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Re: [PATCH 1/3] introduce for_each_process_thread_{break,continue}() helpers

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  Re: [PATCH 1/3] introduce for_each_process_thread_{break,continue}()  helpers Peter Zijlstra <peterz@infradead.org> - 2016-08-02 18:10 +0200
    Re: [PATCH 1/3] introduce for_each_process_thread_{break,continue}()  helpers Oleg Nesterov <oleg@redhat.com> - 2016-08-03 22:40 +0200
      Re: [PATCH 1/3] introduce for_each_process_thread_{break,continue}()  helpers Peter Zijlstra <peterz@infradead.org> - 2016-08-08 14:30 +0200

#1454878 — Re: [PATCH 1/3] introduce for_each_process_thread_{break,continue}() helpers

On Mon, Jul 25, 2016 at 06:23:48PM +0200, Oleg Nesterov wrote:
> +void for_each_process_thread_continue(struct task_struct **p_leader,
> +				      struct task_struct **p_thread)
> +{
> +	struct task_struct *leader = *p_leader, *thread = *p_thread;
> +	struct task_struct *prev, *next;
> +	u64 start_time;
> +
> +	if (pid_alive(thread)) {
> +		/* mt exec could change the leader */
> +		*p_leader = thread->group_leader;
> +	} else if (pid_alive(leader)) {
> +		start_time = thread->start_time;
> +		prev = leader;
> +
> +		for_each_thread(leader, next) {
> +			if (next->start_time > start_time)
> +				break;
> +			prev = next;
> +		}

This,

> +		*p_thread = prev;
> +	} else {
> +		start_time = leader->start_time;
> +		prev = &init_task;
> +
> +		for_each_process(next) {
> +			if (next->start_time > start_time)
> +				break;
> +			prev = next;
> +		}

and this, could be 'SPEND_TOO_MUCH_TIME' all by themselves.

Unlikely though, nor do I really have a better suggestion :/

> +
> +		*p_leader = prev;
> +		/* a new thread can come after that, but this is fine */
> +		*p_thread = list_last_entry(&prev->signal->thread_head,
> +						struct task_struct,
> +						thread_node);
> +	}
> +
> +	put_task_struct(leader);
> +	put_task_struct(thread);
> +}

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#1456019

Thanks for review and sorry for delay, I am travelling till the end of this week.

On 08/02, Peter Zijlstra wrote:
>
> On Mon, Jul 25, 2016 at 06:23:48PM +0200, Oleg Nesterov wrote:
> > +void for_each_process_thread_continue(struct task_struct **p_leader,
> > +				      struct task_struct **p_thread)
> > +{
> > +	struct task_struct *leader = *p_leader, *thread = *p_thread;
> > +	struct task_struct *prev, *next;
> > +	u64 start_time;
> > +
> > +	if (pid_alive(thread)) {
> > +		/* mt exec could change the leader */
> > +		*p_leader = thread->group_leader;
> > +	} else if (pid_alive(leader)) {
> > +		start_time = thread->start_time;
> > +		prev = leader;
> > +
> > +		for_each_thread(leader, next) {
> > +			if (next->start_time > start_time)
> > +				break;
> > +			prev = next;
> > +		}
>
> This,
>
> > +		*p_thread = prev;
> > +	} else {
> > +		start_time = leader->start_time;
> > +		prev = &init_task;
> > +
> > +		for_each_process(next) {
> > +			if (next->start_time > start_time)
> > +				break;
> > +			prev = next;
> > +		}
>
> and this, could be 'SPEND_TOO_MUCH_TIME' all by themselves.
>
> Unlikely though, nor do I really have a better suggestion :/

Yeees, I don't think this can actually hurt "in practice", but I agree, compared
to rcu_lock_break() this is only bounded by PID_MAX_DEFAULT in theory.

Will you agree if I just add the "int max_scan" argument and make it return a boolean
for the start? The caller will need to abort the for_each_process_thread() loop if
_continue() fails.


Probably this is not what we actually want for show_filter_state(), we can make it
better later. We can "embed" the rcu_lock_break() logic into _continue(), or change
break/continue to record the state (leader_start_time, thread_start_time) so that
a "false" return from _continue() means that the caller needs another schedule(),

	struct state state;

        rcu_read_lock();
        for_each_process_thread(p, t) {
                do_something_slow(p, t);

                if (SPENT_TOO_MANY_TIME) {
                        for_each_process_thread_break(p, t, &state);
        another_break:
                        rcu_read_unlock();
                        schedule();
                        rcu_read_lock();
                        if (!for_each_process_thread_continue(&p, &t, LIMIT, &state))
                        	goto another_break;
               	}
       	}
	rcu_read_unlock();

Not sure. I'd like to do something simple for the start. We need to make
show_state_filter() "preemptible" in any case. And even killable, I think.
Not only it can trivially trigger the soft-lockups (at least), it can simply
never finish.

Oleg.

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#1457756

On Wed, Aug 03, 2016 at 10:26:09PM +0200, Oleg Nesterov wrote:
> Not sure. I'd like to do something simple for the start. We need to make
> show_state_filter() "preemptible" in any case. And even killable, I think.
> Not only it can trivially trigger the soft-lockups (at least), it can simply
> never finish.

Yeah, no objection raelly. Just make sure to put a comment explaining
the limitation/issues with whatever you descide to go for.

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