srcu: Expedited grace periods with reduced memory contention
Commit f60d231a87c5 ("srcu: Crude control of expedited grace periods")
introduced a per-srcu_struct atomic counter to track outstanding
requests for grace periods. This works, but represents a memory-contention
bottleneck. This commit therefore uses the srcu_node combining tree
to remove this bottleneck.
This commit adds new ->srcu_gp_seq_needed_exp fields to the
srcu_data, srcu_node, and srcu_struct structures, which track the
farthest-in-the-future grace period that must be expedited, which in
turn requires that all nearer-term grace periods also be expedited.
Requests for expediting start with the srcu_data structure, run up
through the srcu_node tree, and end at the srcu_struct structure.
Note that it may be necessary to expedite a grace period that just
now started, and this is handled by a new srcu_funnel_exp_start()
function, which is invoked when the grace period itself is already
in its way, but when that grace period was not marked as expedited.
A new srcu_get_delay() function returns zero if there is at least one
expedited SRCU grace period in flight, or SRCU_INTERVAL otherwise.
This function is used to calculate delays: Normal grace periods
are allowed to extend in order to cover more requests with a given
grace-period computation, which decreases per-request overhead.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Mike Galbraith <efault@gmx.de>
diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index 3865717..86df48d 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -43,6 +43,7 @@
spinlock_t lock ____cacheline_internodealigned_in_smp;
struct rcu_segcblist srcu_cblist; /* List of callbacks.*/
unsigned long srcu_gp_seq_needed; /* Furthest future GP needed. */
+ unsigned long srcu_gp_seq_needed_exp; /* Furthest future exp GP. */
bool srcu_cblist_invoking; /* Invoking these CBs? */
struct delayed_work work; /* Context for CB invoking. */
struct rcu_head srcu_barrier_head; /* For srcu_barrier() use. */
@@ -63,6 +64,7 @@
/* is > ->srcu_gq_seq. */
unsigned long srcu_data_have_cbs[4]; /* Which srcu_data structs */
/* have CBs for given GP? */
+ unsigned long srcu_gp_seq_needed_exp; /* Furthest future exp GP. */
struct srcu_node *srcu_parent; /* Next up in tree. */
int grplo; /* Least CPU for node. */
int grphi; /* Biggest CPU for node. */
@@ -81,7 +83,7 @@
unsigned int srcu_idx; /* Current rdr array element. */
unsigned long srcu_gp_seq; /* Grace-period seq #. */
unsigned long srcu_gp_seq_needed; /* Latest gp_seq needed. */
- atomic_t srcu_exp_cnt; /* # ongoing expedited GPs. */
+ unsigned long srcu_gp_seq_needed_exp; /* Furthest future exp GP. */
struct srcu_data __percpu *sda; /* Per-CPU srcu_data array. */
unsigned long srcu_barrier_seq; /* srcu_barrier seq #. */
struct mutex srcu_barrier_mutex; /* Serialize barrier ops. */
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 72b6cce..4b98e6f 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -72,6 +72,7 @@
snp->srcu_have_cbs[i] = 0;
snp->srcu_data_have_cbs[i] = 0;
}
+ snp->srcu_gp_seq_needed_exp = 0;
snp->grplo = -1;
snp->grphi = -1;
if (snp == &sp->node[0]) {
@@ -102,6 +103,7 @@
rcu_segcblist_init(&sdp->srcu_cblist);
sdp->srcu_cblist_invoking = false;
sdp->srcu_gp_seq_needed = sp->srcu_gp_seq;
+ sdp->srcu_gp_seq_needed_exp = sp->srcu_gp_seq;
sdp->mynode = &snp_first[cpu / levelspread[level]];
for (snp = sdp->mynode; snp != NULL; snp = snp->srcu_parent) {
if (snp->grplo < 0)
@@ -135,7 +137,6 @@
mutex_init(&sp->srcu_gp_mutex);
sp->srcu_idx = 0;
sp->srcu_gp_seq = 0;
- atomic_set(&sp->srcu_exp_cnt, 0);
sp->srcu_barrier_seq = 0;
mutex_init(&sp->srcu_barrier_mutex);
atomic_set(&sp->srcu_barrier_cpu_cnt, 0);
@@ -143,6 +144,7 @@
if (!is_static)
sp->sda = alloc_percpu(struct srcu_data);
init_srcu_struct_nodes(sp, is_static);
+ sp->srcu_gp_seq_needed_exp = 0;
smp_store_release(&sp->srcu_gp_seq_needed, 0); /* Init done. */
return sp->sda ? 0 : -ENOMEM;
}
@@ -307,6 +309,18 @@
#define SRCU_INTERVAL 1
+/*
+ * Return grace-period delay, zero if there are expedited grace
+ * periods pending, SRCU_INTERVAL otherwise.
+ */
+static unsigned long srcu_get_delay(struct srcu_struct *sp)
+{
+ if (ULONG_CMP_LT(READ_ONCE(sp->srcu_gp_seq),
+ READ_ONCE(sp->srcu_gp_seq_needed_exp)))
+ return 0;
+ return SRCU_INTERVAL;
+}
+
/**
* cleanup_srcu_struct - deconstruct a sleep-RCU structure
* @sp: structure to clean up.
@@ -318,7 +332,8 @@
{
int cpu;
- WARN_ON_ONCE(atomic_read(&sp->srcu_exp_cnt));
+ if (WARN_ON(!srcu_get_delay(sp)))
+ return; /* Leakage unless caller handles error. */
if (WARN_ON(srcu_readers_active(sp)))
return; /* Leakage unless caller handles error. */
flush_delayed_work(&sp->work);
@@ -444,15 +459,14 @@
* schedule this invocation on the corresponding CPUs.
*/
static void srcu_schedule_cbs_snp(struct srcu_struct *sp, struct srcu_node *snp,
- unsigned long mask)
+ unsigned long mask, unsigned long delay)
{
int cpu;
for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
if (!(mask & (1 << (cpu - snp->grplo))))
continue;
- srcu_schedule_cbs_sdp(per_cpu_ptr(sp->sda, cpu),
- atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
+ srcu_schedule_cbs_sdp(per_cpu_ptr(sp->sda, cpu), delay);
}
}
@@ -467,6 +481,7 @@
*/
static void srcu_gp_end(struct srcu_struct *sp)
{
+ unsigned long cbdelay;
bool cbs;
unsigned long gpseq;
int idx;
@@ -481,8 +496,11 @@
spin_lock_irq(&sp->gp_lock);
idx = rcu_seq_state(sp->srcu_gp_seq);
WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
+ cbdelay = srcu_get_delay(sp);
rcu_seq_end(&sp->srcu_gp_seq);
gpseq = rcu_seq_current(&sp->srcu_gp_seq);
+ if (ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, gpseq))
+ sp->srcu_gp_seq_needed_exp = gpseq;
spin_unlock_irq(&sp->gp_lock);
mutex_unlock(&sp->srcu_gp_mutex);
/* A new grace period can start at this point. But only one. */
@@ -497,12 +515,14 @@
cbs = snp->srcu_have_cbs[idx] == gpseq;
snp->srcu_have_cbs[idx] = gpseq;
rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
+ if (ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, gpseq))
+ snp->srcu_gp_seq_needed_exp = gpseq;
mask = snp->srcu_data_have_cbs[idx];
snp->srcu_data_have_cbs[idx] = 0;
spin_unlock_irq(&snp->lock);
if (cbs) {
smp_mb(); /* GP end before CB invocation. */
- srcu_schedule_cbs_snp(sp, snp, mask);
+ srcu_schedule_cbs_snp(sp, snp, mask, cbdelay);
}
}
@@ -517,25 +537,52 @@
srcu_gp_start(sp);
spin_unlock_irq(&sp->gp_lock);
/* Throttle expedited grace periods: Should be rare! */
- srcu_reschedule(sp, atomic_read(&sp->srcu_exp_cnt) &&
- rcu_seq_ctr(gpseq) & 0xf
- ? 0
- : SRCU_INTERVAL);
+ srcu_reschedule(sp, rcu_seq_ctr(gpseq) & 0x3ff
+ ? 0 : SRCU_INTERVAL);
} else {
spin_unlock_irq(&sp->gp_lock);
}
}
/*
+ * Funnel-locking scheme to scalably mediate many concurrent expedited
+ * grace-period requests. This function is invoked for the first known
+ * expedited request for a grace period that has already been requested,
+ * but without expediting. To start a completely new grace period,
+ * whether expedited or not, use srcu_funnel_gp_start() instead.
+ */
+static void srcu_funnel_exp_start(struct srcu_struct *sp, struct srcu_node *snp,
+ unsigned long s)
+{
+ unsigned long flags;
+
+ for (; snp != NULL; snp = snp->srcu_parent) {
+ if (rcu_seq_done(&sp->srcu_gp_seq, s) ||
+ ULONG_CMP_GE(READ_ONCE(snp->srcu_gp_seq_needed_exp), s))
+ return;
+ spin_lock_irqsave(&snp->lock, flags);
+ if (ULONG_CMP_GE(snp->srcu_gp_seq_needed_exp, s)) {
+ spin_unlock_irqrestore(&snp->lock, flags);
+ return;
+ }
+ WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
+ spin_unlock_irqrestore(&snp->lock, flags);
+ }
+ spin_lock_irqsave(&sp->gp_lock, flags);
+ if (!ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
+ sp->srcu_gp_seq_needed_exp = s;
+ spin_unlock_irqrestore(&sp->gp_lock, flags);
+}
+
+/*
* Funnel-locking scheme to scalably mediate many concurrent grace-period
* requests. The winner has to do the work of actually starting grace
* period s. Losers must either ensure that their desired grace-period
* number is recorded on at least their leaf srcu_node structure, or they
* must take steps to invoke their own callbacks.
*/
-static void srcu_funnel_gp_start(struct srcu_struct *sp,
- struct srcu_data *sdp,
- unsigned long s)
+static void srcu_funnel_gp_start(struct srcu_struct *sp, struct srcu_data *sdp,
+ unsigned long s, bool do_norm)
{
unsigned long flags;
int idx = rcu_seq_ctr(s) % ARRAY_SIZE(sdp->mynode->srcu_have_cbs);
@@ -554,13 +601,20 @@
spin_unlock_irqrestore(&snp->lock, flags);
if (snp == sdp->mynode && snp_seq != s) {
smp_mb(); /* CBs after GP! */
- srcu_schedule_cbs_sdp(sdp, 0);
+ srcu_schedule_cbs_sdp(sdp, do_norm
+ ? SRCU_INTERVAL
+ : 0);
+ return;
}
+ if (!do_norm)
+ srcu_funnel_exp_start(sp, snp, s);
return;
}
snp->srcu_have_cbs[idx] = s;
if (snp == sdp->mynode)
snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
+ if (!do_norm && ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, s))
+ snp->srcu_gp_seq_needed_exp = s;
spin_unlock_irqrestore(&snp->lock, flags);
}
@@ -573,6 +627,8 @@
*/
smp_store_release(&sp->srcu_gp_seq_needed, s); /*^^^*/
}
+ if (!do_norm && ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
+ sp->srcu_gp_seq_needed_exp = s;
/* If grace period not already done and none in progress, start it. */
if (!rcu_seq_done(&sp->srcu_gp_seq, s) &&
@@ -580,9 +636,7 @@
WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
srcu_gp_start(sp);
queue_delayed_work(system_power_efficient_wq, &sp->work,
- atomic_read(&sp->srcu_exp_cnt)
- ? 0
- : SRCU_INTERVAL);
+ srcu_get_delay(sp));
}
spin_unlock_irqrestore(&sp->gp_lock, flags);
}
@@ -597,7 +651,7 @@
for (;;) {
if (srcu_readers_active_idx_check(sp, idx))
return true;
- if (--trycount + !!atomic_read(&sp->srcu_exp_cnt) <= 0)
+ if (--trycount + !srcu_get_delay(sp) <= 0)
return false;
udelay(SRCU_RETRY_CHECK_DELAY);
}
@@ -650,10 +704,11 @@
* srcu_read_lock(), and srcu_read_unlock() that are all passed the same
* srcu_struct structure.
*/
-void call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
- rcu_callback_t func)
+void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
+ rcu_callback_t func, bool do_norm)
{
unsigned long flags;
+ bool needexp = false;
bool needgp = false;
unsigned long s;
struct srcu_data *sdp;
@@ -672,16 +727,28 @@
sdp->srcu_gp_seq_needed = s;
needgp = true;
}
+ if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
+ sdp->srcu_gp_seq_needed_exp = s;
+ needexp = true;
+ }
spin_unlock_irqrestore(&sdp->lock, flags);
if (needgp)
- srcu_funnel_gp_start(sp, sdp, s);
+ srcu_funnel_gp_start(sp, sdp, s, do_norm);
+ else if (needexp)
+ srcu_funnel_exp_start(sp, sdp->mynode, s);
+}
+
+void call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
+ rcu_callback_t func)
+{
+ __call_srcu(sp, rhp, func, true);
}
EXPORT_SYMBOL_GPL(call_srcu);
/*
* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
*/
-static void __synchronize_srcu(struct srcu_struct *sp)
+static void __synchronize_srcu(struct srcu_struct *sp, bool do_norm)
{
struct rcu_synchronize rcu;
@@ -697,7 +764,7 @@
check_init_srcu_struct(sp);
init_completion(&rcu.completion);
init_rcu_head_on_stack(&rcu.head);
- call_srcu(sp, &rcu.head, wakeme_after_rcu);
+ __call_srcu(sp, &rcu.head, wakeme_after_rcu, do_norm);
wait_for_completion(&rcu.completion);
destroy_rcu_head_on_stack(&rcu.head);
}
@@ -714,18 +781,7 @@
*/
void synchronize_srcu_expedited(struct srcu_struct *sp)
{
- bool do_norm = rcu_gp_is_normal();
-
- check_init_srcu_struct(sp);
- if (!do_norm) {
- atomic_inc(&sp->srcu_exp_cnt);
- smp_mb__after_atomic(); /* increment before GP. */
- }
- __synchronize_srcu(sp);
- if (!do_norm) {
- smp_mb__before_atomic(); /* GP before decrement. */
- WARN_ON_ONCE(atomic_dec_return(&sp->srcu_exp_cnt) < 0);
- }
+ __synchronize_srcu(sp, rcu_gp_is_normal());
}
EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
@@ -773,7 +829,7 @@
if (rcu_gp_is_expedited())
synchronize_srcu_expedited(sp);
else
- __synchronize_srcu(sp);
+ __synchronize_srcu(sp, true);
}
EXPORT_SYMBOL_GPL(synchronize_srcu);
@@ -1008,14 +1064,13 @@
sp = container_of(work, struct srcu_struct, work.work);
srcu_advance_state(sp);
- srcu_reschedule(sp, atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
+ srcu_reschedule(sp, srcu_get_delay(sp));
}
EXPORT_SYMBOL_GPL(process_srcu);
void srcutorture_get_gp_data(enum rcutorture_type test_type,
- struct srcu_struct *sp, int *flags,
- unsigned long *gpnum,
- unsigned long *completed)
+ struct srcu_struct *sp, int *flags,
+ unsigned long *gpnum, unsigned long *completed)
{
if (test_type != SRCU_FLAVOR)
return;
