rcu: Introduce grace-period sequence numbers

This commit adds grace-period sequence numbers (->gp_seq) to the
rcu_state, rcu_node, and rcu_data structures, and updates them.
It also checks for consistency between rsp->gpnum and rsp->gp_seq.
These ->gp_seq counters will eventually replace the existing ->gpnum
and ->completed counters, allowing a single memory access to determine
whether or not a grace period is in progress and if so, which one.
This in turn will enable changes that will reduce ->lock contention on
the leaf rcu_node structures.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a9a4a26..467cd8e 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -97,6 +97,7 @@ struct rcu_state sname##_state = { \
        .gp_state = RCU_GP_IDLE, \
        .gpnum = 0UL - 300UL, \
        .completed = 0UL - 300UL, \
+       .gp_seq = (0UL - 300UL) << RCU_SEQ_CTR_SHIFT, \
        .barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
        .name = RCU_STATE_NAME(sname), \
        .abbr = sabbr, \
@@ -1849,6 +1850,8 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
                WRITE_ONCE(rdp->gpwrap, false);
                rcu_gpnum_ovf(rnp, rdp);
        }
+       if (rdp->gp_seq != rnp->gp_seq)
+               rdp->gp_seq = rnp->gp_seq;
        return ret;
 }
 
@@ -1910,7 +1913,10 @@ static bool rcu_gp_init(struct rcu_state *rsp)
        /* Advance to a new grace period and initialize state. */
        record_gp_stall_check_time(rsp);
        /* Record GP times before starting GP, hence smp_store_release(). */
+       WARN_ON_ONCE(rsp->gpnum << RCU_SEQ_CTR_SHIFT != rsp->gp_seq);
        smp_store_release(&rsp->gpnum, rsp->gpnum + 1);
+       smp_mb(); /* Pairs with barriers in stall-warning code. */
+       rcu_seq_start(&rsp->gp_seq);
        trace_rcu_grace_period(rsp->name, rsp->gpnum, TPS("start"));
        raw_spin_unlock_irq_rcu_node(rnp);
 
@@ -1984,6 +1990,7 @@ static bool rcu_gp_init(struct rcu_state *rsp)
                WRITE_ONCE(rnp->gpnum, rsp->gpnum);
                if (WARN_ON_ONCE(rnp->completed != rsp->completed))
                        WRITE_ONCE(rnp->completed, rsp->completed);
+               WRITE_ONCE(rnp->gp_seq, rsp->gp_seq);
                if (rnp == rdp->mynode)
                        (void)__note_gp_changes(rsp, rnp, rdp);
                rcu_preempt_boost_start_gp(rnp);
@@ -2050,6 +2057,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 {
        unsigned long gp_duration;
        bool needgp = false;
+       unsigned long new_gp_seq;
        struct rcu_data *rdp;
        struct rcu_node *rnp = rcu_get_root(rsp);
        struct swait_queue_head *sq;
@@ -2079,12 +2087,15 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
         * all of the rcu_node structures before the beginning of the next
         * grace period is recorded in any of the rcu_node structures.
         */
+       new_gp_seq = rsp->gp_seq;
+       rcu_seq_end(&new_gp_seq);
        rcu_for_each_node_breadth_first(rsp, rnp) {
                raw_spin_lock_irq_rcu_node(rnp);
                if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
                        dump_blkd_tasks(rnp, 10);
                WARN_ON_ONCE(rnp->qsmask);
                WRITE_ONCE(rnp->completed, rsp->gpnum);
+               WRITE_ONCE(rnp->gp_seq, new_gp_seq);
                rdp = this_cpu_ptr(rsp->rda);
                if (rnp == rdp->mynode)
                        needgp = __note_gp_changes(rsp, rnp, rdp) || needgp;
@@ -2098,10 +2109,11 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
                rcu_gp_slow(rsp, gp_cleanup_delay);
        }
        rnp = rcu_get_root(rsp);
-       raw_spin_lock_irq_rcu_node(rnp); /* Order GP before ->completed update. */
+       raw_spin_lock_irq_rcu_node(rnp); /* GP before rsp->gp_seq update. */
 
        /* Declare grace period done. */
        WRITE_ONCE(rsp->completed, rsp->gpnum);
+       rcu_seq_end(&rsp->gp_seq);
        trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
        rsp->gp_state = RCU_GP_IDLE;
        /* Check for GP requests since above loop. */
@@ -3612,6 +3624,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
        rdp->beenonline = true;  /* We have now been online. */
        rdp->gpnum = rnp->completed; /* Make CPU later note any new GP. */
        rdp->completed = rnp->completed;
+       rdp->gp_seq = rnp->gp_seq;
        rdp->cpu_no_qs.b.norm = true;
        rdp->rcu_qs_ctr_snap = per_cpu(rcu_dynticks.rcu_qs_ctr, cpu);
        rdp->core_needs_qs = false;
@@ -3991,6 +4004,7 @@ static void __init rcu_init_one(struct rcu_state *rsp)
                                                   &rcu_fqs_class[i], fqs[i]);
                        rnp->gpnum = rsp->gpnum;
                        rnp->completed = rsp->completed;
+                       rnp->gp_seq = rsp->gp_seq;
                        rnp->completedqs = rsp->completed;
                        rnp->qsmask = 0;
                        rnp->qsmaskinit = 0;

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 3c19421..50a28d1 100644 (file)
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -87,6 +87,7 @@ struct rcu_node {
        unsigned long completed; /* Last GP completed for this node. */
                                /*  This will either be equal to or one */
                                /*  behind the root rcu_node's gpnum. */
+       unsigned long gp_seq;   /* Track rsp->rcu_gp_seq. */
        unsigned long completedqs; /* All QSes done for this node. */
        unsigned long qsmask;   /* CPUs or groups that need to switch in */
                                /*  order for current grace period to proceed.*/
@@ -211,6 +212,7 @@ struct rcu_data {
                                        /*  in order to detect GP end. */
        unsigned long   gpnum;          /* Highest gp number that this CPU */
                                        /*  is aware of having started. */
+       unsigned long   gp_seq;         /* Track rsp->rcu_gp_seq counter. */
        unsigned long   rcu_qs_ctr_snap;/* Snapshot of rcu_qs_ctr to check */
                                        /*  for rcu_all_qs() invocations. */
        union rcu_noqs  cpu_no_qs;      /* No QSes yet for this CPU. */
@@ -343,6 +345,7 @@ struct rcu_state {
                                                /* Subject to priority boost. */
        unsigned long gpnum;                    /* Current gp number. */
        unsigned long completed;                /* # of last completed gp. */
+       unsigned long gp_seq;                   /* Grace-period sequence #. */
        struct task_struct *gp_kthread;         /* Task for grace periods. */
        struct swait_queue_head gp_wq;          /* Where GP task waits. */
        short gp_flags;                         /* Commands for GP task. */