On Tue, Jun 23, 2015 at 11:26:26AM -0700, Paul E. McKenney wrote:
> On Tue, Jun 23, 2015 at 08:04:11PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 23, 2015 at 10:30:38AM -0700, Paul E. McKenney wrote:
> > > Good, you don't need this because you can check for dynticks later.
> > > You will need to check for offline CPUs.
> >
> > get_online_cpus()
> > for_each_online_cpus() {
> > ...
> > }
> >
> > is what the new code does.
>
> Ah, I missed that this was not deleted.
But get_online_cpus() will re-introduce a deadlock.
Thanx, Paul
> > > > - /*
> > > > - * Each pass through the following loop attempts to force a
> > > > - * context switch on each CPU.
> > > > - */
> > > > - while (try_stop_cpus(cma ? cm : cpu_online_mask,
> > > > - synchronize_sched_expedited_cpu_stop,
> > > > - NULL) == -EAGAIN) {
> > > > - put_online_cpus();
> > > > - atomic_long_inc(&rsp->expedited_tryfail);
> > > > -
> > > > - /* Check to see if someone else did our work for us. */
> > > > - s = atomic_long_read(&rsp->expedited_done);
> > > > - if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) {
> > > > - /* ensure test happens before caller kfree */
> > > > - smp_mb__before_atomic(); /* ^^^ */
> > > > - atomic_long_inc(&rsp->expedited_workdone1);
> > > > - free_cpumask_var(cm);
> > > > - return;
> > >
> > > Here you lose batching. Yeah, I know that synchronize_sched_expedited()
> > > is -supposed- to be used sparingly, but it is not cool for the kernel
> > > to melt down just because some creative user found a way to heat up a
> > > code path. Need a mutex_trylock() with a counter and checking for
> > > others having already done the needed work.
> >
> > I really think you're making that expedited nonsense far too accessible.
>
> This has nothing to do with accessibility and everything to do with
> robustness. And with me not becoming the triage center for too many
> non-RCU bugs.
>
> > But it was exactly that trylock I was trying to get rid of.
>
> OK. Why, exactly?
>
> > > And we still need to be able to drop back to synchronize_sched()
> > > (AKA wait_rcu_gp(call_rcu_sched) in this case) in case we have both a
> > > creative user and a long-running RCU-sched read-side critical section.
> >
> > No, a long-running RCU-sched read-side is a bug and we should fix that,
> > its called a preemption-latency, we don't like those.
>
> Yes, we should fix them. No, they absolutely must not result in a
> meltdown of some unrelated portion of the kernel (like RCU), particularly
> if this situation occurs on some system running a production workload
> that doesn't happen to care about preemption latency.
>
> > > > + for_each_online_cpu(cpu) {
> > > > + struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
> > > >
> > > > - /* Recheck to see if someone else did our work for us.
> > > > */
> > > > - s = atomic_long_read(&rsp->expedited_done);
> > > > - if (ULONG_CMP_GE((ulong)s, (ulong)firstsnap)) {
> > > > - /* ensure test happens before caller kfree */
> > > > - smp_mb__before_atomic(); /* ^^^ */
> > > > - atomic_long_inc(&rsp->expedited_workdone2);
> > > > - free_cpumask_var(cm);
> > > > - return;
> > > > - }
> > > > + /* Offline CPUs, idle CPUs, and any CPU we run on are
> > > > quiescent. */
> > > > + if (!(atomic_add_return(0, &rdtp->dynticks) & 0x1))
> > > > + continue;
> > >
> > > Let's see... This does work for idle CPUs and for nohz_full CPUs running
> > > in userspace.
> > >
> > > It does not work for the current CPU, so the check needs an additional
> > > check against raw_smp_processor_id(), which is easy enough to add.
> >
> > Right, realized after I send it out, but it _should_ work for the
> > current cpu too. Just pointless doing it.
>
> OK, and easily fixed up in any case.
>
> > > There always has been a race window involving CPU hotplug.
> >
> > There is no hotplug race, the entire thing has get_online_cpus() held
> > across it.
>
> Which I would like to get rid of, but not urgent.
>
> > > > + stop_one_cpu(cpu, synchronize_sched_expedited_cpu_stop,
> > > > NULL);
> > >
> > > My thought was to use smp_call_function_single(), and to have the function
> > > called recheck dyntick-idle state, avoiding doing a set_tsk_need_resched()
> > > if so.
> >
> > set_tsk_need_resched() is buggy and should not be used.
>
> OK, what API is used for this purpose?
>
> > > This would result in a single pass through schedule() instead
> > > of stop_one_cpu()'s double context switch. It would likely also require
> > > some rework of rcu_note_context_switch(), which stop_one_cpu() avoids
> > > the need for.
> >
> > _IF_ you're going to touch rcu_note_context_switch(), you might as well
> > use a completion, set it for the number of CPUs that need a resched,
> > spray resched-IPI and have rcu_note_context_switch() do a complete().
> >
> > But I would really like to avoid adding code to
> > rcu_note_context_switch(), because we run that on _every_ single context
> > switch.
>
> I believe that I can rework the current code to get the effect without
> increased overhead, given that I have no intention of adding the
> complete(). Adding the complete -would- add overhead to that fastpath.
>
> Thanx, Paul
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