
> (unsigned long)(-300L*HZ)
>
> so that the number is first sign-extended and then casted to unsigned.
Sorry, I now went patching the code, and discovered I was a total moron:
/*
* Have the 32 bit jiffies value wrap 5 minutes after boot
* so jiffies wrap bugs show up earlier.
*/
#define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))
It does not say "jiffies wrap", but the "32 bit values" of jiffies wrap...
So, the x86_64 value must actually be the one you have seen - the problem is
that this trigger some kind of bug in UML - do we have some extra 32-bit var?
However, I have two suggestion
paolo [~/Admin/kernel/6/linux-2.6.11: 02:47 (2)] $ grep INITIAL_JIFFIES `find
arch/um/`
paolo [~/Admin/kernel/6/linux-2.6.11: 02:47 (2)] $ grep INITIAL_JIFFIES `find
include/asm-um`
paolo [~/Admin/kernel/6/linux-2.6.11: 02:47 (2)] $ grep INITIAL_JIFFIES `find
include/linux`
include/linux/jiffies.h:#define INITIAL_JIFFIES ((unsigned long)(unsigned int)
(-300*HZ))
For stolen headers:
$ grep INITIAL_JIFFIES `find include/asm-i386`
$ grep INITIAL_JIFFIES `find include/asm-generic`
I.e. we don't use INITIAL_JIFFIES, it seems. Please point out the problem in
my search!
Instead, for i386:
paolo [~/Admin/kernel/6/linux-2.6.11: 02:48 (2)] $ grep INITIAL_JIFFIES
arch/i386/kernel/*.c
arch/i386/kernel/time.c:u64 jiffies_64 = INITIAL_JIFFIES;
arch/i386/kernel/time.c: xtime.tv_nsec = (INITIAL_JIFFIES % HZ) *
(NSEC_PER_SEC / HZ);
arch/i386/kernel/time.c: xtime.tv_nsec = (INITIAL_JIFFIES % HZ) *
(NSEC_PER_SEC / HZ);
- I've compared the link scripts, and discovered this code:
from arch/um/kernel/{dyn,uml}.lds.S and arch/i386/kernel/vmlinux.lds.S
jiffies = jiffies_64;
in arch/x86_64/kernel/vmlinux.lds.S:
jiffies_64 = jiffies;
I'm going to check if I see what is the difference. This is just a note, it
seems that it could be the same thing, after some analisys - in both cases
the sources define the jiffies_64 var.
However, I didn't want to miss it anyway, just in case.
--
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729
http://www.user-mode-linux.org/~blaisorblade
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