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Subject[PATCH tip/core/rcu 2/6] rcu: prevent RCU IPI storms in presence of high call_rcu() load
Date
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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