Messages in this thread | | | Date | Mon, 29 Jul 2013 11:36:05 +0800 | From | Lai Jiangshan <> | Subject | Re: [PATCH RFC nohz_full 7/7] nohz_full: Force RCU's grace-period kthreads onto timekeeping CPU |
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On 07/27/2013 07:19 AM, Paul E. McKenney wrote: > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > > Because RCU's quiescent-state-forcing mechanism is used to drive the > full-system-idle state machine, and because this mechanism is executed > by RCU's grace-period kthreads, this commit forces these kthreads to > run on the timekeeping CPU (tick_do_timer_cpu). To do otherwise would > mean that the RCU grace-period kthreads would force the system into > non-idle state every time they drove the state machine, which would > be just a bit on the futile side. > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > Cc: Frederic Weisbecker <fweisbec@gmail.com> > Cc: Steven Rostedt <rostedt@goodmis.org> > --- > kernel/rcutree.c | 1 + > kernel/rcutree.h | 1 + > kernel/rcutree_plugin.h | 20 +++++++++++++++++++- > 3 files changed, 21 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index aa6d96e..fe83085 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -1286,6 +1286,7 @@ static int rcu_gp_init(struct rcu_state *rsp) > struct rcu_data *rdp; > struct rcu_node *rnp = rcu_get_root(rsp); > > + rcu_bind_gp_kthread(); > raw_spin_lock_irq(&rnp->lock); > rsp->gp_flags = 0; /* Clear all flags: New grace period. */
bind the gp thread when RCU_GP_FLAG_INIT ...
> > diff --git a/kernel/rcutree.h b/kernel/rcutree.h > index e0de5dc..49dac99 100644 > --- a/kernel/rcutree.h > +++ b/kernel/rcutree.h > @@ -560,6 +560,7 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle, > static bool is_sysidle_rcu_state(struct rcu_state *rsp); > static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle, > unsigned long maxj); > +static void rcu_bind_gp_kthread(void); > static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp); > > #endif /* #ifndef RCU_TREE_NONCORE */ > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h > index ff84bed..f65d9c2 100644 > --- a/kernel/rcutree_plugin.h > +++ b/kernel/rcutree_plugin.h > @@ -2544,7 +2544,7 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle, > if (!*isidle || rdp->rsp != rcu_sysidle_state || > cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu) > return; > - /* WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu); */ > + WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);
but call rcu_sysidle_check_cpu() when RCU_GP_FLAG_FQS.
In this time, the thread may not be bound to tick_do_timer_cpu, the WARN_ON_ONCE() may be wrong.
Does any other code ensure the gp thread bound on tick_do_timer_cpu which I missed?
> > /* Pick up current idle and NMI-nesting counter and check. */ > cur = atomic_read(&rdtp->dynticks_idle); > @@ -2570,6 +2570,20 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp) > } > > /* > + * Bind the grace-period kthread for the sysidle flavor of RCU to the > + * timekeeping CPU. > + */ > +static void rcu_bind_gp_kthread(void) > +{ > + int cpu = ACCESS_ONCE(tick_do_timer_cpu); > + > + if (cpu < 0 || cpu >= nr_cpu_ids) > + return; > + if (raw_smp_processor_id() != cpu) > + set_cpus_allowed_ptr(current, cpumask_of(cpu)); > +} > + > +/* > * Return a delay in jiffies based on the number of CPUs, rcu_node > * leaf fanout, and jiffies tick rate. The idea is to allow larger > * systems more time to transition to full-idle state in order to > @@ -2767,6 +2781,10 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp) > return false; > } > > +static void rcu_bind_gp_kthread(void) > +{ > +} > + > static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle, > unsigned long maxj) > {
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