Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

Oleg Nesterov
2014-08-14T18:36:45Z

On 08/14, Oleg Nesterov wrote:
>
> But just for record, the "lockless" version doesn't look that bad to me,
>
> void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
> {
> struct signal_struct *sig = tsk->signal;
> bool lockless, is_dead;
> struct task_struct *t;
> unsigned long flags;
> u64 exec;
>
> lockless = true;
> is_dead = !lock_task_sighand(p, &flags);
> retry:
> times->utime = sig->utime;
> times->stime = sig->stime;
> times->sum_exec_runtime = exec = sig->sum_sched_runtime;
> if (is_dead)
> return;
>
> if (lockless)
> unlock_task_sighand(p, &flags);
>
> rcu_read_lock();
> for_each_thread(tsk, t) {
> cputime_t utime, stime;
> task_cputime(t, &utime, &stime);
> times->utime += utime;
> times->stime += stime;
> times->sum_exec_runtime += task_sched_runtime(t);
> }
> rcu_read_unlock();
>
> if (lockless) {
> lockless = false;
> is_dead = !lock_task_sighand(p, &flags);
> if (is_dead || exec != sig->sum_sched_runtime)
> goto retry;
> }
> unlock_task_sighand(p, &flags);
> }
>
> The obvious problem is that we should shift lock_task_sighand() from the
> callers to thread_group_cputime() first, or add thread_group_cputime_lockless()
> and change the current users one by one.

OTOH, it is simple to convert do_sys_times() and posix_cpu_clock_get_task()
to use the lockless version, and avoid the new stats_lock and other changes
it needs.

> And of course, stats_lock is more generic.

Yes, this is true in any case.

So I simply do not know.

Oleg.