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

[Jason Low <jason.low2@hp.com>]

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;
	}