rcu: Rename cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs()
Commit e31d28b6ab
("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>
This commit is contained in:
parent
6fba2b3767
commit
cee4393989
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@ -188,13 +188,13 @@ static inline void exit_tasks_rcu_finish(void) { }
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#endif /* #else #ifdef CONFIG_TASKS_RCU */
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/**
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* cond_resched_rcu_qs - Report potential quiescent states to RCU
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* cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
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*
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* This macro resembles cond_resched(), except that it is defined to
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* report potential quiescent states to RCU-tasks even if the cond_resched()
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* machinery were to be shut off, as some advocate for PREEMPT kernels.
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*/
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#define cond_resched_rcu_qs() \
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#define cond_resched_tasks_rcu_qs() \
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do { \
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if (!cond_resched()) \
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rcu_note_voluntary_context_switch_lite(current); \
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@ -369,7 +369,7 @@ static bool __maybe_unused torturing_tasks(void)
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*/
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static void rcu_perf_wait_shutdown(void)
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{
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cond_resched_rcu_qs();
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cond_resched_tasks_rcu_qs();
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if (atomic_read(&n_rcu_perf_writer_finished) < nrealwriters)
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return;
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while (!torture_must_stop())
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@ -1234,10 +1234,10 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
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}
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/*
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* Has this CPU encountered a cond_resched_rcu_qs() since the
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* beginning of the grace period? For this to be the case,
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* the CPU has to have noticed the current grace period. This
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* might not be the case for nohz_full CPUs looping in the kernel.
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* Has this CPU encountered a cond_resched() since the beginning
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* of the grace period? For this to be the case, the CPU has to
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* have noticed the current grace period. This might not be the
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* case for nohz_full CPUs looping in the kernel.
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*/
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jtsq = jiffies_till_sched_qs;
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ruqp = per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, rdp->cpu);
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@ -2049,7 +2049,7 @@ static bool rcu_gp_init(struct rcu_state *rsp)
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rnp->level, rnp->grplo,
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rnp->grphi, rnp->qsmask);
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raw_spin_unlock_irq_rcu_node(rnp);
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cond_resched_rcu_qs();
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cond_resched_tasks_rcu_qs();
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WRITE_ONCE(rsp->gp_activity, jiffies);
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}
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@ -2151,7 +2151,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
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sq = rcu_nocb_gp_get(rnp);
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raw_spin_unlock_irq_rcu_node(rnp);
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rcu_nocb_gp_cleanup(sq);
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cond_resched_rcu_qs();
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cond_resched_tasks_rcu_qs();
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WRITE_ONCE(rsp->gp_activity, jiffies);
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rcu_gp_slow(rsp, gp_cleanup_delay);
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}
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@ -2202,7 +2202,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
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/* Locking provides needed memory barrier. */
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if (rcu_gp_init(rsp))
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break;
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cond_resched_rcu_qs();
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cond_resched_tasks_rcu_qs();
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WRITE_ONCE(rsp->gp_activity, jiffies);
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WARN_ON(signal_pending(current));
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trace_rcu_grace_period(rsp->name,
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@ -2247,7 +2247,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
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trace_rcu_grace_period(rsp->name,
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READ_ONCE(rsp->gpnum),
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TPS("fqsend"));
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cond_resched_rcu_qs();
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cond_resched_tasks_rcu_qs();
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WRITE_ONCE(rsp->gp_activity, jiffies);
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ret = 0; /* Force full wait till next FQS. */
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j = jiffies_till_next_fqs;
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@ -2260,7 +2260,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
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}
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} else {
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/* Deal with stray signal. */
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cond_resched_rcu_qs();
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cond_resched_tasks_rcu_qs();
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WRITE_ONCE(rsp->gp_activity, jiffies);
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WARN_ON(signal_pending(current));
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trace_rcu_grace_period(rsp->name,
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@ -2782,7 +2782,7 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *rsp))
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struct rcu_node *rnp;
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rcu_for_each_leaf_node(rsp, rnp) {
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cond_resched_rcu_qs();
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cond_resched_tasks_rcu_qs();
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mask = 0;
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raw_spin_lock_irqsave_rcu_node(rnp, flags);
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if (rnp->qsmask == 0) {
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@ -1598,7 +1598,7 @@ static int rcu_oom_notify(struct notifier_block *self,
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for_each_online_cpu(cpu) {
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smp_call_function_single(cpu, rcu_oom_notify_cpu, NULL, 1);
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cond_resched_rcu_qs();
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cond_resched_tasks_rcu_qs();
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}
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/* Unconditionally decrement: no need to wake ourselves up. */
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@ -2227,7 +2227,7 @@ static int rcu_nocb_kthread(void *arg)
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cl++;
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c++;
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local_bh_enable();
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cond_resched_rcu_qs();
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cond_resched_tasks_rcu_qs();
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list = next;
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}
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trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1);
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@ -624,7 +624,7 @@ EXPORT_SYMBOL_GPL(call_rcu_tasks);
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* grace period has elapsed, in other words after all currently
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* executing rcu-tasks read-side critical sections have elapsed. These
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* read-side critical sections are delimited by calls to schedule(),
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* cond_resched_rcu_qs(), idle execution, userspace execution, calls
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* cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls
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* to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
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*
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* This is a very specialized primitive, intended only for a few uses in
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@ -574,7 +574,7 @@ void stutter_wait(const char *title)
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{
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int spt;
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cond_resched_rcu_qs();
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cond_resched_tasks_rcu_qs();
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spt = READ_ONCE(stutter_pause_test);
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for (; spt; spt = READ_ONCE(stutter_pause_test)) {
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if (spt == 1) {
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@ -159,13 +159,13 @@ static int benchmark_event_kthread(void *arg)
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* wants to run, schedule in, but if the CPU is idle,
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* we'll keep burning cycles.
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*
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* Note the _rcu_qs() version of cond_resched() will
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* Note the tasks_rcu_qs() version of cond_resched() will
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* notify synchronize_rcu_tasks() that this thread has
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* passed a quiescent state for rcu_tasks. Otherwise
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* this thread will never voluntarily schedule which would
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* block synchronize_rcu_tasks() indefinitely.
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*/
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cond_resched();
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cond_resched_tasks_rcu_qs();
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}
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return 0;
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