linux-kernel - Re: [PATCH 1/3] introduce for_each_process_thread

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Date:	Wed, 3 Aug 2016 22:26:09 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Anderson <anderson@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Wang Shu <shuwang@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] introduce for_each_process_thread_{break,continue}()
 helpers

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.