Re: [PATCH] Provide u64 version of jiffies_to_usecs() in kernel/tsacct.c

[H. Peter Anvin]

Andrew Morton wrote:
> On Fri, 28 Dec 2007 13:26:07 -0800 (PST) Jonathan Lim <jlim@xxxxxxx> wrote:
>
> > It's possible that the values used in and returned from jiffies_to_usecs() are
> > incorrect because of truncation when variables of type u64 are involved.  So a
> > function specific to that type is used instead.
> >
> > Diff'd against: linux/kernel/git/stable/linux-2.6.23.y.git
> >
> > Signed-off-by: Jonathan Lim <jlim@xxxxxxx>
> >
> > --- a/kernel/tsacct.c	2007-12-28 11:58:05.182065029 -0800
> > +++ b/kernel/tsacct.c	2007-12-28 11:57:37.949013675 -0800
> > @@ -71,6 +71,17 @@ void bacct_add_tsk(struct taskstats *sta
> >  
> >  #ifdef CONFIG_TASK_XACCT
> >  
> > +static inline u64 jiffies_to_usecs_u64(const u64 j)
> > +{
> > +#if HZ <= USEC_PER_SEC && !(USEC_PER_SEC % HZ)
> > +	return (USEC_PER_SEC / HZ) * j;
> > +#elif HZ > USEC_PER_SEC && !(HZ % USEC_PER_SEC)
> > +	return (j + (HZ / USEC_PER_SEC) - 1)/(HZ / USEC_PER_SEC);
> > +#else
> > +	return (j * USEC_PER_SEC) / HZ;
> > +#endif
> > +}
> > +
> >  #define KB 1024
> >  #define MB (1024*KB)
> >  /*
> > @@ -81,8 +92,8 @@ void xacct_add_tsk(struct taskstats *sta
> >  	struct mm_struct *mm;
> >  
> >  	/* convert pages-jiffies to Mbyte-usec */
> > -	stats->coremem = jiffies_to_usecs(p->acct_rss_mem1) * PAGE_SIZE / MB;
> > -	stats->virtmem = jiffies_to_usecs(p->acct_vm_mem1) * PAGE_SIZE / MB;
> > +	stats->coremem = jiffies_to_usecs_u64(p->acct_rss_mem1) * PAGE_SIZE / MB;
> > +	stats->virtmem = jiffies_to_usecs_u64(p->acct_vm_mem1) * PAGE_SIZE / MB;
> >  	mm = get_task_mm(p);
> >  	if (mm) {
> >  		/* adjust to KB unit */
>
> Fair enough.  But I guess that new function should be a kernel-wide thing
> because surely other users will turn up.
>
> Peter has been working on the accuracy of some of these conversion
> functions and might need to know about this change?

Yes, the function should be coded using the new #defines produced by 
timeconst.h; that way you end up avoiding a possible overflow in the 
multiplication.

I believe all three cases can be folded, then, to:

	return (j*HZ_TO_USEC_NUM + HZ_TO_USEC_DEN-1) / HZ_TO_USEC_DEN;

I would also like to observe that the roundoff behaviour of the function 
 above is inconsistent; in case 2 it will round up, but in case 3 it 
will round down.  The line proposed above has round up behaviour.

	-hpa
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