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>

2 files changed, 98 insertions, 41 deletions

diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index 3865717df124..86df48d3e97b 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -43,6 +43,7 @@ struct srcu_data {
         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 @@ struct srcu_node {
                                                 /*  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 @@ struct srcu_struct {
         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 72b6cce5f591..4b98e6f45166 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -72,6 +72,7 @@ static void init_srcu_struct_nodes(struct srcu_struct *sp, bool is_static)
                         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 @@ static void init_srcu_struct_nodes(struct srcu_struct *sp, bool is_static)
                 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 @@ static int init_srcu_struct_fields(struct srcu_struct *sp, bool is_static)
         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 @@ static int init_srcu_struct_fields(struct srcu_struct *sp, bool is_static)
         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 @@ static bool srcu_readers_active(struct srcu_struct *sp)
 
 #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 @@ void cleanup_srcu_struct(struct srcu_struct *sp)
 {
         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 @@ static void srcu_schedule_cbs_sdp(struct srcu_data *sdp, unsigned long delay)
  * 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_schedule_cbs_snp(struct srcu_struct *sp, struct srcu_node *snp,
  */
 static void srcu_gp_end(struct srcu_struct *sp)
 {
+        unsigned long cbdelay;
         bool cbs;
         unsigned long gpseq;
         int idx;
@@ -481,8 +496,11 @@ static void srcu_gp_end(struct srcu_struct *sp)
         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 @@ static void srcu_gp_end(struct srcu_struct *sp)
                         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 @@ static void srcu_gp_end(struct srcu_struct *sp)
                 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 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp,
                         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 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp,
                  */
                 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 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp,
                 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 @@ static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
         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 @@ static void srcu_flip(struct srcu_struct *sp)
  * 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 @@ void call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
                 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 @@ static void __synchronize_srcu(struct srcu_struct *sp)
         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 @@ static void __synchronize_srcu(struct srcu_struct *sp)
  */
 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 @@ void synchronize_srcu(struct srcu_struct *sp)
         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 @@ void process_srcu(struct work_struct *work)
         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;