Messages in this thread Patch in this message | | | Date | Mon, 25 Mar 2019 08:52:20 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v3] rcu: Allow to eliminate softirq processing from rcutree |
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On Mon, Mar 25, 2019 at 08:08:00AM -0700, Paul E. McKenney wrote: > On Mon, Mar 25, 2019 at 09:41:29AM -0400, Joel Fernandes wrote: > > On Sun, Mar 24, 2019 at 04:42:11PM -0700, Paul E. McKenney wrote: > > > On Sat, Mar 23, 2019 at 09:10:02AM -0700, Paul E. McKenney wrote: > > > > On Fri, Mar 22, 2019 at 05:25:19PM -0700, Paul E. McKenney wrote: > > > > > On Fri, Mar 22, 2019 at 07:48:19PM -0400, Joel Fernandes wrote: > > > > > > On Wed, Mar 20, 2019 at 10:13:33PM +0100, Sebastian Andrzej Siewior wrote: > > > > > > > Running RCU out of softirq is a problem for some workloads that would > > > > > > > like to manage RCU core processing independently of other softirq > > > > > > > work, for example, setting kthread priority. This commit therefore > > > > > > > introduces the `rcunosoftirq' option which moves the RCU core work > > > > > > > from softirq to a per-CPU/per-flavor SCHED_OTHER kthread named rcuc. > > > > > > > The SCHED_OTHER approach avoids the scalability problems that appeared > > > > > > > with the earlier attempt to move RCU core processing to from softirq > > > > > > > to kthreads. That said, kernels built with RCU_BOOST=y will run the > > > > > > > rcuc kthreads at the RCU-boosting priority. > > > > > > [snip] > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > > > > index 0f31b79eb6761..05a1e42fdaf10 100644 > > > > > > > --- a/kernel/rcu/tree.c > > > > > > > +++ b/kernel/rcu/tree.c > > > > > > > @@ -51,6 +51,12 @@ > > > > > > > #include <linux/tick.h> > > > > > > > #include <linux/sysrq.h> > > > > > > > #include <linux/kprobes.h> > > > > > > > +#include <linux/gfp.h> > > > > > > > +#include <linux/oom.h> > > > > > > > +#include <linux/smpboot.h> > > > > > > > +#include <linux/jiffies.h> > > > > > > > +#include <linux/sched/isolation.h> > > > > > > > +#include "../time/tick-internal.h" > > > > > > > > > > > > > > #include "tree.h" > > > > > > > #include "rcu.h" > > > > > > > @@ -92,6 +98,9 @@ struct rcu_state rcu_state = { > > > > > > > /* Dump rcu_node combining tree at boot to verify correct setup. */ > > > > > > > static bool dump_tree; > > > > > > > module_param(dump_tree, bool, 0444); > > > > > > > +/* Move RCU_SOFTIRQ to rcuc kthreads. */ > > > > > > > +static bool use_softirq = 1; > > > > > > > +module_param(use_softirq, bool, 0444); > > > > > > > /* Control rcu_node-tree auto-balancing at boot time. */ > > > > > > > static bool rcu_fanout_exact; > > > > > > > module_param(rcu_fanout_exact, bool, 0444); > > > > > > > @@ -2253,7 +2262,7 @@ void rcu_force_quiescent_state(void) > > > > > > > EXPORT_SYMBOL_GPL(rcu_force_quiescent_state); > > > > > > > > > > > > > > /* Perform RCU core processing work for the current CPU. */ > > > > > > > -static __latent_entropy void rcu_core(struct softirq_action *unused) > > > > > > > +static __latent_entropy void rcu_core(void) > > > > > > > { > > > > > > > unsigned long flags; > > > > > > > struct rcu_data *rdp = raw_cpu_ptr(&rcu_data); > > > > > > > @@ -2295,6 +2304,34 @@ static __latent_entropy void rcu_core(struct softirq_action *unused) > > > > > > > trace_rcu_utilization(TPS("End RCU core")); > > > > > > > } > > > > > > > > > > > > > > +static void rcu_core_si(struct softirq_action *h) > > > > > > > +{ > > > > > > > + rcu_core(); > > > > > > > +} > > > > > > > + > > > > > > > +static void rcu_wake_cond(struct task_struct *t, int status) > > > > > > > +{ > > > > > > > + /* > > > > > > > + * If the thread is yielding, only wake it when this > > > > > > > + * is invoked from idle > > > > > > > + */ > > > > > > > + if (t && (status != RCU_KTHREAD_YIELDING || is_idle_task(current))) > > > > > > > + wake_up_process(t); > > > > > > > +} > > > > > > > + > > > > > > > +static void invoke_rcu_core_kthread(void) > > > > > > > +{ > > > > > > > + struct task_struct *t; > > > > > > > + unsigned long flags; > > > > > > > + > > > > > > > + local_irq_save(flags); > > > > > > > + __this_cpu_write(rcu_data.rcu_cpu_has_work, 1); > > > > > > > + t = __this_cpu_read(rcu_data.rcu_cpu_kthread_task); > > > > > > > + if (t != NULL && t != current) > > > > > > > + rcu_wake_cond(t, __this_cpu_read(rcu_data.rcu_cpu_kthread_status)); > > > > > > > + local_irq_restore(flags); > > > > > > > +} > > > > > > > + > > > > > > > /* > > > > > > > * Schedule RCU callback invocation. If the running implementation of RCU > > > > > > > * does not support RCU priority boosting, just do a direct call, otherwise > > > > > > > @@ -2306,19 +2343,95 @@ static void invoke_rcu_callbacks(struct rcu_data *rdp) > > > > > > > { > > > > > > > if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) > > > > > > > return; > > > > > > > - if (likely(!rcu_state.boost)) { > > > > > > > - rcu_do_batch(rdp); > > > > > > > - return; > > > > > > > - } > > > > > > > - invoke_rcu_callbacks_kthread(); > > > > > > > + if (rcu_state.boost || !use_softirq) > > > > > > > + invoke_rcu_core_kthread(); > > > > > > > + rcu_do_batch(rdp); > > > > > > > > > > > > Shouldn't there be an else before the rcu_do_batch? If we are waking up the > > > > > > rcuc thread, then that will do the rcu_do_batch when it runs right? > > > > > > > > > > > > Something like: > > > > > > if (rcu_state.boost || !use_softirq) > > > > > > invoke_rcu_core_kthread(); > > > > > > else > > > > > > rcu_do_batch(rdp); > > > > > > > > > > > > Previous code similarly had a return; also. > > > > > > > > > > I believe that you are correct, so I will give it a shot. Good eyes! > > > > > > > > Yet rcutorture disagrees. Actually, if we are using rcuc kthreads, this > > > > is only ever invoked from within tha thread, so the only check we need is > > > > for the scheduler being operational. I am therefore trying this one out. > > > > > > > > Thoughts? > > > > > > And rcutorture likes this one, though at this point this function should > > > be pulled into its sole callsite. ;-) > > > > Great, I'm glad the testing is going well. > > Which reminds me... I have been assuming that Frederic Weisbecker's > split-softirq patches were stalled for the time being. > > http://lkml.kernel.org/r/20190228171242.32144-1-frederic@kernel.org > > If those were to show up soonish, perhaps that would allow per-softirq > control of priority. > > My thought is not to wait, but I figured I should mention it. > > > By the way I enlightened that jitter.sh script about CPU offline issues as > > well (sent patch last week). Let me know if you agree with it. > > I just sent a reply. Still trying to remember why I excluded CPU 0. ;-) > > Perhaps because of issues with single-CPU rcutorture runs?
I also considered and rejected the following patch because it actually can make sense to build with CONFIG_RCU_BOOST but still use softirq, for example, when SCHED_IDLE tasks might get stuck in RCU read-side critical sections. But then I noticed that rcu_spawn_core_kthreads() unconditionally creates the rcuc kthreads if CONFIG_RCU_BOOST.
So I either need to apply the patch below, or I need to remove the "!IS_ENABLED(CONFIG_RCU_BOOST)" from the "if" statement in rcu_spawn_core_kthreads(). The question is "do we allow CONFIG_RCU_BOOST kernels to use RCU_SOFTIRQ?" Some plusses and minuses:
+ Supports the SCHED_IDLE use case for CONFIG_RCU_BOOST without slowing down other workloads. This might be important given RCU flavor consolidation
- Another configuration combination to test and maintain.
So I am leaning towards ditching the patch below in favor of updating the "if" condition in rcu_spawn_core_kthreads().
Thoughts?
Thanx, Paul
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diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index a17034ee4d3d..5782fe9ac27d 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -99,8 +99,12 @@ struct rcu_state rcu_state = { static bool dump_tree; module_param(dump_tree, bool, 0444); /* By default, use RCU_SOFTIRQ instead of rcuc kthreads. */ +#ifdef CONFIG_RCU_BOOST +static const bool use_softirq = 0; +#else /* #ifdef CONFIG_RCU_BOOST */ static bool use_softirq = 1; module_param(use_softirq, bool, 0444); +#endif /* #else #ifdef CONFIG_RCU_BOOST */ /* Control rcu_node-tree auto-balancing at boot time. */ static bool rcu_fanout_exact; module_param(rcu_fanout_exact, bool, 0444);
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