From de30ad512a668b56e7ad7a5a7c379d7c5d138a94 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 26 Apr 2018 11:52:09 -0700 Subject: [PATCH] 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 --- kernel/rcu/tree.c | 16 +++++++++++++++- kernel/rcu/tree.h | 3 +++ 2 files changed, 18 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index a9a4a26..467cd8e 100644 --- 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 --- 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. */ -- 2.7.4