Re: [RFD 4/9] Make total_forks per-cgroup

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Wed, 28 Sep 2011 12:35:24 +0200
Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Wed, 2011-09-28 at 10:13 +0200, Martin Schwidefsky wrote:
> > On Wed, 28 Sep 2011 00:00:37 +0200
> > Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > 
> > > On Fri, 2011-09-23 at 19:20 -0300, Glauber Costa wrote:
> > > > @@ -1039,6 +1035,8 @@ static void posix_cpu_timers_init(struct task_struct *tsk)
> > > >         INIT_LIST_HEAD(&tsk->cpu_timers[2]);
> > > >  }
> > > >  
> > > > +struct task_group *task_group(struct task_struct *p);
> > > 
> > > That doesn't appear to be actually used in this file..
> > > 
> > > Also, since there's already a for_each_possible_cpu() loop in that
> > > proc/stat function, would it yield some code improvement to make
> > > total_forks a cpu_usage_stat?
> > > 
> > > I guess the whole cputime64_t crap gets in the way of that being
> > > natural... 
> > > 
> > > We could of course kill off the cputime64_t thing, its pretty pointless
> > > and its a u64 all over the board. I think Martin or Heiko created this
> > > stuff (although I might be wrong, my git tree doesn't go back that far).
> > 
> > The reason to introduce cputime_t has been that different architecture
> > needed differently sized integers for their respective representation
> > of cputime. On x86-32 the number of ticks is recorded in a u32, on s390
> > we needed a u64 for the cpu timer values. cputime64_t is needed for
> > cpustat and other sums of cputime that would overflow a cputime_t
> > (in particular on x86-32 with the u32 cputime_t and the u64 cputime64_t).
> > 
> > Now we would convert everything to u64 but that would cause x86-32 to
> > use 64-bit arithmetic for the tick counter. If that is acceptable I
> > can't say.
> 
> Right, so the main point was about cputime64_t, we might as well use a
> u64 for that throughout and ditch the silly cputime64_$op() accessors
> and write normal code.
> 
> But even if cputime_t differs between 32 and 64 bit machines, there is
> no reason actually use cputime_add(), C can do this.
> 
> The only reason to use things like cputime_add() is if you use a non
> simple type, but that doesn't seem to be the case.
> 
> So I think we can simplify the code lots by doing away with cputime64_t
> and all the cputime_*() functions. We can keep cputime_t, or we can use
> unsigned long, which I think will end up doing pretty much the same.
> 
> That is, am I missing some added value of all this cputime*() foo?

C can do the math as long as the encoding of the cputime is simple enough.
Can we demand that a cputime value needs to be an integral type ?

What I did when I wrote all that stuff is to define cputime_t as a struct
that contains a single u64. That way I found all the places in the kernel
that used a cputime and could convert the code accordingly.

My fear is that if the cputime_xxx operations are removed, code will
sneak in again that just uses an unsigned long instead of a cputime_t.
That would break any arch that requires something bigger than a u32 for
its cputime. I really have to find my old debugging patch and see if we
already have bit rot in regard to cputime_t.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.