Re: [RFC PATCH] sched, timer: Use atomics for thread_group_cputimer stats

Peter Zijlstra
2015-01-23T20:08:35Z

On Fri, Jan 23, 2015 at 11:23:36AM -0800, Jason Low wrote:
> On Fri, 2015-01-23 at 10:25 +0100, Peter Zijlstra wrote:
> > On Thu, Jan 22, 2015 at 07:31:53PM -0800, Jason Low wrote:
> > > +static void update_gt_cputime(struct thread_group_cputimer *a, struct task_cputime *b)
> > > {
> > > + if (b->utime > atomic64_read(&a->utime))
> > > + atomic64_set(&a->utime, b->utime);
> > >
> > > + if (b->stime > atomic64_read(&a->stime))
> > > + atomic64_set(&a->stime, b->stime);
> > >
> > > + if (b->sum_exec_runtime > atomic64_read(&a->sum_exec_runtime))
> > > + atomic64_set(&a->sum_exec_runtime, b->sum_exec_runtime);
> > > }
> >
> > See something like this is not safe against concurrent adds.
>
> How about something like:
>
> u64 a_utime, a_stime, a_sum_exec_runtime;
>
> retry_utime:
> a_utime = atomic64_read(&a->utime);
> if (b->utime > a_utime) {
> if (atomic64_cmpxchg(&a->utime, a_utime, b->utime) != a_utime)
> goto retry_utime;
> }
>
> retry_stime:
> a_stime = atomic64_read(&a->stime);
> if (b->stime > a_stime) {
> if (atomic64_cmpxchg(&a->stime, a_stime, b->stime) != a_stime)
> goto retry_stime;
> }
>
> retry_sum_exec_runtime:
> a_sum_exec_runtime = atomic64_read(&a->sum_exec_runtime);
> if (b->sum_exec_runtime > a_sum_exec_runtime) {
> if (atomic64_cmpxchg(&a->sum_exec_runtime, a_sum_exec_runtime,
> b->sum_exec_runtime) != a_sum_exec_runtime)
> goto retry_sum_exec_runtime;
> }

Disgusting, at least use an inline or macro to avoid repeating it :-)

Also, does anyone care about performance on 32bit systems? There's a few
where atomic64 is abysmal.