rcu: Rename cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs()

Commit e31d28b6ab8f ("trace: Eliminate cond_resched_rcu_qs() in favor
of cond_resched()") substituted cond_resched() for the earlier call
to cond_resched_rcu_qs().  However, the new-age cond_resched() does
not do anything to help RCU-tasks grace periods because (1) RCU-tasks
is only enabled when CONFIG_PREEMPT=y and (2) cond_resched() is a
complete no-op when preemption is enabled.  This situation results
in hangs when running the trace benchmarks.

A number of potential fixes were discussed on LKML
(https://lkml.kernel.org/r/20180224151240.0d63a059@vmware.local.home),
including making cond_resched() not be a no-op; making cond_resched()
not be a no-op, but only when running tracing benchmarks; reverting
the aforementioned commit (which works because cond_resched_rcu_qs()
does provide an RCU-tasks quiescent state; and adding a call to the
scheduler/RCU rcu_note_voluntary_context_switch() function.  All were
deemed unsatisfactory, either due to added cond_resched() overhead or
due to magic functions inviting cargo culting.

This commit renames cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs(),
which provides a clear hint as to what this function is doing and
why and where it should be used, and then replaces the call to
cond_resched() with cond_resched_tasks_rcu_qs() in the trace benchmark's
benchmark_event_kthread() function.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>

1 files changed, 10 insertions, 10 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2a734692a581..c4db0e20b035 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1234,10 +1234,10 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
         }
 
         /*
-         * Has this CPU encountered a cond_resched_rcu_qs() since the
-         * beginning of the grace period?  For this to be the case,
-         * the CPU has to have noticed the current grace period.  This
-         * might not be the case for nohz_full CPUs looping in the kernel.
+         * Has this CPU encountered a cond_resched() since the beginning
+         * of the grace period?  For this to be the case, the CPU has to
+         * have noticed the current grace period.  This might not be the
+         * case for nohz_full CPUs looping in the kernel.
          */
         jtsq = jiffies_till_sched_qs;
         ruqp = per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, rdp->cpu);
@@ -2049,7 +2049,7 @@ static bool rcu_gp_init(struct rcu_state *rsp)
                                             rnp->level, rnp->grplo,
                                             rnp->grphi, rnp->qsmask);
                 raw_spin_unlock_irq_rcu_node(rnp);
-                cond_resched_rcu_qs();
+                cond_resched_tasks_rcu_qs();
                 WRITE_ONCE(rsp->gp_activity, jiffies);
         }
 
@@ -2151,7 +2151,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
                 sq = rcu_nocb_gp_get(rnp);
                 raw_spin_unlock_irq_rcu_node(rnp);
                 rcu_nocb_gp_cleanup(sq);
-                cond_resched_rcu_qs();
+                cond_resched_tasks_rcu_qs();
                 WRITE_ONCE(rsp->gp_activity, jiffies);
                 rcu_gp_slow(rsp, gp_cleanup_delay);
         }
@@ -2202,7 +2202,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
                         /* Locking provides needed memory barrier. */
                         if (rcu_gp_init(rsp))
                                 break;
-                        cond_resched_rcu_qs();
+                        cond_resched_tasks_rcu_qs();
                         WRITE_ONCE(rsp->gp_activity, jiffies);
                         WARN_ON(signal_pending(current));
                         trace_rcu_grace_period(rsp->name,
@@ -2247,7 +2247,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
                                 trace_rcu_grace_period(rsp->name,
                                                        READ_ONCE(rsp->gpnum),
                                                        TPS("fqsend"));
-                                cond_resched_rcu_qs();
+                                cond_resched_tasks_rcu_qs();
                                 WRITE_ONCE(rsp->gp_activity, jiffies);
                                 ret = 0; /* Force full wait till next FQS. */
                                 j = jiffies_till_next_fqs;
@@ -2260,7 +2260,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
                                 }
                         } else {
                                 /* Deal with stray signal. */
-                                cond_resched_rcu_qs();
+                                cond_resched_tasks_rcu_qs();
                                 WRITE_ONCE(rsp->gp_activity, jiffies);
                                 WARN_ON(signal_pending(current));
                                 trace_rcu_grace_period(rsp->name,
@@ -2782,7 +2782,7 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *rsp))
         struct rcu_node *rnp;
 
         rcu_for_each_leaf_node(rsp, rnp) {
-                cond_resched_rcu_qs();
+                cond_resched_tasks_rcu_qs();
                 mask = 0;
                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
                 if (rnp->qsmask == 0) {