Re: [PATCH RFC tip/core/rcu] Parallelize and economize NOCB kthread wakeups

Paul E. McKenney
2014-07-02T16:56:08Z

On Wed, Jul 02, 2014 at 09:46:19AM -0400, Rik van Riel wrote:
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> On 07/02/2014 08:34 AM, Peter Zijlstra wrote:
> > On Fri, Jun 27, 2014 at 07:20:38AM -0700, Paul E. McKenney wrote:
> >> An 80-CPU system with a context-switch-heavy workload can require
> >> so many NOCB kthread wakeups that the RCU grace-period kthreads
> >> spend several tens of percent of a CPU just awakening things.
> >> This clearly will not scale well: If you add enough CPUs, the RCU
> >> grace-period kthreads would get behind, increasing grace-period
> >> latency.
> >>
> >> To avoid this problem, this commit divides the NOCB kthreads into
> >> leaders and followers, where the grace-period kthreads awaken the
> >> leaders each of whom in turn awakens its followers. By default,
> >> the number of groups of kthreads is the square root of the number
> >> of CPUs, but this default may be overridden using the
> >> rcutree.rcu_nocb_leader_stride boot parameter. This reduces the
> >> number of wakeups done per grace period by the RCU grace-period
> >> kthread by the square root of the number of CPUs, but of course
> >> by shifting those wakeups to the leaders. In addition, because
> >> the leaders do grace periods on behalf of their respective
> >> followers, the number of wakeups of the followers decreases by up
> >> to a factor of two. Instead of being awakened once when new
> >> callbacks arrive and again at the end of the grace period, the
> >> followers are awakened only at the end of the grace period.
> >>
> >> For a numerical example, in a 4096-CPU system, the grace-period
> >> kthread would awaken 64 leaders, each of which would awaken its
> >> 63 followers at the end of the grace period. This compares
> >> favorably with the 79 wakeups for the grace-period kthread on an
> >> 80-CPU system.
> >
> > Urgh, how about we kill the entire nocb nonsense and try again?
> > This is getting quite rediculous.
>
> Some observations.
>
> First, the rcuos/N threads are NOT bound to CPU N at all, but are
> free to float through the system.

I could easily bind each to its home CPU by default for CONFIG_NO_HZ_FULL=n.
For CONFIG_NO_HZ_FULL=y, they get bound to the non-nohz_full= CPUs.

> Second, the number of RCU callbacks at the end of each grace period
> is quite likely to be small most of the time.
>
> This suggests that on a system with N CPUs, it may be perfectly
> sufficient to have a much smaller number of rcuos threads.
>
> One thread can probably handle the RCU callbacks for as many as
> 16, or even 64 CPUs...

In many cases, one thread could handle the RCU callbacks for way more
than that. In other cases, a single CPU could keep a single rcuo kthread
quite busy. So something dynamic ends up being required.

But I suspect that the real solution here is to adjust the Kconfig setup
between NO_HZ_FULL and RCU_NOCB_CPU_ALL so that you have to specify boot
parameters to get callback offloading on systems built with NO_HZ_FULL.
Then add some boot-time code so that any CPU that has nohz_full= is
forced to also have rcu_nocbs= set. This would have the good effect
of applying callback offloading only to those workloads for which it
was specifically designed, but allowing those workloads to gain the
latency-reduction benefits of callback offloading.

I do freely confess that I was hoping that callback offloading might one
day completely replace RCU_SOFTIRQ, but that hope now appears to be at
best premature.

Something like the attached patch. Untested, probably does not even build.

Thanx, Paul

------------------------------------------------------------------------

rcu: Don't offload callbacks unless specifically requested

<more here soon>

Not-yet-signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/init/Kconfig b/init/Kconfig
index 9d76b99af1b9..9332d33346ac 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -737,7 +737,7 @@ choice

config RCU_NOCB_CPU_NONE
bool "No build_forced no-CBs CPUs"
- depends on RCU_NOCB_CPU && !NO_HZ_FULL
+ depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
help
This option does not force any of the CPUs to be no-CBs CPUs.
Only CPUs designated by the rcu_nocbs= boot parameter will be
@@ -751,7 +751,7 @@ config RCU_NOCB_CPU_NONE

config RCU_NOCB_CPU_ZERO
bool "CPU 0 is a build_forced no-CBs CPU"
- depends on RCU_NOCB_CPU && !NO_HZ_FULL
+ depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
help
This option forces CPU 0 to be a no-CBs CPU, so that its RCU
callbacks are invoked by a per-CPU kthread whose name begins
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 58fbb8204d15..3b150bfcce3d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2473,6 +2473,9 @@ static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)

if (rcu_nocb_mask == NULL)
return;
+#ifdef CONFIG_NO_HZ_FULL
+ cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
+#endif /* #ifdef CONFIG_NO_HZ_FULL */
if (ls == -1) {
ls = int_sqrt(nr_cpu_ids);
rcu_nocb_leader_stride = ls;