From cee4393989333795ae04dc9f3b83a578afe3fca6 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 2 Mar 2018 16:35:27 -0800 Subject: [PATCH] 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 Signed-off-by: Paul E. McKenney Tested-by: Nicholas Piggin --- include/linux/rcupdate.h | 4 ++-- kernel/rcu/rcuperf.c | 2 +- kernel/rcu/tree.c | 20 ++++++++++---------- kernel/rcu/tree_plugin.h | 4 ++-- kernel/rcu/update.c | 2 +- kernel/torture.c | 2 +- kernel/trace/trace_benchmark.c | 4 ++-- 7 files changed, 19 insertions(+), 19 deletions(-) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 36360d07f25b..19d235fefdb9 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -188,13 +188,13 @@ static inline void exit_tasks_rcu_finish(void) { } #endif /* #else #ifdef CONFIG_TASKS_RCU */ /** - * cond_resched_rcu_qs - Report potential quiescent states to RCU + * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU * * This macro resembles cond_resched(), except that it is defined to * report potential quiescent states to RCU-tasks even if the cond_resched() * machinery were to be shut off, as some advocate for PREEMPT kernels. */ -#define cond_resched_rcu_qs() \ +#define cond_resched_tasks_rcu_qs() \ do { \ if (!cond_resched()) \ rcu_note_voluntary_context_switch_lite(current); \ diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c index 777e7a6a0292..e232846516b3 100644 --- a/kernel/rcu/rcuperf.c +++ b/kernel/rcu/rcuperf.c @@ -369,7 +369,7 @@ static bool __maybe_unused torturing_tasks(void) */ static void rcu_perf_wait_shutdown(void) { - cond_resched_rcu_qs(); + cond_resched_tasks_rcu_qs(); if (atomic_read(&n_rcu_perf_writer_finished) < nrealwriters) return; while (!torture_must_stop()) 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) { diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 12774c4fb546..dfc42c3d52d4 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -1598,7 +1598,7 @@ static int rcu_oom_notify(struct notifier_block *self, for_each_online_cpu(cpu) { smp_call_function_single(cpu, rcu_oom_notify_cpu, NULL, 1); - cond_resched_rcu_qs(); + cond_resched_tasks_rcu_qs(); } /* Unconditionally decrement: no need to wake ourselves up. */ @@ -2227,7 +2227,7 @@ static int rcu_nocb_kthread(void *arg) cl++; c++; local_bh_enable(); - cond_resched_rcu_qs(); + cond_resched_tasks_rcu_qs(); list = next; } trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1); diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c index 68fa19a5e7bd..e401960c7f51 100644 --- a/kernel/rcu/update.c +++ b/kernel/rcu/update.c @@ -624,7 +624,7 @@ EXPORT_SYMBOL_GPL(call_rcu_tasks); * grace period has elapsed, in other words after all currently * executing rcu-tasks read-side critical sections have elapsed. These * read-side critical sections are delimited by calls to schedule(), - * cond_resched_rcu_qs(), idle execution, userspace execution, calls + * cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched(). * * This is a very specialized primitive, intended only for a few uses in diff --git a/kernel/torture.c b/kernel/torture.c index 37b94012a3f8..3de1efbecd6a 100644 --- a/kernel/torture.c +++ b/kernel/torture.c @@ -574,7 +574,7 @@ void stutter_wait(const char *title) { int spt; - cond_resched_rcu_qs(); + cond_resched_tasks_rcu_qs(); spt = READ_ONCE(stutter_pause_test); for (; spt; spt = READ_ONCE(stutter_pause_test)) { if (spt == 1) { diff --git a/kernel/trace/trace_benchmark.c b/kernel/trace/trace_benchmark.c index 22fee766081b..80e0b2aca703 100644 --- a/kernel/trace/trace_benchmark.c +++ b/kernel/trace/trace_benchmark.c @@ -159,13 +159,13 @@ static int benchmark_event_kthread(void *arg) * wants to run, schedule in, but if the CPU is idle, * we'll keep burning cycles. * - * Note the _rcu_qs() version of cond_resched() will + * Note the tasks_rcu_qs() version of cond_resched() will * notify synchronize_rcu_tasks() that this thread has * passed a quiescent state for rcu_tasks. Otherwise * this thread will never voluntarily schedule which would * block synchronize_rcu_tasks() indefinitely. */ - cond_resched(); + cond_resched_tasks_rcu_qs(); } return 0;