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SubjectRe: EXP rcu: Revert expedited GP parallelization cleverness
On 2018-11-01 16:30:31 [-0700], Paul E. McKenney wrote:
> > (Commit 258ba8e089db23f760139266c232f01bad73f85c from linux-rcu)
> >
> > This commit reverts a series of commits starting with fcc635436501 ("rcu:
> > Make expedited GPs handle CPU 0 being offline") and its successors, thus
> > queueing each rcu_node structure's expedited grace-period initialization
> > work on the first CPU of that rcu_node structure.
> >
> > Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> >
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index 0b2c2ad69629..a0486414edb4 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -472,7 +472,6 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
> > static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
> > smp_call_func_t func)
> > {
> > - int cpu;
> > struct rcu_node *rnp;
> >
> > trace_rcu_exp_grace_period(rsp->name, rcu_exp_gp_seq_endval(rsp), TPS("reset"));
> > @@ -494,13 +493,7 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
> > continue;
> > }
> > INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
> > - preempt_disable();
> > - cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask);
> > - /* If all offline, queue the work on an unbound CPU. */
> > - if (unlikely(cpu > rnp->grphi))
> > - cpu = WORK_CPU_UNBOUND;
> > - queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
> > - preempt_enable();
> > + queue_work_on(rnp->grplo, rcu_par_gp_wq, &rnp->rew.rew_work);
> > rnp->exp_need_flush = true;
> > }
>
> How about instead changing the earlier "if" statement to read as follows?
>
> if (!READ_ONCE(rcu_par_gp_wq) ||
> rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
> rcu_is_last_leaf_node(rnp) ||
> IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) {
> /* No workqueues yet or last leaf, do direct call. */
> sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
> continue;
> }
>
> This just adds the "|| IS_ENABLED(CONFIG_PREEMPT_RT_FULL)" to the "if"
> condition.
>
> The advantage of this approach is that it leaves the parallelization
> alone for mainline, and avoids the overhead of the workqueues for -rt.

I don't oppose to the workqueue approach. It is just preempt_disable() +
workqueue don't work on -RT. And if I remember correctly, we can't take
CPU hotplug lock for other reasons (which woould make the
preempt_disable() go away). Also the original argument why that patch
went in was not solid so I though removing the extra complexity would be
a good thing.
However using sync_rcu_exp_select_node_cpus() (based von v4.20-rc1)
should work on -RT from what I can see. And performance wise it should
not matter for -RT because the whole synchronize_.*_expedited() is
disabled on -RT anyway. So it should be used only during boot-up.

> Thanx, Paul

Sebastian

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