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. ;-) Thanx, Paul > ------------------------------------------------------------------------ > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 76d6c0902f66..8d6ebc0944ec 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2333,18 +2333,16 @@ static void invoke_rcu_core_kthread(void) > } > > /* > - * Schedule RCU callback invocation. If the running implementation of RCU > - * does not support RCU priority boosting, just do a direct call, otherwise > - * wake up the per-CPU kernel kthread. Note that because we are running > - * on the current CPU with softirqs disabled, the rcu_cpu_kthread_task > - * cannot disappear out from under us. > + * Do RCU callback invocation. Not that if we are running !use_softirq, > + * we are already in the rcuc kthread. If callbacks are offloaded, then > + * ->cblist is always empty, so we don't get here. Therefore, we only > + * ever need to check for the scheduler being operational (some callbacks > + * do wakeups, so we do need the scheduler). > */ > static void invoke_rcu_callbacks(struct rcu_data *rdp) > { > if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) > return; > - if (rcu_state.boost || !use_softirq) > - invoke_rcu_core_kthread(); > rcu_do_batch(rdp); > } >