Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer to improve scalability

Oleg Nesterov
2015-03-02T19:42:53Z

Well, I forgot everything about this code, but let me ask anyway ;)

On 03/02, Jason Low wrote:
>
> -static void update_gt_cputime(struct task_cputime *a, struct task_cputime *b)
> +static inline void __update_gt_cputime(atomic64_t *cputime, u64 sum_cputime)
> {
> - if (b->utime > a->utime)
> - a->utime = b->utime;
> -
> - if (b->stime > a->stime)
> - a->stime = b->stime;
> + u64 curr_cputime;
> + /*
> + * Set cputime to sum_cputime if sum_cputime > cputime. Use cmpxchg
> + * to avoid race conditions with concurrent updates to cputime.
> + */
> +retry:
> + curr_cputime = atomic64_read(cputime);
> + if (sum_cputime > curr_cputime) {
> + if (atomic64_cmpxchg(cputime, curr_cputime, sum_cputime) != curr_cputime)
> + goto retry;
> + }
> +}
>
> - if (b->sum_exec_runtime > a->sum_exec_runtime)
> - a->sum_exec_runtime = b->sum_exec_runtime;
> +static void update_gt_cputime(struct thread_group_cputimer *cputimer, struct task_cputime *sum)
> +{
> + __update_gt_cputime(&cputimer->utime, sum->utime);
> + __update_gt_cputime(&cputimer->stime, sum->stime);
> + __update_gt_cputime(&cputimer->sum_exec_runtime, sum->sum_exec_runtime);
> }

And this is called if !cputimer_running().

So who else can update these atomic64_t's ? The caller is called under ->siglock.
IOW, do we really need to cmpxchg/retry ?

Just curious, I am sure I missed something.

> @@ -222,13 +239,10 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
> * it.
> */
> thread_group_cputime(tsk, &sum);
> - raw_spin_lock_irqsave(&cputimer->lock, flags);
> - cputimer->running = 1;
> - update_gt_cputime(&cputimer->cputime, &sum);
> - } else
> - raw_spin_lock_irqsave(&cputimer->lock, flags);
> - *times = cputimer->cputime;
> - raw_spin_unlock_irqrestore(&cputimer->lock, flags);
> + update_gt_cputime(cputimer, &sum);
> + ACCESS_ONCE(cputimer->running) = 1;

WRITE_ONCE() looks better... but it is not clear to me why do we need it
at all.

Oleg.