On 07/27/2013 07:19 AM, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul...@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 <paul...@linux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweis...@gmail.com>
> Cc: Steven Rostedt <rost...@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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