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Subject[PATCH tip/core/rcu 2/5] rcu: React to callback overload by aggressively seeking quiescent states
Date
From: "Paul E. McKenney" <paulmck@kernel.org>

In default configutions, RCU currently waits at least 100 milliseconds
before asking cond_resched() and/or resched_rcu() for help seeking
quiescent states to end a grace period. But 100 milliseconds can be
one good long time during an RCU callback flood, for example, as can
happen when user processes repeatedly open and close files in a tight
loop. These 100-millisecond gaps in successive grace periods during a
callback flood can result in excessive numbers of callbacks piling up,
unnecessarily increasing memory footprint.

This commit therefore asks cond_resched() and/or resched_rcu() for help
as early as the first FQS scan when at least one of the CPUs has more
than 20,000 callbacks queued, a number that can be changed using the new
rcutree.qovld kernel boot parameter. An auxiliary qovld_calc variable
is used to avoid acquisition of locks that have not yet been initialized.
Early tests indicate that this reduces the RCU-callback memory footprint
during rcutorture floods by from 50% to 4x, depending on configuration.

Reported-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reported-by: Tejun Heo <tj@kernel.org>
[ paulmck: Fix bug located by Qian Cai. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Dexuan Cui <decui@microsoft.com>
Tested-by: Qian Cai <cai@lca.pw>
---
Documentation/admin-guide/kernel-parameters.txt | 9 +++
kernel/rcu/tree.c | 75 +++++++++++++++++++++++--
kernel/rcu/tree.h | 4 ++
kernel/rcu/tree_plugin.h | 2 +
4 files changed, 86 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index dbc22d6..dbd4b4a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3980,6 +3980,15 @@
Set threshold of queued RCU callbacks below which
batch limiting is re-enabled.

+ rcutree.qovld= [KNL]
+ Set threshold of queued RCU callbacks beyond which
+ RCU's force-quiescent-state scan will aggressively
+ enlist help from cond_resched() and sched IPIs to
+ help CPUs more quickly reach quiescent states.
+ Set to less than zero to make this be set based
+ on rcutree.qhimark at boot time and to zero to
+ disable more aggressive help enlistment.
+
rcutree.rcu_idle_gp_delay= [KNL]
Set wakeup interval for idle CPUs that have
RCU callbacks (RCU_FAST_NO_HZ=y).
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 31d01f8..48fba22 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -150,6 +150,7 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
static void invoke_rcu_core(void);
static void rcu_report_exp_rdp(struct rcu_data *rdp);
static void sync_sched_exp_online_cleanup(int cpu);
+static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp);

/* rcuc/rcub kthread realtime priority */
static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
@@ -410,10 +411,15 @@ static long blimit = DEFAULT_RCU_BLIMIT;
static long qhimark = DEFAULT_RCU_QHIMARK;
#define DEFAULT_RCU_QLOMARK 100 /* Once only this many pending, use blimit. */
static long qlowmark = DEFAULT_RCU_QLOMARK;
+#define DEFAULT_RCU_QOVLD_MULT 2
+#define DEFAULT_RCU_QOVLD (DEFAULT_RCU_QOVLD_MULT * DEFAULT_RCU_QHIMARK)
+static long qovld = DEFAULT_RCU_QOVLD; /* If this many pending, hammer QS. */
+static long qovld_calc = -1; /* No pre-initialization lock acquisitions! */

module_param(blimit, long, 0444);
module_param(qhimark, long, 0444);
module_param(qlowmark, long, 0444);
+module_param(qovld, long, 0444);

static ulong jiffies_till_first_fqs = ULONG_MAX;
static ulong jiffies_till_next_fqs = ULONG_MAX;
@@ -1072,7 +1078,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
rnhqp = &per_cpu(rcu_data.rcu_need_heavy_qs, rdp->cpu);
if (!READ_ONCE(*rnhqp) &&
(time_after(jiffies, rcu_state.gp_start + jtsq * 2) ||
- time_after(jiffies, rcu_state.jiffies_resched))) {
+ time_after(jiffies, rcu_state.jiffies_resched) ||
+ rcu_state.cbovld)) {
WRITE_ONCE(*rnhqp, true);
/* Store rcu_need_heavy_qs before rcu_urgent_qs. */
smp_store_release(ruqp, true);
@@ -1089,8 +1096,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
* So hit them over the head with the resched_cpu() hammer!
*/
if (tick_nohz_full_cpu(rdp->cpu) &&
- time_after(jiffies,
- READ_ONCE(rdp->last_fqs_resched) + jtsq * 3)) {
+ (time_after(jiffies, READ_ONCE(rdp->last_fqs_resched) + jtsq * 3) ||
+ rcu_state.cbovld)) {
WRITE_ONCE(*ruqp, true);
resched_cpu(rdp->cpu);
WRITE_ONCE(rdp->last_fqs_resched, jiffies);
@@ -1704,8 +1711,9 @@ static void rcu_gp_fqs_loop(void)
*/
static void rcu_gp_cleanup(void)
{
- unsigned long gp_duration;
+ int cpu;
bool needgp = false;
+ unsigned long gp_duration;
unsigned long new_gp_seq;
bool offloaded;
struct rcu_data *rdp;
@@ -1751,6 +1759,12 @@ static void rcu_gp_cleanup(void)
needgp = __note_gp_changes(rnp, rdp) || needgp;
/* smp_mb() provided by prior unlock-lock pair. */
needgp = rcu_future_gp_cleanup(rnp) || needgp;
+ // Reset overload indication for CPUs no longer overloaded
+ if (rcu_is_leaf_node(rnp))
+ for_each_leaf_node_cpu_mask(rnp, cpu, rnp->cbovldmask) {
+ rdp = per_cpu_ptr(&rcu_data, cpu);
+ check_cb_ovld_locked(rdp, rnp);
+ }
sq = rcu_nocb_gp_get(rnp);
raw_spin_unlock_irq_rcu_node(rnp);
rcu_nocb_gp_cleanup(sq);
@@ -2299,10 +2313,13 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
struct rcu_data *rdp;
struct rcu_node *rnp;

+ rcu_state.cbovld = rcu_state.cbovldnext;
+ rcu_state.cbovldnext = false;
rcu_for_each_leaf_node(rnp) {
cond_resched_tasks_rcu_qs();
mask = 0;
raw_spin_lock_irqsave_rcu_node(rnp, flags);
+ rcu_state.cbovldnext |= !!rnp->cbovldmask;
if (rnp->qsmask == 0) {
if (!IS_ENABLED(CONFIG_PREEMPT_RCU) ||
rcu_preempt_blocked_readers_cgp(rnp)) {
@@ -2584,6 +2601,48 @@ static void rcu_leak_callback(struct rcu_head *rhp)
}

/*
+ * Check and if necessary update the leaf rcu_node structure's
+ * ->cbovldmask bit corresponding to the current CPU based on that CPU's
+ * number of queued RCU callbacks. The caller must hold the leaf rcu_node
+ * structure's ->lock.
+ */
+static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp)
+{
+ raw_lockdep_assert_held_rcu_node(rnp);
+ if (qovld_calc <= 0)
+ return; // Early boot and wildcard value set.
+ if (rcu_segcblist_n_cbs(&rdp->cblist) >= qovld_calc)
+ WRITE_ONCE(rnp->cbovldmask, rnp->cbovldmask | rdp->grpmask);
+ else
+ WRITE_ONCE(rnp->cbovldmask, rnp->cbovldmask & ~rdp->grpmask);
+}
+
+/*
+ * Check and if necessary update the leaf rcu_node structure's
+ * ->cbovldmask bit corresponding to the current CPU based on that CPU's
+ * number of queued RCU callbacks. No locks need be held, but the
+ * caller must have disabled interrupts.
+ *
+ * Note that this function ignores the possibility that there are a lot
+ * of callbacks all of which have already seen the end of their respective
+ * grace periods. This omission is due to the need for no-CBs CPUs to
+ * be holding ->nocb_lock to do this check, which is too heavy for a
+ * common-case operation.
+ */
+static void check_cb_ovld(struct rcu_data *rdp)
+{
+ struct rcu_node *const rnp = rdp->mynode;
+
+ if (qovld_calc <= 0 ||
+ ((rcu_segcblist_n_cbs(&rdp->cblist) >= qovld_calc) ==
+ !!(READ_ONCE(rnp->cbovldmask) & rdp->grpmask)))
+ return; // Early boot wildcard value or already set correctly.
+ raw_spin_lock_rcu_node(rnp);
+ check_cb_ovld_locked(rdp, rnp);
+ raw_spin_unlock_rcu_node(rnp);
+}
+
+/*
* Helper function for call_rcu() and friends. The cpu argument will
* normally be -1, indicating "currently running CPU". It may specify
* a CPU only if that CPU is a no-CBs CPU. Currently, only rcu_barrier()
@@ -2626,6 +2685,7 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
rcu_segcblist_init(&rdp->cblist);
}

+ check_cb_ovld(rdp);
if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))
return; // Enqueued onto ->nocb_bypass, so just leave.
/* If we get here, rcu_nocb_try_bypass() acquired ->nocb_lock. */
@@ -3814,6 +3874,13 @@ void __init rcu_init(void)
rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
WARN_ON(!rcu_par_gp_wq);
srcu_init();
+
+ /* Fill in default value for rcutree.qovld boot parameter. */
+ /* -After- the rcu_node ->lock fields are initialized! */
+ if (qovld < 0)
+ qovld_calc = DEFAULT_RCU_QOVLD_MULT * qhimark;
+ else
+ qovld_calc = qovld;
}

#include "tree_stall.h"
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 0c87e4c..9dc2ec0 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -68,6 +68,8 @@ struct rcu_node {
/* Online CPUs for next expedited GP. */
/* Any CPU that has ever been online will */
/* have its bit set. */
+ unsigned long cbovldmask;
+ /* CPUs experiencing callback overload. */
unsigned long ffmask; /* Fully functional CPUs. */
unsigned long grpmask; /* Mask to apply to parent qsmask. */
/* Only one bit will be set in this mask. */
@@ -321,6 +323,8 @@ struct rcu_state {
atomic_t expedited_need_qs; /* # CPUs left to check in. */
struct swait_queue_head expedited_wq; /* Wait for check-ins. */
int ncpus_snap; /* # CPUs seen last time. */
+ u8 cbovld; /* Callback overload now? */
+ u8 cbovldnext; /* ^ ^ next time? */

unsigned long jiffies_force_qs; /* Time at which to invoke */
/* force_quiescent_state(). */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index c6ea81c..0be8fad 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -56,6 +56,8 @@ static void __init rcu_bootup_announce_oddness(void)
pr_info("\tBoot-time adjustment of callback high-water mark to %ld.\n", qhimark);
if (qlowmark != DEFAULT_RCU_QLOMARK)
pr_info("\tBoot-time adjustment of callback low-water mark to %ld.\n", qlowmark);
+ if (qovld != DEFAULT_RCU_QOVLD)
+ pr_info("\tBoot-time adjustment of callback overload leval to %ld.\n", qovld);
if (jiffies_till_first_fqs != ULONG_MAX)
pr_info("\tBoot-time adjustment of first FQS scan delay to %ld jiffies.\n", jiffies_till_first_fqs);
if (jiffies_till_next_fqs != ULONG_MAX)
--
2.9.5
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