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SubjectRe: [PATCH tip/core/rcu 2/6] rcu: prevent RCU IPI storms in presence of high call_rcu() load
Testing this on top of my vfs-scale patches, a 64-way 32-node ia64
system runs the parallel open/close microbenchmark ~30 times faster
and is scaling linearly now. Single thread performance is not
noticably changed. Thanks very much Paul.

On Wed, Oct 14, 2009 at 10:15:55AM -0700, Paul E. McKenney wrote:
> From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> As the number of callbacks on a given CPU rises, invoke
> force_quiescent_state() only every blimit number of callbacks
> (defaults to 10,000), and even then only if no other CPU has invoked
> force_quiescent_state() in the meantime.
>
> Reported-by: Nick Piggin <npiggin@suse.de>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> kernel/rcutree.c | 29 ++++++++++++++++++++++++-----
> kernel/rcutree.h | 4 ++++
> 2 files changed, 28 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 705f02a..ddbf111 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -958,7 +958,7 @@ static void rcu_offline_cpu(int cpu)
> * Invoke any RCU callbacks that have made it to the end of their grace
> * period. Thottle as specified by rdp->blimit.
> */
> -static void rcu_do_batch(struct rcu_data *rdp)
> +static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
> {
> unsigned long flags;
> struct rcu_head *next, *list, **tail;
> @@ -1011,6 +1011,13 @@ static void rcu_do_batch(struct rcu_data *rdp)
> if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark)
> rdp->blimit = blimit;
>
> + /* Reset ->qlen_last_fqs_check trigger if enough CBs have drained. */
> + if (rdp->qlen == 0 && rdp->qlen_last_fqs_check != 0) {
> + rdp->qlen_last_fqs_check = 0;
> + rdp->n_force_qs_snap = rsp->n_force_qs;
> + } else if (rdp->qlen < rdp->qlen_last_fqs_check - qhimark)
> + rdp->qlen_last_fqs_check = rdp->qlen;
> +
> local_irq_restore(flags);
>
> /* Re-raise the RCU softirq if there are callbacks remaining. */
> @@ -1224,7 +1231,7 @@ __rcu_process_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
> }
>
> /* If there are callbacks ready, invoke them. */
> - rcu_do_batch(rdp);
> + rcu_do_batch(rsp, rdp);
> }
>
> /*
> @@ -1288,10 +1295,20 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
> rcu_start_gp(rsp, nestflag); /* releases rnp_root->lock. */
> }
>
> - /* Force the grace period if too many callbacks or too long waiting. */
> - if (unlikely(++rdp->qlen > qhimark)) {
> + /*
> + * Force the grace period if too many callbacks or too long waiting.
> + * Enforce hysteresis, and don't invoke force_quiescent_state()
> + * if some other CPU has recently done so. Also, don't bother
> + * invoking force_quiescent_state() if the newly enqueued callback
> + * is the only one waiting for a grace period to complete.
> + */
> + if (unlikely(++rdp->qlen > rdp->qlen_last_fqs_check + qhimark)) {
> rdp->blimit = LONG_MAX;
> - force_quiescent_state(rsp, 0);
> + if (rsp->n_force_qs == rdp->n_force_qs_snap &&
> + *rdp->nxttail[RCU_DONE_TAIL] != head)
> + force_quiescent_state(rsp, 0);
> + rdp->n_force_qs_snap = rsp->n_force_qs;
> + rdp->qlen_last_fqs_check = rdp->qlen;
> } else if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)
> force_quiescent_state(rsp, 1);
> local_irq_restore(flags);
> @@ -1523,6 +1540,8 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptable)
> rdp->beenonline = 1; /* We have now been online. */
> rdp->preemptable = preemptable;
> rdp->passed_quiesc_completed = lastcomp - 1;
> + rdp->qlen_last_fqs_check = 0;
> + rdp->n_force_qs_snap = rsp->n_force_qs;
> rdp->blimit = blimit;
> spin_unlock(&rnp->lock); /* irqs remain disabled. */
>
> diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> index b40ac57..599161f 100644
> --- a/kernel/rcutree.h
> +++ b/kernel/rcutree.h
> @@ -167,6 +167,10 @@ struct rcu_data {
> struct rcu_head *nxtlist;
> struct rcu_head **nxttail[RCU_NEXT_SIZE];
> long qlen; /* # of queued callbacks */
> + long qlen_last_fqs_check;
> + /* qlen at last check for QS forcing */
> + unsigned long n_force_qs_snap;
> + /* did other CPU force QS recently? */
> long blimit; /* Upper limit on a processed batch */
>
> #ifdef CONFIG_NO_HZ
> --
> 1.5.2.5


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