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SubjectRe: [PATCH tip/core/rcu 03/23] rcu: Move RCU grace-period cleanup into kthread
On Thu, Aug 30, 2012 at 11:18:18AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>
> As a first step towards allowing grace-period cleanup to be preemptible,
> this commit moves the RCU grace-period cleanup into the same kthread
> that is now used to initialize grace periods. This is needed to keep
> scheduling latency down to a dull roar.
>
> Reported-by: Mike Galbraith <mgalbraith@suse.de>
> Reported-by: Dimitri Sivanich <sivanich@sgi.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> kernel/rcutree.c | 112 ++++++++++++++++++++++++++++++------------------------
> 1 files changed, 62 insertions(+), 50 deletions(-)
>
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index ef56aa3..9fad21c 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1045,6 +1045,7 @@ rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
> static int rcu_gp_kthread(void *arg)
> {
> unsigned long flags;
> + unsigned long gp_duration;
> struct rcu_data *rdp;
> struct rcu_node *rnp;
> struct rcu_state *rsp = arg;
> @@ -1135,6 +1136,65 @@ static int rcu_gp_kthread(void *arg)
> rsp->fqs_state = RCU_SIGNAL_INIT;
> raw_spin_unlock_irqrestore(&rnp->lock, flags);
> put_online_cpus();
> +
> + /* Handle grace-period end. */
> + rnp = rcu_get_root(rsp);
> + for (;;) {
> + wait_event_interruptible(rsp->gp_wq,
> + !ACCESS_ONCE(rnp->qsmask) &&
> + !rcu_preempt_blocked_readers_cgp(rnp));
> + if (!ACCESS_ONCE(rnp->qsmask) &&
> + !rcu_preempt_blocked_readers_cgp(rnp))
> + break;
> + flush_signals(current);
> + }
> +
> + raw_spin_lock_irqsave(&rnp->lock, flags);
> + gp_duration = jiffies - rsp->gp_start;
> + if (gp_duration > rsp->gp_max)
> + rsp->gp_max = gp_duration;
> +
> + /*
> + * We know the grace period is complete, but to everyone else
> + * it appears to still be ongoing. But it is also the case
> + * that to everyone else it looks like there is nothing that
> + * they can do to advance the grace period. It is therefore
> + * safe for us to drop the lock in order to mark the grace
> + * period as completed in all of the rcu_node structures.
> + *
> + * But if this CPU needs another grace period, it will take
> + * care of this while initializing the next grace period.
> + * We use RCU_WAIT_TAIL instead of the usual RCU_DONE_TAIL
> + * because the callbacks have not yet been advanced: Those
> + * callbacks are waiting on the grace period that just now
> + * completed.
> + */
> + if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
> + raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> +
> + /*
> + * Propagate new ->completed value to rcu_node
> + * structures so that other CPUs don't have to
> + * wait until the start of the next grace period
> + * to process their callbacks.
> + */
> + rcu_for_each_node_breadth_first(rsp, rnp) {
> + /* irqs already disabled. */
> + raw_spin_lock(&rnp->lock);
> + rnp->completed = rsp->gpnum;
> + /* irqs remain disabled. */
> + raw_spin_unlock(&rnp->lock);
> + }
> + rnp = rcu_get_root(rsp);
> + raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> + }
> +
> + rsp->completed = rsp->gpnum; /* Declare grace period done. */
> + trace_rcu_grace_period(rsp->name, rsp->completed, "end");
> + rsp->fqs_state = RCU_GP_IDLE;
> + if (cpu_needs_another_gp(rsp, rdp))
> + rsp->gp_flags = 1;
> + raw_spin_unlock_irqrestore(&rnp->lock, flags);
> }
> return 0;
> }
> @@ -1182,57 +1242,9 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
> static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
> __releases(rcu_get_root(rsp)->lock)
> {
> - unsigned long gp_duration;
> - struct rcu_node *rnp = rcu_get_root(rsp);
> - struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
> -
> WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
> -
> - /*
> - * Ensure that all grace-period and pre-grace-period activity
> - * is seen before the assignment to rsp->completed.
> - */
> - smp_mb(); /* See above block comment. */
> - gp_duration = jiffies - rsp->gp_start;
> - if (gp_duration > rsp->gp_max)
> - rsp->gp_max = gp_duration;
> -
> - /*
> - * We know the grace period is complete, but to everyone else
> - * it appears to still be ongoing. But it is also the case
> - * that to everyone else it looks like there is nothing that
> - * they can do to advance the grace period. It is therefore
> - * safe for us to drop the lock in order to mark the grace
> - * period as completed in all of the rcu_node structures.
> - *
> - * But if this CPU needs another grace period, it will take
> - * care of this while initializing the next grace period.
> - * We use RCU_WAIT_TAIL instead of the usual RCU_DONE_TAIL
> - * because the callbacks have not yet been advanced: Those
> - * callbacks are waiting on the grace period that just now
> - * completed.
> - */
> - if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
> - raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> -
> - /*
> - * Propagate new ->completed value to rcu_node structures
> - * so that other CPUs don't have to wait until the start
> - * of the next grace period to process their callbacks.
> - */
> - rcu_for_each_node_breadth_first(rsp, rnp) {
> - raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> - rnp->completed = rsp->gpnum;
> - raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> - }
> - rnp = rcu_get_root(rsp);
> - raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> - }
> -
> - rsp->completed = rsp->gpnum; /* Declare the grace period complete. */
> - trace_rcu_grace_period(rsp->name, rsp->completed, "end");
> - rsp->fqs_state = RCU_GP_IDLE;
> - rcu_start_gp(rsp, flags); /* releases root node's rnp->lock. */
> + raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
> + wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */
> }
>
> /*
> --
> 1.7.8
>


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