Messages in this thread | | | Date | Wed, 28 Sep 2011 14:42:18 +0200 | From | Martin Schwidefsky <> | Subject | Re: [RFD 4/9] Make total_forks per-cgroup |
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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.
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