Re: finding out the value of HZ from userspace

[Micha Feigin <michf@post.tau.ac.il>]

On Sat, Mar 13, 2004 at 08:38:52PM +0100, Arjan van de Ven wrote:
> On Sat, Mar 13, 2004 at 11:34:37AM -0800, John Reiser wrote:
> > Arjan van de Ven wrote:
> > >On Thu, 2004-03-11 at 15:17, Micha Feigin wrote:
> > >
> > >>Is it possible to find out what the kernel's notion of HZ is from user
> > >>space?
> > >>It seem to change from system to system and between 2.4 (100 on i386)
> > >>to 2.6 (1000 on i386).
> > >
> > >
> > >if you can see 1000 from userspace that is a bad kernel bug; can you say
> > >where you find something in units of 1000 ?
> > 
> > create_elf_tables() in fs/binfmt_elf.c tells every ELF execve():
> >         NEW_AUX_ENT(AT_CLKTCK, CLOCKS_PER_SEC);
> > which can be found by crawling through the stack above the pointer
> > to the last environment variable.
> 
> Ugh that should say 100 on x86....
> but..
> param.h:# define USER_HZ        100             /* .. some user interfaces are in "ticks" */
> param.h:# define CLOCKS_PER_SEC (USER_HZ)       /* like times() */
> .....
> that looks like 100 to me.
> 

When dealing with bdflush and a few other interfaces the values need to
be in jiffies which requires knowledge of the kernels notion of HZ not
userspace.

The other option is to try to push a change to make the interface in
centisecs instead of jiffies, question is if it will catch.